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Measurement of elastic φ photoproduction at HERA
Abstract
The production of φ mesons in the reaction e+p → e+φp (gf → K+K−) at a median Q2 of 10−4 GeV2 h been studied with the ZEUS detector at HERA. The differential φ photoproduction cross section has an exponential shape and has been determined in the kinematic range 0.1 < |t| < 0.5 GeV2 and 60 < W < 80 GeV. An integrated cross sect σγp→φp = 0.96±0.19−0.18+0.21μb has been obtained by extrapolating to t = 0. When compared to lower energy data the results show a weak energy dependence of both σγp→φp and the slope of the t distribution. The φ decay angular distributions are consistent with s-channel helicity conservation. From lower energies to HERA energies, the features of φ photoproduction are compatible with those of a soft diffractive process.
... Our dataset consists of differential cross section measurements for exclusive ω, ϕ-photoproduction. These include high precision measurements with the CEBAF Large Acceptance Spectrometer (CLAS) [69,70], which go from threshold up to √ s ≈ 2.8 GeV; older measurements [71][72][73], which extend up to √ s ≈ 20 GeV; and from ZEUS at much larger energies, √ s ≈ 70 − 94 GeV [74][75][76]. This latter is particularly important for determining the pomeron contribution in γp → ωp, which is not well-constrained from low-energy data alone. ...
... However, it is reassuring that our model successfully describes other datasets as well. These include data from SLAC [71] and Daresbury [72,88], Fermilab [73], and even ZEUS [74][75][76] at much larger energy (shown in figures 11 and 12). Thus, we have confidence in the accuracy of our model as a function of √ s, except for near threshold. ...
... Shaded bands show our phenomenological model (90% confidence intervals) for the γp → ωp, ϕp differential cross sections, for different center-of-mass energies √ s. Data points are experimental results obtained at Fermilab [73] (upper panels) and from ZEUS (lower panels, left [74], middle [75], and right [76]). Black points are included in our fit, gray points are not. ...
A bstract
New GeV-scale dark forces coupling predominantly to quarks offer novel signatures that can be produced directly and searched for at high-luminosity colliders. We compute the photon-proton and electron-proton cross sections for producing a GeV-scale gauge boson arising from a U(1) B gauge symmetry. Our calculation relies on vector meson dominance and a phenomenological model for diffractive scattering used for vector-meson photoproduction. The parameters of our phenomenological model are fixed by performing a Markov Chain Monte Carlo fit to existing exclusive photoproduction data for ω and ϕ mesons. Our approach can be generalized to other GeV-scale dark gauge forces.
... The vector meson production from linearly polarized photons also possesses distinctive signature in the asymmetries of the decay angular distributions. Continuous efforts [8][9][10][11][12][13][14] has been made for more than fifty years to utilize the linearly polarized photons in vector meson photoproduction as a parity filter for the exchange of particles in t channel, which is an effective tool to separate the natural from unnatural parity exchange in t channel. In this letter, we exploit the linearly polarized photons to investigate the interference effect for the vector meson photoproduction in heavy-ion collisions and demonstrate a periodic oscillation with transverse momentum for the asymmetries of the decay angular distributions. ...
... where P γ is the degree of linear polarization, and Φ is the angle between the photon polarization plane and the ρ 0 production plane. The helicity nonflip assumption is consistent with the spin density matrix element measurements for ρ 0 from STAR [13] and ZEUS [12]. As revealed in Eq. 1, the linearly polarized photons result in the 2ndorder modulations in azimuth between the vector meson momentum and the π ± momentum. ...
... Here, A is the final nuclear state containing products of the nuclear disintegration. The approach gives: (12) where σ in ρ 0 N = σ ρ 0 N − σ 2 ρ 0 N /(16πB ρ 0 ) is the inelastic ρ 0nucleon cross section, and B ρ 0 is the slope of the t dependence of the γp → ρ 0 p scattering [19]. ...
The linearly polarized quasi-real photons from the highly Lorentz-contracted Coulomb fields of relativistic heavy ions can fluctuate to quark-antiquark pairs, scatter off a target nucleus and emerge as vector mesons. In the process, the two colliding nuclei can switch roles to act as photon emitter or target, forming a double-slit interference pattern. The product from photoproduction inherits the photon polarization states, leading to the asymmetries of the decay angular distributions. In this letter, we study the interference effect in polarization dimension from the asymmetries of the decay angular distributions for photoprodution in heavy-ion collisions and find a periodic oscillation with the transverse momentum of vector meson, which could reasonably explain the transverse momentum dependence of the 2nd-order modulation in azimuth for the $\rho^{0}$ decay observed by the STAR collaboration.
... under "EPS09" in figure 4 presenting the uncertainties of the nuclear modification. Note that the two LO pQCD calculations carry ad hoc normalisation factors of the cross-section determined using high-energy HERA data [12,29]. Both of them predict well the shapes of the data for both J/ψ and ψ(2S) production as a function of y * in figure 4. A slightly larger (smaller) p * T is predicted for J/ψ (ψ(2S)) production than the data in figure 5. ...
... Various calculations within the framework of the colour-dipole model [11,[34][35][36][37][38][39] are also compared in figure 4 for both J/ψ (left) and ψ(2S) (right). All these models inherit parameters determined by previous HERA data [3,4,29]. ...
A bstract
The cross-sections of exclusive (coherent) photoproduction J/ψ and ψ (2S) mesons in ultra-peripheral PbPb collisions at a nucleon-nucleon centre-of-mass energy of 5 . 02 TeV are measured using a data sample corresponding to an integrated luminosity of 228 ± 10 μb − 1 , collected by the LHCb experiment in 2018. The differential cross-sections are measured separately as a function of transverse momentum and rapidity in the nucleus-nucleus centre-of-mass frame for J/ψ and ψ (2S) mesons. The integrated cross-sections are measured to be $$ {\sigma}_{J/\psi}^{\textrm{coh}} $$ σ J / ψ coh = 5 . 965 ± 0 . 059 ± 0 . 232 ± 0 . 262 mb and $$ {\sigma}_{\psi (2S)}^{\textrm{coh}} $$ σ ψ 2 S coh = 0 . 923 ± 0 . 086 ± 0 . 028 ± 0 . 040 mb, where the first listed uncertainty is statistical, the second systematic and the third due to the luminosity determination. The cross-section ratio is measured to be $$ {\sigma}_{\psi (2S)}^{\textrm{coh}}/{\sigma}_{J/\psi}^{\textrm{coh}} $$ σ ψ 2 S coh / σ J / ψ coh = 0 . 155 ± 0 . 014 ± 0 . 003, where the first uncertainty is statistical and the second is systematic. These results are compatible with theoretical predictions.
... For the case of J=ψ, these results are similar to a parametrization used in [22]. In Fig. 2, we show as an example the differential cross section for ϕ photoproduction, comparing our simple fit with the data taken by the ZEUS Collaboration at HERA [23]. ...
... Differential cross section for diffractive photoproduction of ϕ mesons. Data are from the ZEUS Collaboration at HERA[23]. ...
We calculate distributions of different vector mesons in purely exclusive (pp→ppV) and semiexclusive (pp→pXV) processes with the electromagnetic dissociation of a proton. The cross section for the electromagnetic dissociation is expressed through electromagnetic structure functions of the proton. We include the transverse momentum distribution of initial photons in the associated flux. Contributions of the exclusive and semiexclusive processes are compared for different vector mesons (V=ϕ, J/ψ, ϒ). We discuss how the relative contribution of the semiexclusive processes depends on the mass of the vector meson as well as on different kinematical variables of the vector meson (y, pt). The ratio of semiexclusive to exclusive contributions are shown and compared for different mesons in different variables.
... The vector meson production from linearly polarized photons also possesses a distinctive signature in the asymmetries of the decay angular distributions. Continuous efforts [19][20][21][22][23][24][25][26] has been made for more than 50 years to utilize the linearly polarized photons in vector meson photoproduction as a parity filter for the exchange of particles in the t channel, which is an effective tool to separate the natural from unnatural parity exchange in the t channel. Since the photons are in a linear polarization state, the interference can reveal itself in polarization space, inducing an asymmetry in the decay angular distributions. ...
... Under the helicity no-flip assumption, the vector meson inherits the photon polarization state, which is fully linearly polarized. The helicity conservation assumption has been investigated by various experimental measurements [20][21][22][23][24][25]. Following Ref. [27] and the derivation in the Appendix, the decay angular distribution of vector meson to two spinless products (e.g., ρ 0 → π þ þ π − ) is ...
The linearly polarized quasireal photons from the highly Lorentz-contracted Coulomb fields of relativistic heavy ions can fluctuate to quark-antiquark pairs, scatter off a target nucleus, and emerge as vector mesons. In the process, the two colliding nuclei can serve as slits, forming a Young’s double-slit experiment. In addition to the well-known double-slit interference pattern in position space, a similar interference pattern may be expected in polarization space due to the linear polarization of the colliding photons. In this paper, we investigate the interference effect in polarization space as revealed by the asymmetries of the decay angular distribution for vector meson photoprodution in heavy-ion collisions. We find a periodic oscillation pattern with transverse momentum, which can reasonably explain the second-order modulation in azimuth for the ρ0 decay observed by the STAR Collaboration.
... In (2.12) the coupling parameters a Pϕϕ and b Pϕϕ have dimensions GeV −3 and GeV −1 , respectively. In [57] we have fixed the coupling parameters of the tensor Pomeron to the ϕ meson based on the HERA experimental data for the γp → ϕp reaction [79,80]. However, the ω-ϕ mixing effect was not taken into account there. ...
... Right panel: The differential cross section dσ=djtj for the γp → ϕp process. We show the ZEUS data at low jtj (at W γp ¼ 70 GeV and the squared photon virtuality Q 2 ¼ 0 GeV 2 , solid marks, [79]) and at higher jtj (at W γp ¼ 94 GeV and Q 2 < 0.01 GeV 2 , open circles, [80]). Again, the results for the two parameter sets, set A (B8) and set B (B9), are presented. ...
We explore the possibility of observing odderon exchange in the pp→ppK+K− and pp→ppμ+μ− reactions at the LHC. We consider the central exclusive production (CEP) of the ϕ(1020) resonance decaying into K+K− and μ+μ−. We compare the purely diffractive contribution (odderon-Pomeron fusion) to the photoproduction contribution (photon-Pomeron fusion). The theoretical results are calculated within the tensor-Pomeron and vector-odderon model for soft reactions. We include absorptive corrections at the amplitude level. In order to fix the coupling constants for the photon-Pomeron fusion contribution we discuss the reactions γp→ωp and γp→ϕp including ϕ−ω mixing. We compare our results for these reactions with the available data, especially those from HERA. Our coupling constants for the Pomeron-odderon-ϕ vertex are taken from an analysis of the WA102 data for the pp→ppϕ reaction. We show that the odderon-exchange contribution significantly improves the description of the pp azimuthal correlations and the dPt “glueball-filter variable” dependence of ϕ CEP measured by WA102. To describe the low-energy data more accurately we consider also subleading processes with Reggeized vector-meson exchanges. However, they do not play a significant role at the LHC. We present predictions for two possible types of measurements: at midrapidity and with forward measurement of protons (relevant for ATLAS-ALFA or CMS-TOTEM), and at forward rapidities and without measurement of protons (relevant for LHCb). We discuss the influence of experimental cuts on the integrated cross sections and on various differential distributions. With the corresponding LHC data one should be able to get a decisive answer concerning the presence of an odderon-Pomeron fusion contribution in single ϕ CEP.
... The data in the right panel is from the LEPS data [3]. [4,[22][23][24][25][26][27][28], where the data points named as Dey 14 are obtained by integrating over the differential cross sections given in Ref. [4]. Our model favors the CLAS data [4] and data of Ref. [27] as well. ...
... Data are from Refs. [22,31]. ...
We investigate the role driven by the scalar meson σ exchange in the photoproduction of the vector meson ϕ(1020) off a proton by using a Reggeized model. Based on the π0(135)+σ(500)+f2(1270)+Pomeron exchanges, we demonstrate that the σ exchange plays the role to reproduce the bump structure at the forward angle in the differential cross section as well as the peaking behavior in the total cross section observed in the CLAS Collaboration. We also discuss the possible observation of the scaled cross section s7dσ/dt at the production angle θ=90° from the CLAS data. It is found that the axial vector meson f1(1285) exchange with the trajectory αf1(t)=0.028t+0.9±0.2 arising from the axial anomaly of the QCD vacuum plays the role to clarify the scaling up to 5 GeV.
... In the ideal case (ignoring the size and density distribution of nuclei), the polarization direction of the scattered photons is fully aligned with the impact parameter for coherent photoproduction in heavy-ion collisions. Under the helicity no-flip assumption [16][17][18][19][20][21], the produced vector meson inherits the linear polarization state, which leads to the preferential orientation of the decay angle along the direction of polarization. This offers us an opportunity to determine the reaction plane via the decay asymmetry of the vector meson from coherent photoproduction, where the reaction plane is spanned by the impact parameter and the beam axis. ...
The collective observables play critical roles in probing the properties of quark-gluon-plasma created in relativistic heavy-ion collisions, in which the information on initial collision geometry is crucial. However, the initial collision geometry, e.g., the reaction plane, cannot be directly extracted in the experiment. In this paper, we demonstrate the idea of determining the reaction plane via the feature of linear polarization of the coherent photoproduction process and discuss the advantages of the proposed approach in comparison with traditional methods.
... In the ideal case (ignoring the size and density distribution of nuclei), the polarization direction of the scattered photons is fully aligned with the impact parameter for coherent photoproduction in heavy-ion collisions. Under the helicity no-flip assumption [16][17][18][19][20][21], the produced vector meson inherits the linear polarization state, which leads to the preferential orientation of the decay angle along the direction of polarization. This offers us an opportunity to determine the reaction plane via the decay asymmetry of the vector meson from coherent photoproduction, where the reaction plane is spanned by the impact parameter and the beam axis. ...
The collective observables play critical roles in probing the properties of quark-gluon-plasma created in relativistic heavy-ion collisions, in which the information on initial collision geometry is crucial. However, the initial collision geometry, e.g., the reaction plane, cannot be directly extracted in the experiment. In this paper, we demonstrate the idea of determining the reaction plane via the feature of linear polarization of the coherent photoproduction process and discuss the advantages of the proposed approach in comparison with traditional methods.
... The elastic scattering process in photoproduction is often referred to as exclusive vector-meson production for which there are nowadays a good amount of data from HERA experiments, see e.g. [273][274][275][276][277][278][279]. The SaS parameterization tends to provide a good description for low-mass vector-meson production, e.g. in case of ρ 0 , but underestimates higher-mass states such as the J/ψ by a large margin. ...
This manual describes the Pythia event generator, the most recent version of an evolving physics tool used to answer fundamental questions in particle physics. The program is most often used to generate high-energy-physics collision “events”, i.e. sets of particles produced in association with the collision of two incoming high-energy particles, but has several uses beyond that. The guiding philosophy is to produce and re-produce properties of experimentally obtained collisions as accurately as possible. The program includes a wide ranges of reactions within and beyond the Standard Model, and extending to heavy ion physics. Emphasis is put on phenomena where strong interactions play a major role. The manual contains both pedagogical and practical components. All included physics models are described in enough detail to allow the user to obtain a cursory overview of used assumptions and approximations, enabling an informed evaluation of the program output. A number of the most central algorithms are described in enough detail that the main results of the program can be reproduced independently, allowing further development of existing models or the addition of new ones. Finally, a chapter dedicated fully to the user is included towards the end, providing pedagogical examples of standard use cases, and a detailed description of a number of external interfaces. The program code, the online manual, and the latest version of this print manual can be found on the Pythia web page: https://www.pythia.org/.
... The solid curves are calculated from using the Pom-DL model. Data are from Refs.[17,[35][36][37][95][96][97][98][99][100][101][102][103][104][105][106]. ...
The $J/\Psi$ photo-production reactions on the nucleon can provide information on the roles of gluons in determining the $J/\Psi$-nucleon ($J/\Psi$-N) interactions and the structure of the nucleon. The information on the $J/\Psi$-N interactions is needed to test lattice QCD (LQCD) calculations and to understand the nucleon resonances such as $N^*(P_c)$ recently reported by the LHCb Collaboration. In addition, it is also needed to investigate the production of nuclei with hidden charms and to extract the gluon distributions in nuclei. The main purpose of this article is to review six reaction models of $\gamma + p \rightarrow J/\Psi +p$ reactions which have been and can be applied to analyze the data from Thomas Jefferson National Accelerator Facility (JLab). The formulae for each model are given and used to obtain the results to show the extent to which the available data can be described. The models presented include the Pomeron-exchange model of Donnachie and Landshoff (Pom-DL) and its extensions to include $J/\Psi$-N potentials extracted from LQCD (Pom-pot) and to also use the constituent quark model (CQM) to account for the quark substructure of $J/\Psi$ (Pom-CQM). The other three models are developed from applying the perturbative QCD approach to calculate the two-gluon exchange using the generalized parton distribution (GPD) of the nucleon (GPD-based), two- and three-gluon exchanges using the parton distribution of the nucleon ($2g+3g$), and the exchanges of scalar ($0^{++}$) and tensor ($2^{++}$) glueballs within the holographic formulation (holog). The results of investigating the excitation of the nucleon resonances $N^*(P_c)$ in the $\gamma + p \rightarrow J/\Psi +p$ reactions are also given.
... refs. [274][275][276][277][278][279][280]. The SaS parameterization tends to provide a good description for low-mass vector-meson production, e.g. in case of ρ 0 , but underestimates higher-mass states such as the J/ψ by a large margin. ...
This manual describes the PYTHIA 8.3 event generator, the most recent version of an evolving physics tool used to answer fundamental questions in particle physics. The program is most often used to generate high-energy-physics collision "events", i.e. sets of particles produced in association with the collision of two incoming high-energy particles, but has several uses beyond that. The guiding philosophy is to produce and reproduce properties of experimentally obtained collisions as accurately as possible. The program includes a wide ranges of reactions within and beyond the Standard Model, and extending to heavy ion physics. Emphasis is put on phenomena where strong interactions play a major role. The manual contains both pedagogical and practical components. All included physics models are described in enough detail to allow the user to obtain a cursory overview of used assumptions and approximations, enabling an informed evaluation of the program output. A number of the most central algorithms are described in enough detail that the main results of the program can be reproduced independently, allowing further development of existing models or the addition of new ones. Finally, a chapter dedicated fully to the user is included towards the end, providing pedagogical examples of standard use cases, and a detailed description of a number of external interfaces. The program code, the online manual, and the latest version of this print manual can be found on the PYTHIA web page: https://www.pythia.org/
... In (3.12) the coupling parameters a Pϕϕ and b Pϕϕ have dimensions GeV −3 and GeV −1 , respectively. In [20] we have fixed the coupling parameters of the tensor Pomeron to the ϕ meson based on the HERA experimental data for the γp → ϕp reaction [62,63]. We take the coupling constants a Pϕϕ ¼ 0.49 GeV −3 and b Pϕϕ ¼ 4.27 GeV −1 from Table II of [20] (see also Sec. ...
We present a study of the exclusive pp→ppK+K−K+K− reaction at high energies. We consider diffractive mechanisms with the intermediate ϕϕ state with its decay into the K+K−K+K− system. We include the ϕ(1020)t^/u^-channel exchanges and the f2(2340)s-channel exchange mechanism. This f2 state is a candidate for a tensor glueball. We discuss the possibility to use the pp→ppϕϕ process in identifying the Odderon exchange. An upper limit for the POϕ coupling is extracted from the WA102 experimental data. The amplitudes for the processes are formulated within the tensor-Pomeron and vector-Odderon approach. We adjust parameters of our model to the WA102 data and present several predictions for the ALICE, ATLAS, CMS, and LHCb experiments. Integrated cross sections of order of a few nb are obtained, including the experimental cuts relevant for the LHC experiments. The distributions in the four-kaon invariant mass, rapidity distance between the two ϕ mesons, special “glueball filter variable,” and proton-proton relative azimuthal angle are presented. The distribution in rapidity difference of both ϕ-mesons could shed light on the f2(2340)→ϕϕ coupling, not known at present. We discuss the possible role of the f0(2100), η(2225), and X(2500) resonances observed in the ϕϕ channel in radiative decays of J/ψ. Using typical kinematic cuts for LHC experiments, we find from our model that the Odderon-exchange contribution should be distinguishable from other contributions for large rapidity distance between the ϕ mesons and in the region of large four-kaon invariant masses. At least, it should be possible to derive an upper limit on the Odderon contribution in this reaction.
... For this purpose we can use so called "standard candle" processes, i.e. events which have the same theoretical ingredients for the calculations. For low central masses we can use processes: [63]; p + p → p + M + p, M = {qq} (light meson) or "glueball" [10]- [14], M = hh (dihadron system) [15]. From the first principles (covariant reggeization approach [59]) we can write the general structure of the vertex for different cases. ...
The case of the low invariant mass exclusive central diffractive production
is considered in the general theoretical framework. It is shown that
diffractive patterns (differential cross-sections on variables like transfer
momenta squared, the azimuthal angle between final hadrons and their
combinations) can serve as a unique tool to explore the picture of the $pp$
interaction and falsify theoretical models. Basic kinematical and dynamical
properties of the process are considered in detail. As an example,
visualizations of diffractive patterns in the model with three pomerons for
processes $p+p\to p+R+p$ and $p+p\to p+\pi^{+}\pi^{-}+p$ are presented.
... The nearly real photons produced by HERA's electron beam extend the energy range for photoproduction processes by more than one order of magnitude, up to the equivalent of tens of TeV in fixed target experiments, providing a large kinematic range for studying the properties of the photon. The H1 and ZEUS collaborations have begun to explore this domain, e.g. with measurements of the total γp cross section [1,2,3] and vector meson production [2,4,5,6,7]. ...
The cross section for the elastic photoproduction of ϱ0 mesons (γp → ϱ0p) has been measured with the H1 detector at HERA for two average photon-proton centre-of-mass energies of 55 and 187 GeV. The lower energy point was measured by observing directly the ϱ0 decay giving a cross section of 9.1 ± 0.9 (stat.) ± 2.5 (syst.) μb. The logarithmic slope parameter of the differential cross section, dσ/dt, is found to be 10.9 ± 2.4 (stat.) ± 1.1 (syst.) GeV−2. The ϱ0 decay polar angular distribution is found to be consistent with s-channel helicity conservation. The higher energy cross section was determined from analysis of the lower part of the hadronic invariant mass spectrum of diffractive photoproduction and found to be 13.6 ± 0.8 (stat.) ± 2.4 (syst.) μb.
... for low central masses (M ∼ 1 GeV, nonperturbative Pomeron-Pomeron fusion) [74]; -p + p → p + M + p, M = {qq} (light meson) or "glueball" [33]- [37], M = hh (dihadron system) [45]; ...
We consider the general theoretical framework to study exclusive double
diffractive events (EDDE). It is a powerful tool to study the picture of the
$pp$ interaction. Basic kinematical and dynamical properties of the process,
and also normalization of parameters via standard processes like the exclusive
vector meson production (EVMP), are considered in detail. As an example,
calculations of the cross-sections in the model with three pomerons for the
process $p+p\to p+M+p$ are presented for Tevatron and LHC energies.
K + K − pairs may be produced in photonuclear collisions, either from the decays of photoproduced ϕ ( 1020 ) mesons or directly as nonresonant K + K − pairs. Measurements of K + K − photoproduction probe the couplings between the ϕ ( 1020 ) and charged kaons with photons and nuclear targets. The kaon-proton scattering occurs at energies far above those available elsewhere. We present the first measurement of coherent photoproduction of K + K − pairs on lead ions in ultraperipheral collisions using the ALICE detector, including the first investigation of direct K + K − production. There is significant K + K − production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range 1.1 < M K K < 1.4 GeV / c 2 above the ϕ ( 1020 ) resonance, for rapidity | y K K | < 0.8 and p T , K K < 0.1 GeV / c , the measured coherent photoproduction cross section is d σ / d y = 3.37 ± 0.61 ( stat ) ± 0.15 ( syst ) mb . The center-of-mass energy per nucleon of the photon-nucleus (Pb) system W γ Pb , n ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for ϕ ( 1020 ) photoproduction alone. The mass spectrum is fit to a cocktail consisting of ϕ ( 1020 ) decays, direct K + K − photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct K + K − photoproduction are presented.
© 2024 CERN, for the ALICE Collaboration 2024 CERN
There are two mass generating mechanisms in the standard model of particle physics (SM). One is related to the Higgs boson and fairly well understood. The other is embedded in quantum chromodynamics (QCD), the SM’s strong interaction piece; and although responsible for emergence of the roughly 1 GeV mass scale that characterises the proton and hence all observable matter, the source and impacts of this emergent hadronic mass (EHM) remain puzzling. As bound states seeded by a valence-quark and -antiquark, pseudoscalar mesons present a simpler problem in quantum field theory than that associated with the nucleon. Consequently, there is a large array of robust predictions for pion and kaon properties whose empirical validation will provide a clear window onto many effects of both mass generating mechanisms and the constructive interference between them. This has now become significant because new-era experimental facilities, in operation, construction, or planning, are capable of conducting such tests and thereby contributing greatly to resolving the puzzles of EHM. These aspects of experiment, phenomenology, and theory, along with contemporary successes and challenges, are reviewed herein. In addition to providing an overview of the experimental status, we focus on recent progress made using continuum Schwinger function methods and lattice-regularized QCD. Advances made using other theoretical tools are also sketched. Our primary goal is to highlight the potential gains that can accrue from a coherent effort aimed at finally reaching an understanding of the character and structure of Nature’s Nambu–Goldstone modes.
We discuss central exclusive diffractive production of light mesons in the reactions pp→ppπ+π−$pp \to pp{\pi ^ + }{\pi ^ - }$ and pp→ppK+K−$pp \to pp{K^ + }{K^ - }$. The calculation is based on a tensor-pomeron approach. We include a purely diffractive dipion continuum, and the scalar f0(980), f0(1500), f0(1710) and tensor f2(1770), f′2(1525)${f^{\prime}_2}(1525)$ resonances decaying into pseudoscalar meson pairs. We include also photoproduction mechanisms for the nonresonant (Drell-Söding) and the ϕ(1020) resonance contributions. The theoretical results are compared with existing CDF experimental data and predictions for being carried out LHC experiments are presented. The distributions in dimeson invariant mass and in a special "glueball filter variable" including the interference effects of resonance and dimeson continuum are presented.
We discuss diffractive photon production of vector mesons in holographic QCD. At large s, the QCD scattering amplitudes are reduced to the scattering of pairs of dipoles exchanging a closed string or a pomeron. We use the holographic construction in a 5-dimensional anti-deSitter space (AdS5) to describe both the intrinsic dipole distribution in each hadron, and the pomeron exchange. Our results for the heavy meson photon production are made explicit and compared to some existing experiments.
We present a study of the central exclusive production of the $K^{+} K^{-}$ pairs in proton-proton collisions at high energies. We consider diffractive mechanisms including the $K^{+} K^{-}$ continuum, the dominant scalar $f_{0}(980)$, $f_{0}(1500)$, $f_{0}(1710)$ and tensor $f_{2}(1270)$, $f'_{2}(1525)$ resonances decaying into the $K^{+} K^{-}$ pairs. We include also photoproduction mechanisms for the non-resonant (Drell-S\"{o}ding) and the $\phi(1020)$ resonance contributions. The theoretical results are calculated within the tensor-pomeron approach including both pomeron and reggeon exchanges. Predictions for planned or current experiments at RHIC and LHC are presented. We discuss the influence of the experimental cuts on the integrated cross section and on various differential distributions for outgoing particles. The distributions in two-kaon invariant mass, in a special "glueball filter variable", as well as examples of angular distributions in the $K^{+}K^{-}$ rest frame are presented. We compare the $\phi(1020)$ and continuum photoproduction contributions to the $f_{0}(980)$ and continuum diffractive contributions and discuss whether the $\phi(1020)$ resonance could be extracted experimentally. For the determination of some model parameters we also include a discussion of $K$-nucleon scattering, in particular total cross sections, and of $\phi(1020)$ photoproduction.
We make predictions for the cross sections of coherent photoproduction of ρ,ϕ,J/ψ,ψ(2S), and Υ(1S) mesons in Pb-Pb ultraperipheral collisions (UPCs) at sNN=5.02TeV in the kinematics of run 2 at the Large Hadron Collider extending the approaches successfully describing the available Pb-Pb UPC data at sNN=2.76TeV. Our results illustrate the important roles of hadronic fluctuations of the photon and inelastic nuclear shadowing in photoproduction of light vector mesons on nuclei and the large leading twist nuclear gluon shadowing in photoproduction of quarkonia on nuclei. We show that the ratio of ψ(2S) and J/ψ photoproduction cross sections in Pb-Pb UPCs is largely determined by the ratio of these cross sections on the proton. We also argue that UPCs with electromagnetic excitations of the colliding ions followed by the forward neutron emission allows one to significantly increase the range of photon energies accessed in vector meson photoproduction on nuclei.
The problem of the timelike Pomeron coupling to light mesons and photons is considered in light of available data on high-energy meson-proton scattering. Possible correspondence of $f_2(1950)$ meson to the ground state of Pomeron is argued.
We study the vector meson electro-production off the proton in a QCD inspired model. A calculation of the differential cross section is performed for the J/psi, phi meson off the proton. The theoretical results are consistent with the experimental data, and remind us to consider the contribution from the tensor glueball and Odderon to the differential cross section. Since gluons interact among themselves via self-interaction, the gluons can form a glueball with quantum numbers I(G), J(PC) = 0(+), 2(++), with a decay width Gamma(t) = 100 MeV and mass of m(G) = 2.23 GeV. The three gluons can form a three gluon color bound state with charge conjugation quantum number C = -1. This study is quite important to verify the validity of QCD and to search for new particles (tensor glueball and Odderon) as well as quest for new physics.
A first measurement is presented of exclusive photoproduction of \(\rho ^0\) mesons associated with leading neutrons at HERA. The data were taken with the H1 detector in the years 2006 and 2007 at a centre-of-mass energy of \(\sqrt{s}=319\) GeV and correspond to an integrated luminosity of 1.16 pb\(^{-1}\). The \(\rho ^0\) mesons with transverse momenta \(p_T<1\) GeV are reconstructed from their decays to charged pions, while leading neutrons carrying a large fraction of the incoming proton momentum, \(x_L>0.35\), are detected in the Forward Neutron Calorimeter. The phase space of the measurement is defined by the photon virtuality \(Q^2 < 2\) GeV\(^2\), the total energy of the photon–proton system \(20 < W_{\gamma p}< 100\) GeV and the polar angle of the leading neutron \(\theta _n < 0.75\) mrad. The cross section of the reaction \(\gamma p \rightarrow \rho ^0 n \pi ^+\) is measured as a function of several variables. The data are interpreted in terms of a double peripheral process, involving pion exchange at the proton vertex followed by elastic photoproduction of a \(\rho ^0\) meson on the virtual pion. In the framework of one-pion-exchange dominance the elastic cross section of photon-pion scattering, \(\sigma ^\mathrm{el}(\gamma \pi ^+ \rightarrow \rho ^0\pi ^+)\), is extracted. The value of this cross section indicates significant absorptive corrections for the exclusive reaction \(\gamma p \rightarrow \rho ^0 n \pi ^+\).
We review the status of the data and phenomenology in the Generalized Parton
Distribution approach of Deep Virtual Meson Production.
We present a range of physics results for central exclusive production
processes at the LHC, using the new SuperChic 2 Monte Carlo event generator.
This includes significant theoretical improvements and updates, most
importantly a fully differential treatment of the soft survival factor, as well
as a greater number of generated processes. We provide an overview of the
latest theoretical framework, and consider in detail a selection of final
states, namely exclusive 2 and 3 jets, photoproduced vector mesons, two-photon
initiated muon and $W$ boson pairs and heavy $\chi_{c,b}$ quarkonia.
The cross section for the elastic photoproduction of rho(0) mesons (gamma p --> rho(0)p) has been measured with the H1 detector at HERA for two average photon-proton centre-of-mass energies of 55 and 187 GeV. The lower energy point was measured by observing directly the rho(0) decay giving a cross section of 9.1 +/- 0.9 (stat.) +/- 2.5 (syst.) mu b. The logarithmic slope parameter of the differential cross section, d sigma/dt, is found to be 10.9 +/- 2.4 (stat.) +/- 1.1 (syst.) GeV-2. The rho(0) decay polar angular distribution is found to be consistent with s-channel helicity conservation, The higher energy cross section was determined from analysis of the lower part of the hadronic invariant mass spectrum of diffractive photoproduction and found to be 13.6 +/- 0.8 (stat.) +/- 2.4(syst.) mu b.
A selection of hard exclusive measurements (deeply virtual Compton Scattering and meson production) from past, present and future experimental programs are reviewed. The goal is the determination of the Generalized Parton Distributions which offer one of the most complete description of the partonic structure of the nucleon.
Studies of diffractive events performed by the H1 and ZEUS
Collaborations at the HERA ep collider are presented. The data of vector
meson and photon production, inclusive deep inelastic scattering and
hadronic final states with open charm and jets are confronted with the
predictions of the resolved Pomeron and colour dipole models.
We study the tensor structure function b1(x,Q2) in
deep inelastic scattering of an electron from a polarized deuteron
target. We model the electron-nucleon cross section at the starting
point for Q2 evolution by vector-meson dominance. Shadowing
due to the double scattering of vector mesons, along with the presence
of a d-state admixture in a ground-state deuteron wave function gives
rise to a nonvanishing contribution to b1(x,Q2).
We find a large enhancement at low x in qualitative agreement with other
recent estimates of double-scattering contributions to
b1(x,Q2). If the model is valid, it should apply
within the range of present fixed-target experiments.
High-statistics measurements of differential cross sections and spin density
matrix elements for the reaction $\gamma p \to \phi p$ have been made using the
CLAS detector at Jefferson Lab. We cover center-of-mass energies ($\sqrt{s}$)
from 1.97 to 2.84 GeV, with an extensive coverage in the $\phi$ production
angle. The high statistics of the data sample made it necessary to carefully
account for the interplay between the $\phi$ natural lineshape and effects of
the detector resolution, that are found to be comparable in magnitude. We study
both the charged- ($\phi \to K^+ K^-$) and neutral- ($\phi \to K^0_S K^0_L$)
$K\overline{K}$ decay modes of the $\phi$. Further, for the charged mode, we
differentiate between the cases where the final $K^-$ track is directly
detected or its momentum reconstructed as the total missing momentum in the
event. The two charged-mode topologies and the neutral-mode have different
resolutions and are calibrated against each other. Extensive usage is made of
kinematic fitting to improve the reconstructed $\phi$ mass resolution. Our
final results are reported in 10- and mostly 30-MeV-wide $\sqrt{s}$ bins for
the charged- and the neutral-mode, respectively. Possible effects from $K^+
\Lambda^\ast$ channels with $p K\overline{K}$ final-states are discussed. These
present results constitute the most precise and extensive $\phi$
photoproduction measurements to date and in conjunction with the $\omega$
photoproduction results recently published by CLAS, will greatly improve our
understanding of low energy vector meson photoproduction.
Recent measurements of the diffractive production of neutral vector
mesons at HERA are reviewed. The issue of transition between `soft' and
`hard' underlying production mechanism is discussed in context of the
phenomenology of Regge theory and the predictions of the perturbative
QCD.
We compare the contributions from quark and from gluon exchange to the
exclusive process γ∗p→ρ0p.
We present evidence that the gluon contribution is substantial for
values of the Bjorken variable xB around 0.1.
Spin-parity analyses of the ωπ system in the reaction
γp→(ωπ)p for photon laboratory energies from 20 to
70 GeV have shown that production of the
JP=1+b1(1235) meson dominates, with a
JP=1- background at the level of 20%. Using
vector-meson dominance arguments, this background is shown to be
consistent with the data on
e+e-→ωπ. The energy dependence
of the data imply that the mechanism is a combination of reggeon and
pomeron exchange. Assuming that the latter is relevant only for the
JP=1- component and extrapolating to W=200
GeV, it is argued that this accounts for most of the preliminary
ωπ signal observed by the H1 Collaboration in the same
reaction. A residual peak can be ascribed to the b1(1235),
which requires a quark spin-flip from pomeron exchange. Precisely the
same mechanism occurs in the reaction πp→a1(1260)p.
A Pomeron model applicable to both "`soft" and "hard" processes is suggested
and tested against the high-energy data from virtual photon-induced reactions.
The Pomeron is universal, containing two terms, a "soft" and a "hard" one,
whose relative weight varies with $\widetilde {Q^2}=Q^2+M_V^2$, where $Q^2$ is
the virtuality of the incoming photon and $M_V$ is the mass of the produced
vector particle. With a small number of adjustable parameters, the model fits
all available data on vector meson production and deeply virtual Compton
scattering from HERA. Furthermore, we attempt to apply the model to
hadron-induced reactions, by using high-energy data from proton-proton
scattering.
A two-component model is developed for diffractive electroproduction of
ρ0, φ, and J/ψ, based on nonperturbative and
perturbative two-gluon exchange. This provides a common kinematical
structure for nonperturbative and perturbative effects, and allows the
role of the vector-meson vertex functions to be explored independently
of the production dynamics. A good global description of the
vector-meson data is obtained.
A compilation of the last H1 and ZEUS results for vector meson
production in photoproduction on HERA is presented.
The Q
2 dependence of diffractive ρ meson electroproduction has been analyzed in the present paper. This dependence arises from the virtual photon–vector meson–reggeon vertex that is represented by a triangle of a quark loop. Integration over the quark loop produces a form involving a Spence function that is symmetric in the external masses connected to the triangle. In the Regge model, the vertex is considered a residue of the Regge pole. As the parameterization of the Regge residue is given by an exponential function of t times the residue at t=0 then, the vertex is reduced to an elementary function. Two pomeron models have also been adopted in the analysis. A reasonable agreement with the data has been achieved.
Recent results from the ZEUS experiment are reviewed. This report concentrates on structure functions at very low Q2, cross sections at very high Q2, searches beyond the standard model, jets in DIS and photoproduction, and cross sections and event shapes of diffractive processes.
Protons and antiprotons at collider energies are a source of high energy Weizsaecker-Williams photons. This opens up a possibility to study at the LHC exclusive photoproduction of heavy vector mesons at energies much larger than possible at the HERA accelerator.We present selected results on the production of vector mesons {rho}, {omega}, {phi}, J/{Psi} and {Upsilon}. We show distributions in rapidity, transverse momentum of mesons and azimuthal angle between outgoing protons for RHIC, Tevatron and LHC energies. The absorption effects are discussed.The amplitude for {gamma}p{yields}Vp is calculated in a pQCD k{sub T}-factorization approach with an unintegrated gluon distribution constrained by inclusive deep-inelastic structure function. The total cross section for diffractive meson (virtual) photo-production as a function of energy and photon virtuality is calculated and compared to HERA data. We also discuss the ratio of the first radial excitation state (2S) to the ground state (1S) in diffractive J/{Psi} and {Upsilon} production.
Photoproduction process γp → Vp has been mesured recently at HERA. We compare our results with the recent data. The corresponding amplitude is used to predict the cross section for exclusive production in pp and $p\bar p$ collisions. Here we present distribution in rapidity and transverse momentum of vector mesons for Tevatron and LHC energies. The absorption effects are discussed.
The amplitude for γp→ϕp is calculated in a pQCD kT-factorization approach. The total cross section for this process is compared with HERA data. Total cross section, as a function of photon–proton energy and photon virtuality, is calculated. We also discuss the ratio of σL/σT and the dependence on the mass of the strange quark. The amplitude for γp→ϕp is used to predict the cross section for exclusive photoproduction of ϕ meson in proton–proton collisions. Absorption effects are included. The results for RHIC, Tevatron and LHC energies are presented.
We calculate the differential cross section and single-polarization
observables for the reaction γ p --> Φ p within
perturbative QCD, treating the proton as a quark-diquark system. The
phenomenological couplings of gauge bosons to (spatially extended)
diquarks and the quark-diquark distribution amplitude of the proton are
adopted from previous investigations of baryon form factors and
two-photon processes. Going beyond leading order, we take into account
hadron-mass effects by means of a systematic expansion in the small
parameter (hadron mass/ photon energy). With the Φ-meson
distribution amplitude taken from the literature our predictions for the
differential cross section at \|t\|>~4 GeV2 seem to
provide a reasonable extrapolation of the low-t data and are also
comparable in magnitude with the results of a two-gluon exchange model
in which the gluons are considered as a remnant of the Pomeron. For
momentum transfers of a few GeV hadron-mass effects appear still to be
sizeable.
Exclusive production of \(\rho^0\) and \(J/\psi\) mesons in e\(^+\)p collisions has been studied with the ZEUS detector in the kinematic range \(0.25 < Q^2 < 50\) GeV\(^2\), \(20 < W < 167\) GeV for the \(\rho^0\) data and \(2 < Q^2 < 40\) GeV\(^2\), \(50 < W < 150\) GeV for the \(J/\psi\) data. Cross sections for exclusive \(\rho^0\) and \(J/\psi\) production have been measured as a function of \(Q^2\), W and t. The spin-density matrix elements \(r^{04}_{00}\), \(r^1_{1-1}\) and Re\(\:r^{5}_{10}\) have been determined for exclusive \(\rho^0\) production as well as \(r^{04}_{00}\) and \(r^{04}_{1-1}\) for exclusive \(J/\psi\) production. The results are discussed in the context of theoretical models invoking soft and hard phenomena.
We studied the exclusive reaction ep⃗e′p′φ using the φ⃗K+K- decay mode. The data were collected using a 4.2 GeV incident electron beam and the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Our experiment covers the range in Q2 from 0.7 to 2.2 GeV2, and W from 2.0 to 2.6 GeV. Taken together with all previous data, we find a consistent picture of φ production on the proton. Our measurement shows the expected decrease of the t slope with the vector-meson formation time cΔτ below 2 fm. At 〈cΔτ〉=0.6 fm, we measure bφ=2.27±0.42 GeV-2. The cross section dependence on W as W0.2±0.1 at Q2=1.3 GeV2 was determined by comparison with φ production at HERA after correcting for threshold effects. This is the same dependence as observed in photoproduction.
We present a detailed comparison of a variety of predictions for diffractive light vector meson production with the data collected at the DESY HERA collider. All our calculations are performed within a dipole model framework and make use of different models for the meson light-cone wave function. There are no free parameters in any of the scenarios we consider. Generally we find good agreement with the data using rather simple Gaussian motivated wave functions in conjunction with dipole cross sections which have been fitted to other data.
We studied the exclusive reaction ep⃗e′p′φ using the φ⃗K+K- decay mode. The data were collected using a 4.2 GeV incident electron beam and the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Our experiment covers the range in Q2 from 0.7 to 2.2 GeV2, and W from 2.0 to 2.6 GeV. Taken together with all previous data, we find a consistent picture of φ production on the proton. Our measurement shows the expected decrease of the t slope with the vector-meson formation time cΔτ below 2 fm. At 〈cΔτ〉=0.6 fm, we measure bφ=2.27±0.42 GeV-2. The cross section dependence on W as W0.2±0.1 at Q2=1.3 GeV2 was determined by comparison with φ production at HERA after correcting for threshold effects. This is the same dependence as observed in photoproduction.
The amplitude for photoproduction gammap-->Vp is calculated in a pQCD k-factorization approach with an unintegrated gluon distribution by inclusive deep-inelastic structure function. The total cross for gammap-->Vp is compared to HERA data. We also discuss the 2S/1S ratio in difractive J/psi production. The amplitude for gammap-->Vp is used to predict the cross section for exclusive photoproduction in pp collision at Tevatron energies. We included absorption effects. We also discuss the exclusive photoproduction of the electroweak Z0 boson.
High-energy photon interactions are discussed in terms of the hadronic structure of the photon, which is expressed by means of a formulation which is akin to, but somewhat more general than, vector-meson-dominance or specific generalized vector-dominance models. Experiments which demonstrate and yield information about this hadronic structure are discussed critically, and the resulting information is carefully evaluated. Special attention is paid to diffractive processes such as the photoproduction of vector mesons and to photon shadowing effects on nuclei. Relationships to other views of photon interactions, such as the parton model and the space-time description, are also discussed; these views are seen to complement the hadronic structure picture rather than to be in conflict. The general overview is that there is ample evidence which shows that the photon's hadronic structure plays a significant role in its interactions. What further work would most significantly enhance the understanding of the hadronic structure of the photon is also pointed out.
A study of -meson photoproduction by partially polarized photons of energy 20–40 GeV is reported. The production mechanism is found to conserves-channel helicity and to proceed via natural-parity exchange in thet channel. In the photoproduction of high-massK
+
K
– states with photons of energy 20–70 GeV, there is evidence for an enhancement at a mass of 1.76 GeV with width 0.08 GeV.
We present first results on the total photoproduction cross section measurement with the H1 detector at HERA. The data were extracted from low Q2 collisions of 26.7 GeV electrons with 820 GeV protons. The γp total cross section has been measured by two independent methods in the γp center of mass energy range from 90 to 290 GeV. For an average center of mass energy of 195 GeV a value of σtot (γp) = 159 ± 7 (stat.) ± 20 (syst.) μb was obtained.
This paper reports the cross section measurements for the process ep → e J/ψ p for Q2 < 4 GeV2 at GeV, based on an integrated luminosity of about 0.5 pb−1, using the ZEUS detector. The J/ψ was detected in its e+e− and μ+μ− decay modes. The photoproduction cross section was measured to be 52−12+7 ± 10 nb at an average γp centre of mass energy of 67 GeV and 71−20+13±12 nb at 114 GeV. The significant rise of the cross section compared to lower energy measurements is not in agreement with VDM models, but can be described by QCD inspired models if a rise in the gluon momentum density at low x in the proton is assumed.
The exclusive production of ϱ0 mesons in deep inelastic electron-proton scattering has been studied using the ZEUS detector. Cross sections have been measured in the range 7 < Q2 < 25 GeV2 for λ∗p centre of mass (c.m.) energies 40 to 130 GeV. The λ∗p → ϱ0p cross section exhibits a Q−(4.2±0.8−0.5+1.4) dependence and both longitudinally and transversely polarised ϱ0's are observed. The λ∗p → ϱ0p cross section rises strongly with increasing c.m. energy, when compared with NMC data at lower energy, which cannot be explained by production through soft pomeron exchange. The data are compared with perturbative QCD calculations where the rise in the cross section reflects the increase in the gluon density at low x.
Regge theory provides a very simple and economical description of all total cross sections.
We demonstrate that the distinctive features of the forward differential cross section of diffractive leptoproduction of a vector meson can be legitimately calculated in perturbative QCD in terms of the light-cone $q \bar q$ wave function of the vector meson and the gluon distribution of the target. In particular, we calculate the $Q^2$ and nuclear dependence of the diffractive leptoproduction of vector mesons and estimate the cross section. The production of longitudinally polarized vector mesons by longitudinally polarized virtual photons is predicted to be the dominant component, yielding a cross section behaving as $Q^{-6}$. The nuclear dependence of the diffractive cross sections, which follows from a factorization theorem in perturbative QCD, provides important tests of color transparency as well as constraints on the shadowing of the gluon structure functions and the longitudinal structure functions of nuclei. Comment: 32 pages, requires phyzzx.tex, figures can be obtained by sending preprint request to SLAC, minor clarifications and additional references incorporated in revised version, preprint SLAC-PUB-6412
The forward and rear calorimeters of the ZEUS experiment are made of 48 modules with maximum active dimensions of 4.6 m height, 0.2 m width, 7-lambda-depth and maximum weight of 12 t. It consists of 1 X0 uranium plates interleaved with plastic scintillator tiles read out via wavelength shifters and photomultipliers. The mechanical construction, the achieved tolerances as well as the optical and electronics readout are described. Ten of these modules have been tested with electrons, hadrons and muons in the momentum range 15-100 GeV/c. Results on resolution, uniformity and calibration are presented. Our main result is the achieved calibration accuracy of about 1% obtained by using the signal from the uranium radioactivity.
This biennial Review summarizes much of Particle Physics. Using data from previous editions, plus 2205 new measurements from 667 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. This edition features expanded coverage of CP violation in B mesons and of neutrino oscillations. For the first time we cover searches for evidence of extra dimensions (both in the particle listings and in a new review). Another new review is on Grand Unified Theories. A booklet is available containing the Summary Tables and abbreviated versions of some of the other sections of this full Review. All tables, listings, and reviews (and errata) are also available on the Particle Data Group website: http://pdg.lbl.gov.
At the Bonn 2.5 GeV synchrotron the differential photoproduction cross section dsigma/dt of phi mesons has been measured at a photon energy of 2.0 GeV at fibe different t values between 0.23 < t < 0.73 (GeV/c)2. The phi meson was detected by magnetic momentum analysis of both charged decay K mesons and by a time of flight and angle measurement of the coincident recoil proton. We found an exponential behaviour for the t dependence of the cross section. The measured slope of the exponential decrease was b = (4.01 +/- 0.23) (GeV/c-2. This result, combined with previous measurements at higher energies, implies that the slope of the pomeron trajectory is compatible with zero. In addition the experiment yielded a value of the phi mass, mphi = (1019.4 +/- 0.8) MeV and a value of the phi width, Gamma = (4.4 +/- 0.4) MeV.
The reaction γ + p → Φ + p has been measured using a spark chamber spectrometer and a tagged photon beam in the energy range from 4.6 to 6.7 GeV. Approximately 3500 photoproduced elastic Φ-events have been collected in the t-range between tmin and t = −0.4 (GeV/c)2. Cross sections and t-distributions are presented.
Cross section of diffractiveJ/? production in deep inelastic scattering in the Born and the leading-log approximations of perturbative QCD are calculated.
A vector-meson dominance model for high-energy ep inelastic scattering is proposed. We predict that at high values of the missing mass (>2.5 BeV) the ratio of the longitudinal to the transverse cross section is given by xi(q2mrho2[1- (q22mpnu)]2, where xi is expected to be of the order of unity. Scale invariance (in the sense of Bjorken) is satisfied at very high q2. The model may adequately account for the remarkably large cross section recently observed in the deep inelastic region.
DOI:https://doi.org/10.1103/PhysRevLett.16.71
The elastic photoproduction cross sections for rho and phi mesons from protons have been measured from 30 to 180 GeV. The energy dependences agree well with predictions made by using vector-meson dominance and an additive quark model. The rho cross section is approximately constant with energy while the phi cross section rises from 0.5 to 0.7 ..mu..b with increasing energy.
We present results on vector-meson photoproduction via γp→Vp in the LBL-SLAC 82-in. hydrogen bubble chamber exposed to a linearly polarized photon beam at 2.8, 4.7, and 9.3 GeV. We find ρ0 production to have the characteristics of a diffractive process, i.e., a cross section decreasing slowly with energy and a differential cross section with slope of ∼ 6.5 GeV-2. Within errors the ρ0 production amplitudes are entirely due to natural-parity exchange. s-channel helicity is conserved to a high degree in the γ→ρ0 transition. We find evidence for small helicity-flip amplitudes for ππ pairs in the ρ0 region. Photoproduction of ω mesons is separated into its natural- (σN) and unnatural- (σU) parity-exchange contributions. The Eγ and t dependence and the spin density matrix of the unnatural-parity-exchange contribution are consistent with a one-pion-exchange process. The natural-parity-exchange part has characteristics similar to ρ0 production. At 9.3 GeV the ratio of σ(ρ0) to σN(ω) is ∼ 7. The slope of the φ differential cross section is ∼ 4.5 GeV-2, smaller than that of ρ0 and ω production. Natural-parity exchange is the main contributor to φ production. No evidence for higher-mass vector mesons is found in ππ, πππ, or KK̅ final states. The s and t dependences of Compton scattering as calculated from ρ, ω, and φ photoproduction using vector-meson dominance agree with experiment, but the predicted Compton cross section is too small by a factor of 2.
In a study of photoproduction at photon energies between 0.3 and 5.8 GeV
in a hydrogen bubble chamber, approximately 31 000 events with three,
five, or seven outgoing charged particles were analyzed. This paper
gives a survey of the experimental procedure and of resonance production
in quasi-two-body reactions. The cross sections for the processes
γp-->pπ+π-,
γp-->pπ+π+π-π-
,
γp-->pπ+π+π+π-
π-π-, γp-->pV
(V=ρ0,ω,φ), γp-->pη,
γp-->pX0,
γp-->Δ++π-, and
γp-->Δ0π+ have been measured as
a function of the photon energy. For reactions
γp-->pρ0, γp-->pω, and
γp-->Δ++π-, the differential
cross sections and density matrix elements are given for various
intervals of the photon energy. The photoproduction of vector mesons and
Δ isobars is discussed in terms of several models.
We report measurements from elastic photoproduction of ω's on
hydrogen for photon energies between 60 and 225 GeV, elastic φ
photoproduction on hydrogen between 35 and 165 GeV and on deuterium
between 45 and 85 GeV, elastic photoproduction on deuterium of an
enhancement at 1.72 GeV/c2 decaying into
K+K-, and elastic and inelastic photoproduction on
deuterium of pp¯ pairs.
We report a measurement of the diffraction dissociation differential
cross section d2σSD/dM2dt for
p¯p-->p¯X at √s =546 and 1800 GeV,
M2/s<0.2 and 0<=-t<=0.4 GeV2. Our results
are compared to theoretical predictions and to extrapolations from
experimental results at lower energies.
Cross-sections are presented for the ``elastic'' processes
γ+p-->ϱ0 and
γ+p-->ø0 at 8.5 GeV.. The inelastic
contribution to ϱ0 and ø0
photoproduction from hydrogen was eliminated by requiring a proton
recoil in coincidence with the pair-spectrometer. Detailed mass-fits
were made to the rho data using a Söding interference model to
obtain the final lldelastic'' cross-sections.
The simple AQM prediction still fits experimental data to better than 10% suggesti an underlying dynamics describing the meson-baryon difference primarily in terms of the number of constituent quarks. Equally valid fits to the energy dependence of total cross sections from two models predicting very different asymptotic behaviour indicate that present data cannot distinguish between different pomeron models. Meson-nucleon and hyperon-nucleon data at higher energies could clarify finer details of flavor and baryon number dependence supporting a hierarchy of contributions inspired by large Nc QCD: (1) multigluon exchange, (2) planar quark diagrams, (3) nonplanar quark diagrams.
Photon proton cross sections for elastic light vector meson production, σ el νp , inelastic diffractive production, σ nd νp , non-diffractive production, σ d νp , as well as the total cross section, σ tot νp , have been measured at an average υp center of mass energy of 180 GeV with the ZEUS detector at HERA. The resulting values are σ el νp = 18 ± 7 μb, σ d νp = 33 ± 8 μb, σ nd νp = 91 ± 11 μb, and σ tot νp 143 ± 17 μb, where the errors include statistical and systematic errors added in quadrature.
All currently available QCD-inspired models/Monte Carlos for the event structure in high-p
T
physics are described. Included is information on hard interactions, initial and final state radiation, structure functions, beam jets and fragmentation. Results from a number of comparative Monte Carlo runs are presented, which help to illuminate how the physics content in Monte Carlos may be tested. Special emphasis is put on the partial observability of coherence effects in initial state radiation, and on differences in the jet structure of high-p
T
events.
A gas vertex detector, operated with dimethylether (DME) at atmospheric pressure, is presently being built for the ZEUS experiment at HERA. Its main design features, together with the performances of a prototype measured at various operating voltages, particle rates and geometrical conditions on a CERN Proton Synchrotron test beam, are presented. A spatial resolution down to 35 μm and an average wire efficiency of 96% have been achieved, for a 3 mm gas gap relative to each sense wire.
All the symmetry models of strong interactions which have been proposed up to the present are devoid of deep physical foundations. It is suggested that, instead of postulating artificial “higher” symmetries which must be broken anyway within the realm of strong interactions, we take the existing exact symmetries of strong interactions more seriously than before and exploit them to the utmost limit. A new theory of strong interactions is proposed on this basis.Following Yang and Mills we require that the gauge transformations that are associated with the three “internal” conservation laws—baryon conservation, hypercharge conservation, and isospin conservation—be “consistent with the local field concept that underlies the usual physical theories.” In analogy with electromagnetism there emerge three kinds of couplings such that in each case a massive vector field is coupled linearly to the conserved current in question. Each of the three fundamental couplings is characterized by a single universal constant. Since, as Pais has shown, there are no other internal symmetries that are exact, and since any successful theory must be simple, there are no other fundamental strong couplings. Parity conservation in strong interactions follows as the direct consequence of parity conservation of the three fundamental vector couplings. The three vector couplings give rise to corresponding current-current interactions. Yukawa-type couplings of pions and K particles to baryons are “phenomenological,” and may arise, for instance, out of four-baryon current-current interactions along the lines suggested by Fermi and Yang. All the successful features of Chew-Low type meson theories and of relativistic dispersion relations can, in principle, be in accordance with the theory whereas none of the predictions based on relativistic Yukawa-type Lagrangians are meaningful unless is considerably less than unity.Simple and direct experimental tests of the theory should be looked for in those phenomena in which phenomenological Yukawa-type couplings are likely to play unimportant roles. The fundamental isospin current coupling in the static limit gives rise to a short-range repulsion (attraction) between two particles whenever the isospins are parallel (antiparallel). Thus the low-energy s-wave πN interaction should be repulsive in the state and attractive in the state in agreement with observation. In πΣ s-wave scattering the T = 0 state is strongly attractive, and there definitely exists the possibility of an s-wave resonance at energies of the order of the K−p threshold, while the T = 1 πΣ phase shift is likely to remain small; using the K matrix formalism of Dalitz and Tuan, we might be able to compare the “ideal” phase shifts derived in this manner with the “actual” phase shifts deduced from K−p reactions. It is expected that the two-pion system exhibits a resonant behavior in the T = 1 (p-wave) state in agreement with the conjecture of Frazer and Fulco based on the electromagnetic structure of the nucleon. The three pion system is expected to exhibit two T = 0, J = 1 resonances. It is conjectured that the two and one “higher rsonances” in the πN interactions may be due to the two T = 0 3π resonances and the one T = 1 2π resonance predicted by the theory. Multiple pion production is expected at all energies to be more frequent than that predicted on the basis of statistical considerations. The fundamental hypercharge current coupling gives rise to a short-range repulsion (attraction) between two charge-doublet particles when their hypercharges are like (opposite). If the isospin current coupling is effectively weaker than the hypercharge current coupling, the KN “potential” should be repulsive and the “potential” should be attractive, and the charge exchange scattering of K+ and K− should be relatively rare, at least in s states. All these features seem to be in agreement with current experiments. Conditions for the validity of Pais' doublet approximation are discussed. The theory offers a possible explanation for the long-standing problem as to why associated production cross sections are small and K− cross sections are large. The empirical fact that the ratio of to (KΛN) + (KΣN) in NN collisions seems to be about twenty to thirty times larger than simple statistical considerations indicate is not surprising. The fundamental baryonic current coupling gives rise to a short-range repulsion for baryon-baryon interactions and an attraction for baryon-antibaryon interactions. There should be effects similar to those expected from “repulsive cores” for all angular momentum and parity states in both the T = 1 and T = 0, NN interactions at short distances though the T = 1 state may be more repulsive. A simple Thomas-type calculation gives rise to a spin-orbit force of the right sign with not unreasonable order of magnitude. The ΛN and ΣN interactions at short distances should be somewhat less repulsive than the NN interactions. Annihilation cross sections in collisions are expected to be large even in Bev regions in contrast to the predictions of Ball and Chew. The observed large pion multiplicity in annihilations is not mysterious. It is possible to invent a reasonable mechanism which makes the reaction very rare, as recently observed. Fermi-Landau-Heisenberg type theories of high energy collisions are not expected to hold in relativistic NN collisions; instead the theory offers a theoretical justification for the “two-fire-ball model“ of high-energy jets previously proposed on purely phenomenological grounds.Because of the strong short-range attraction between a baryon and antibaryon there exists a mechanism for a baryon-antibaryon pair to form a meson. The dynamical basis of the Fermi-Yang-Sakata-Okun model as well as that of the Goldhaber-Christy model follows naturally from the theory; all the ad hoc assumptions that must be made in order that the compound models work at all can be explained from first principles. It is suggested that one should not ask which elementary particles are “more elementary than others,” and which compound model is right, but rather characterize each particle only by its internal properties such as total hypercharge and mean-square baryonic radius. Although the fundamental couplings of the theory are highly symmetric and universal, it is possible for the three couplings alone to account for the observed mass spectrum. The theory can explain, in a trivial manner, why there are no “elementary” particles with baryon number greater than unity provided that the baryonic current coupling is sufficiently strong. The question of whether or not an |S| = 2 meson exists is a dynamical one (not a group-theoretic one) that depends on the strength of the hypercharge current coupling. A possible reason for the nonexistence of a π0′ (charge-singlet, nonstrange boson) is given. The theory realizes Pais' principles of economy of constants and of a hierarchy of interactions in a natural and elegant manner.It is conjectured that there exists a deep connection between the law of conservation of fermions and the universal V-A weak coupling. In the absence of strong and electromagnetic interactions, baryonic charge, hypercharge, and electric charge all disappear, and only the sign of γ5 can distinguish a fermion from an antifermion, the fermionic charge being diagonalized by γ5; hence 1 + γ5 appears naturally in weak interactions. Parity conservation in strong interactions, parity conservation in electromagnetic interactions, parity nonconservation in weak interactions can all be understood from the single common principle of generalized gauge invariance. It appears that in the future ultimate theory of elementary particles all elementary particle interactions will be manifestations of the five fundamental vector-type couplings corresponding to the five conservation laws of “internal attributes”—baryonic charge, hypercharge, isospin, electric charge, and fermionic charge. Gravity and cosmology are briefly discussed; it is estimated that the Compton wavelength of the graviton is of the order of 108 light years.It is suggested that every conceivable experimental attempt be made to detect directly quantum manifestations of the vector fields introduced in the theory, especially by studying Q values of pions in various combinations in annihilations and in multiple pion production.
The Central Tracking Detector of the ZEUS experiment employs a time difference technique to measure the z coordinate of each hit. The method provides fast, three-dimensional space point measurements which are used as input to all levels of the ZEUS trigger. Such a tracking trigger is essential in order to discriminate against events with vertices lying outside the nominal electron-proton interaction region. Since the beam crossing interval of the HERA collider is 96 ns, all data must be pipelined through the front-end readout electronics. Subsequent data aquisition employs a novel technique which utilizes a network of approximately 120 INMOS transputers to process the data in parallel. The z-by-timing method and its data aquisition have been employed successfully in recording and reconstructing tracks from electron-proton interactions in ZEUS.
The mechanical, electrical and electronic design and construction of the ZEUS central tracking detector are described, together with the chamber monitoring and environmental control. This cylindrical drift chamber is designed for track reconstruction, electron identification and fast event triggering in a high beam-crossing rate, high magnetic field application.
The kinematics of the process ℓN → ℓNV, is studied in the one-photon approximation for unpolarized as well as polarized leptons ℓ. The vector-meson spin density matrix is expressed in terms of the s-channel helicity amplitudes in the hadron c.m.s. and the vector-meson decay angular distribution is discussed. The use of longitudinally polarized lepton beams is found to increase considerably the amount of information that can be deduced from the decay distribution. With longitudinal beam polarization it is possible to separate all 26 observable independent density matrix elements into contributions from natural and unnatural parity exchange in the t-channel, respectively.
We present the mechanical design and construction techniques used in building the barrel calorimeter for the ZEUS detector. The latter is currently under construction for use at the HERA electron-proton colliding beam facility. The calorimeter consists of 32 wedge shaped modules with approximate dimensions of 3×0.5×1.7 m3. The modules use alternate layers of depleted uranium and scintillator with one radiation length sampling. The light is collected via wavelength shifter plates placed on the sloping sides of the modules and read out by photomultiplier tubes located at the outer radius. The unit cell dimensions result in a ratio which yields an optimal energy resolution for hadronic jets. The placing of the structural components and the arrangement of the cracks between modules were chosen to maximize the uniformity of the response. Details of the construction and assembly effort needed to realize the total calorimeter are given. We finally describe the procedures used for transporting the completed modules to DESY and to install them on ZEUS.
The reaction γp→K+K−p has been investigated with photons in the energy range of 20<Eγ<36 GeV and with K+K− pairs in the mass range of MK+K−<2.0 GeV. The production of the φ(1019) contributes with a cross section σ(γp → φp) × BR(φ→K+K−) = 240±6 nb with an additional systematic error of ±20 nb. In the higher mass range of 1.05<MK+K−<2.0 GeV the production of K+K− pairs yields a cross section σ(γp→K+K−p) = 160±8 nb with an additional systematic error of nb.
A method of vertex triggering which uses a time difference measurement to determine the z coordinate of a track is under development for the ZEUS Central Tracking Detector. This incorporates a pipelined readout system. We describe the system and report on its performance.
The formalism necessary to analyze production of vector mesons with polarized photons (γp → pV) is presented in detail. The decay angular distribution of the vector mesons is parameterized by the density matrices ϱα, α = 0, 1, 2, 3. Restrictions on the numerical values of the density matrix elements are derived. From the symmetry properties of the helicity amplitudes, it is shown that at high energies the combinations , to leading order in energy receive only contributions from natural (unnatural) parity exchange in the t-channel. It is shown that this is true in any coordinate system which can be reached from the vector-meson helicity system by a rotation around the normal to the production plane. The values of the density matrices as predicted by various models: elementary 0± exchange, spin independence, helicity conservation, are given.
A fully compensating uranium—scintillator calorimeter was constructed for the ZEUS detector at HERA. Several of the barrel calorimeter modules were subjected to beam tests at Fermilab before shipping them to DESY for installation. The calibrations of the modules used beams of electrons and hadrons, measuring the uniformity of the response, and checking the resolution. The runs also provided opportunity to test a large fraction of the actual ZEUS calorimeter readout system in an integrated beam environment more than one year before HERA turn on. The experiment utilized two computer controlled mechanical structures, one of which was capable of holding up to four modules in order to study shower containment, and a magnetic spectrometer with a high resolution beam tracking system. During two running periods, beams of 6 to 110 GeV containing e, μ, π, and were used. The results show energy resolutions of for hadrons and for electrons, uniformities at the 1% level, energy nonlinearity less than 1%, and equal response for electrons and hadrons.
Title of program: JETSET 6.3 Catalogue number: AATJ Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland (see application form in this issue) Computer for which the program is designed and others on which it is operable: ND, VAX, IBM, CDC and others with a FORTRAN 77 compiler Computer: ND-570; Installation: University of Lund, Lund, Sweden Operating system: SINTRAN III - VSX/500 Programming language used: FORTRAN 77 High speed storage required: 50 Kwords No. of bits in word: 32 Peripherals used: terminal for input, terminal or line printer for output No. of lines in combined program and test deck: 6173
The s and t dependence of phi(1019) photoproduction has been investigated in the incident photon energy range 2.8 to 4.8 GeV. Differential cross-sections and density matrix elements are presented for a t range extending from tmin out to -1.3 (GeV/c)^2. The results are discussed in terms of an effective Rcgge trajectory in the t-channel.
Elastic $\rho^0$ photoproduction has been measured using the ZEUS detector at
HERA. Untagged photoproduction events from $ep$ interactions were used to
measure the reaction $\gamma p \rightarrow \rho^0 p$ ($\rho^0 \rightarrow \pi^+
\pi^-$) at photon-proton centre-of-mass energies between 60 and 80~GeV and
$|t|<0.5$~GeV$^2$, where $t$ is the square of the four-momentum transferred at
the proton vertex. The differential cross section $d\sigma/dM_{\pi\pi}$, where
$M_{\pi\pi}$ is the invariant mass of the two pions, and the integrated cross
section, $\sigma_{\gamma p\rightarrow \rho^0 p}$, are presented; the latter was
measured to be $14.7\pm 0.4~(\mbox{stat.})~\pm2.4~(\mbox{syst.})~\mu\mbox{b}$.
The differential cross section $d\sigma/dt$ has an approximately exponential
shape; a fit of the type $A^{\prime}_{t} \exp{(-b^{\prime}_{t}|t| +
c^{\prime}_{t} t^2)}$ yields a $t$-slope $b^{\prime}_{t}=
9.9~\pm~1.2~(\mbox{stat.})~\pm 1.4~(\mbox{syst.})~\mu\mbox{b}$. The results,
when compared to low energy data, show a weak energy dependence of both
$\sigma_{\gamma p\rightarrow \rho^0 p}$ and of the $t$-slope. The $\rho^0$ is
produced predominantly with transverse polarisation, demonstrating that
$s$-channel helicity conservation holds at these energies.
DESY 95-25 I. [ 81 HI Collab
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paper EPS-0468 submitted to the International Europhysics Conference on High Energy Physics
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