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arXiv:hep-ex/9706009v1 10 Jun 1997
D∗Production in Deep Inelastic Scattering
at HERA
ZEUS Collaboration
Abstract
This paper presents measurements of D∗± production in deep inelastic scattering
from collisions between 27.5 GeV positrons and 820 GeV protons. The data have been
taken with the ZEUS detector at HERA. The decay channel D∗+→(D0→K−π+)π+
(+ c.c.) has been used in the study. The e+pcross section for inclusive D∗± produc-
tion with 5 < Q2<100 GeV2and y < 0.7 is 5.3 ±1.0 ±0.8 nb in the kinematic region
1.3< pT(D∗±)<9.0 GeV and |η(D∗± )|<1.5. Differential cross sections as functions of
pT(D∗±), η(D∗±), Wand Q2are compared with next-to-leading order QCD calculations
based on the photon-gluon fusion production mechanism. After an extrapolation of the
cross section to the full kinematic region in pT(D∗± ) and η(D∗±), the charm contribution
Fc¯c
2(x, Q2) to the proton structure function is determined for Bjorken xbetween 2 ·10−4
and 5 ·10−3.
The ZEUS Collaboration
J. Breitweg, M. Derrick, D. Krakauer, S. Magill, D. Mikunas, B. Musgrave, J. Repond, R. Stanek,
R.L. Talaga, R. Yoshida, H. Zhang
Argonne National Laboratory, Argonne, IL, USA p
M.C.K. Mattingly
Andrews University, Berrien Springs, MI, USA
F. Anselmo, P. Antonioli, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni,
G. Cara Romeo, G. Castellini1, L. Cifarelli2, F. Cindolo, A. Contin, M. Corradi, S. De Pasquale,
I. Gialas3, P. Giusti, G. Iacobucci, G. Laurenti, G. Levi, A. Margotti, T. Massam, R. Nania,
F. Palmonari, A. Pesci, A. Polini, G. Sartorelli, Y. Zamora Garcia4, A. Zichichi
University and INFN Bologna, Bologna, Italy f
C. Amelung, A. Bornheim, I. Brock, K. Cob¨oken, J. Crittenden, R. Deffner, M. Eckert, L. Feld5,
M. Grothe, H. Hartmann, K. Heinloth, L. Heinz, E. Hilger, H.-P. Jakob, U.F. Katz, E. Paul,
M. Pfeiffer, Ch. Rembser, J. Stamm, R. Wedemeyer6
Physikalisches Institut der Universit¨at Bonn, Bonn, Germany c
D.S. Bailey, S. Campbell-Robson, W.N. Cottingham, B. Foster, R. Hall-Wilton, M.E. Hayes,
G.P. Heath, H.F. Heath, D. Piccioni, D.G. Roff, R.J. Tapper
H.H. Wills Physics Laboratory, University of Bristol, Bristol, U.K. o
M. Arneodo7, R. Ayad, M. Capua, A. Garfagnini, L. Iannotti, M. Schioppa, G. Susinno
Calabria University, Physics Dept.and INFN, Cosenza, Italy f
J.Y. Kim, J.H. Lee, I.T. Lim, M.Y. Pac8
Chonnam National University, Kwangju, Korea h
A. Caldwell9, N. Cartiglia, Z. Jing, W. Liu, J.A. Parsons, S. Ritz10, S. Sampson, F. Sciulli,
P.B. Straub, Q. Zhu
Columbia University, Nevis Labs., Irvington on Hudson, N.Y., USA q
P. Borzemski, J. Chwastowski, A. Eskreys, Z. Jakubowski, M.B. Przybycie´n, M. Zachara,
L. Zawiejski
Inst. of Nuclear Physics, Cracow, Poland j
L. Adamczyk, B. Bednarek, K. Jele´n, D. Kisielewska, T. Kowalski, M. Przybycie´n, E. Rulikowska-
Zar¸ebska, L. Suszycki, J. Zaj¸ac
Faculty of Physics and Nuclear Techniques, Academy of Mining and Metallurgy, Cracow, Poland j
Z. Duli´nski, A. Kota´nski
Jagellonian Univ., Dept. of Physics, Cracow, Poland k
I
G. Abbiendi11, L.A.T. Bauerdick, U. Behrens, H. Beier, J.K. Bienlein, G. Cases12 , O. Deppe,
K. Desler, G. Drews, U. Fricke, D.J. Gilkinson, C. Glasman, P. G¨ottlicher, J. Große-Knetter,
T. Haas, W. Hain, D. Hasell, K.F. Johnson13 , M. Kasemann, W. Koch, U. K¨otz, H. Kowal-
ski, J. Labs, L. Lindemann, B. L¨ohr, M. L¨owe14, O. Ma´nczak, J. Milewski, T. Monteiro15,
J.S.T. Ng16, D. Notz, K. Ohrenberg17 , I.H. Park18, A. Pellegrino, F. Pelucchi, K. Piotrzkowski,
M. Roco19, M. Rohde, J. Rold´an, J.J. Ryan, A.A. Savin, U. Schneekloth, F. Selonke, B. Sur-
row, E. Tassi, T. Voß20, D. Westphal, G. Wolf, U. Wollmer21, C. Youngman, A.F. ˙
Zarnecki,
W. Zeuner
Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany
B.D. Burow, H.J. Grabosch, A. Meyer, S. Schlenstedt
DESY-IfH Zeuthen, Zeuthen, Germany
G. Barbagli, E. Gallo, P. Pelfer
University and INFN, Florence, Italy f
G. Maccarrone, L. Votano
INFN, Laboratori Nazionali di Frascati, Frascati, Italy f
A. Bamberger, S. Eisenhardt, P. Markun, T. Trefzger22, S. W¨olfle
Fakult¨at f¨ur Physik der Universit¨at Freiburg i.Br., Freiburg i.Br., Germany c
J.T. Bromley, N.H. Brook, P.J. Bussey, A.T. Doyle, D.H. Saxon, L.E. Sinclair, E. Strickland,
M.L. Utley23, R. Waugh, A.S. Wilson
Dept. of Physics and Astronomy, University of Glasgow, Glasgow, U.K. o
I. Bohnet, N. Gendner, U. Holm, A. Meyer-Larsen, H. Salehi, K. Wick
Hamburg University, I. Institute of Exp. Physics, Hamburg, Germany c
L.K. Gladilin24, D. Horstmann, D. K¸cira, R. Klanner, E. Lohrmann, G. Poelz, W. Schott25,
F. Zetsche
Hamburg University, II. Institute of Exp. Physics, Hamburg, Germany c
T.C. Bacon, I. Butterworth, J.E. Cole, V.L. Harris, G. Howell, B.H.Y. Hung, L. Lamberti26 ,
K.R. Long, D.B. Miller, N. Pavel, A. Prinias27, J.K. Sedgbeer, D. Sideris, A.F. Whitfield28
Imperial College London, High Energy Nuclear Physics Group, London, U.K. o
U. Mallik, S.M. Wang, J.T. Wu
University of Iowa, Physics and Astronomy Dept., Iowa City, USA p
P. Cloth, D. Filges
Forschungszentrum J¨ulich, Institut f¨ur Kernphysik, J¨ulich, Germany
J.I. Fleck29, T. Ishii, M. Kuze, M. Nakao, K. Tokushuku, S. Yamada, Y. Yamazaki30
Institute of Particle and Nuclear Studies, KEK, Tsukuba, Japan g
S.H. An, S.B. Lee, S.W. Nam31, H.S. Park, S.K. Park
Korea University, Seoul, Korea h
F. Barreiro, J.P. Fern´andez, G. Garc´ıa, R. Graciani, J.M. Hern´andez, L. Herv´as, L. Labarga,
M. Mart´ınez, J. del Peso, J. Puga, J. Terr´on, J.F. de Troc´oniz
Univer. Aut´onoma Madrid, Depto de F´ısica Te´or´ıca, Madrid, Spain n
II
F. Corriveau, D.S. Hanna, J. Hartmann, L.W. Hung, J.N. Lim, W.N. Murray, A. Ochs, M. Riv-
eline, D.G. Stairs, M. St-Laurent, R. Ullmann
McGill University, Dept. of Physics, Montr´eal, Qu´ebec, Canada a,b
T. Tsurugai
Meiji Gakuin University, Faculty of General Education, Yokohama, Japan
V. Bashkirov, B.A. Dolgoshein, A. Stifutkin
Moscow Engineering Physics Institute, Mosocw, Russia l
G.L. Bashindzhagyan, P.F. Ermolov, Yu.A. Golubkov, L.A. Khein, N.A. Korotkova, I.A. Ko-
rzhavina, V.A. Kuzmin, O.Yu. Lukina, A.S. Proskuryakov, L.M. Shcheglova, A.V. Shumilin,
A.N. Solomin,
S.A. Zotkin
Moscow State University, Institute of Nuclear Physics, Moscow, Russia m
C. Bokel, M. Botje, N. Br¨ummer, F. Chlebana19, J. Engelen, P. Kooijman, A. Kruse, A. van Sighem,
H. Tiecke, W. Verkerke, J. Vossebeld, M. Vreeswijk, L. Wiggers, E. de Wolf
NIKHEF and University of Amsterdam, Netherlands i
D. Acosta, B. Bylsma, L.S. Durkin, J. Gilmore, C.M. Ginsburg, C.L. Kim, T.Y. Ling, P. Ny-
lander, T.A. Romanowski32
Ohio State University, Physics Department, Columbus, Ohio, USA p
H.E. Blaikley, R.J. Cashmore, A.M. Cooper-Sarkar, R.C.E. Devenish, J.K. Edmonds, N. Harnew,
M. Lancaster33, J.D. McFall, C. Nath, V.A. Noyes27, A. Quadt, O. Ruske, J.R. Tickner, H. Ui-
jterwaal,
R. Walczak, D.S. Waters
Department of Physics, University of Oxford, Oxford, U.K. o
A. Bertolin, R. Brugnera, R. Carlin, F. Dal Corso, U. Dosselli, S. Limentani, M. Morandin,
M. Posocco, L. Stanco, R. Stroili, C. Voci
Dipartimento di Fisica dell’ Universita and INFN, Padova, Italy f
J. Bulmahn, R.G. Feild34 , B.Y. Oh, J.R. Okrasi´nski, J.J. Whitmore
Pennsylvania State University, Dept. of Physics, University Park, PA, USA q
Y. Iga
Polytechnic University, Sagamihara, Japan g
G. D’Agostini, G. Marini, A. Nigro, M. Raso
Dipartimento di Fisica, Univ. ’La Sapienza’ and INFN, Rome, Italy f
J.C. Hart, N.A. McCubbin, T.P. Shah
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, U.K. o
E. Barberis33 , T. Dubbs, C. Heusch, M. Van Hook, W. Lockman, J.T. Rahn, H.F.-W. Sadrozin-
ski,
A. Seiden, D.C. Williams
University of California, Santa Cruz, CA, USA p
III
O. Schwarzer, A.H. Walenta
Fachbereich Physik der Universit¨at-Gesamthochschule Siegen, Germany c
H. Abramowicz, G. Briskin, S. Dagan35, T. Doeker, S. Kananov, A. Levy36
Raymond and Beverly Sackler Faculty of Exact Sciences, School of Physics, Tel-Aviv University,
Tel-Aviv, Israel e
T. Abe, T. Fusayasu, M. Inuzuka, K. Nagano, I. Suzuki, K. Umemori, T. Yamashita
Department of Physics, University of Tokyo, Tokyo, Japan g
R. Hamatsu, T. Hirose, K. Homma, S. Kitamura37 , T. Matsushita, K. Yamauchi
Tokyo Metropolitan University, Dept. of Physics, Tokyo, Japan g
R. Cirio, M. Costa, M.I. Ferrero, S. Maselli, V. Monaco, C. Peroni, M.C. Petrucci, R. Sacchi,
A. Solano, A. Staiano
Universita di Torino, Dipartimento di Fisica Sperimentale and INFN, Torino, Italy f
M. Dardo
II Faculty of Sciences, Torino University and INFN - Alessandria, Italy f
D.C. Bailey, M. Brkic, C.-P. Fagerstroem, G.F. Hartner, K.K. Joo, G.M. Levman, J.F. Martin,
R.S. Orr, S. Polenz, C.R. Sampson, D. Simmons, R.J. Teuscher29
University of Toronto, Dept. of Physics, Toronto, Ont., Canada a
J.M. Butterworth, C.D. Catterall, T.W. Jones, P.B. Kaziewicz, J.B. Lane, R.L. Saunders,
J. Shulman, M.R. Sutton
University College London, Physics and Astronomy Dept., London, U.K. o
B. Lu, L.W. Mo
Virginia Polytechnic Inst. and State University, Physics Dept., Blacksburg, VA, USA q
J. Ciborowski, G. Grzelak38, M. Kasprzak, K. Muchorowski39, R.J. Nowak, J.M. Pawlak,
R. Pawlak, T. Tymieniecka, A.K. Wr´oblewski, J.A. Zakrzewski
Warsaw University, Institute of Experimental Physics, Warsaw, Poland j
M. Adamus
Institute for Nuclear Studies, Warsaw, Poland j
C. Coldewey, Y. Eisenberg35 , D. Hochman, U. Karshon35 , D. Revel35
Weizmann Institute, Nuclear Physics Dept., Rehovot, Israel d
W.F. Badgett, D. Chapin, R. Cross, S. Dasu, C. Foudas, R.J. Loveless, S. Mattingly, D.D. Reeder,
W.H. Smith, A. Vaiciulis, M. Wodarczyk
University of Wisconsin, Dept. of Physics, Madison, WI, USA p
S. Bhadra, W.R. Frisken, M. Khakzad, W.B. Schmidke
York University, Dept. of Physics, North York, Ont., Canada a
IV
1also at IROE Florence, Italy
2now at Univ. of Salerno and INFN Napoli, Italy
3now at Univ. of Crete, Greece
4supported by Worldlab, Lausanne, Switzerland
5now OPAL
6retired
7also at University of Torino and Alexander von Humboldt Fellow at University of Hamburg
8now at Dongshin University, Naju, Korea
9also at DESY and Alexander von Humboldt Fellow
10 Alfred P. Sloan Foundation Fellow
11 supported by an EC fellowship number ERBFMBICT 950172
12 now at SAP A.G., Walldorf
13 visitor from Florida State University
14 now at ALCATEL Mobile Communication GmbH, Stuttgart
15 supported by European Community Program PRAXIS XXI
16 now at DESY-Group FDET
17 now at DESY Computer Center
18 visitor from Kyungpook National University, Taegu, Korea, partially supported by DESY
19 now at Fermi National Accelerator Laboratory (FNAL), Batavia, IL, USA
20 now at NORCOM Infosystems, Hamburg
21 now at Oxford University, supported by DAAD fellowship HSP II-AUFE III
22 now at ATLAS Collaboration, Univ. of Munich
23 now at Clinical Operational Research Unit, University College, London
24 on leave from MSU, supported by the GIF, contract I-0444-176.07/95
25 now a self-employed consultant
26 supported by an EC fellowship
27 PPARC Post-doctoral Fellow
28 now at Conduit Communications Ltd., London, U.K.
29 now at CERN
30 supported by JSPS Postdoctoral Fellowships for Research Abroad
31 now at Wayne State University, Detroit
32 now at Department of Energy, Washington
33 now at Lawrence Berkeley Laboratory, Berkeley
34 now at Yale University, New Haven, CT
35 supported by a MINERVA Fellowship
36 partially supported by DESY
37 present address: Tokyo Metropolitan College of Allied Medical Sciences, Tokyo 116, Japan
38 supported by the Polish State Committee for Scientific Research, grant No. 2P03B09308
39 supported by the Polish State Committee for Scientific Research, grant No. 2P03B09208
V
asupported by the Natural Sciences and Engineering Research Council of
Canada (NSERC)
bsupported by the FCAR of Qu´ebec, Canada
csupported by the German Federal Ministry for Education and Science,
Research and Technology (BMBF), under contract numbers 057BN19P,
057FR19P, 057HH19P, 057HH29P, 057SI75I
dsupported by the MINERVA Gesellschaft f¨ur Forschung GmbH, the German
Israeli Foundation, and the U.S.-Israel Binational Science Foundation
esupported by the German Israeli Foundation, and by the Israel Science
Foundation
fsupported by the Italian National Institute for Nuclear Physics (INFN)
gsupported by the Japanese Ministry of Education, Science and Culture (the
Monbusho) and its grants for Scientific Research
hsupported by the Korean Ministry of Education and Korea Science and Engi-
neering Foundation
isupported by the Netherlands Foundation for Research on Matter (FOM)
jsupported by the Polish State Committee for Scientific Research, grant
No. 115/E-343/SPUB/P03/120/96
ksupported by the Polish State Committee for Scientific Research (grant No. 2
P03B 083 08) and Foundation for Polish-German Collaboration
lpartially supported by the German Federal Ministry for Education and Science,
Research and Technology (BMBF)
msupported by the German Federal Ministry for Education and Science, Re-
search and Technology (BMBF), and the Fund of Fundamental Research of
Russian Ministry of Science and Education and by INTAS-Grant No. 93-63
nsupported by the Spanish Ministry of Education and Science through funds
provided by CICYT
osupported by the Particle Physics and Astronomy Research Council
psupported by the US Department of Energy
qsupported by the US National Science Foundation
VI
1 Introduction
In neutral current deep inelastic scattering (DIS), ep →eX, charmed quarks are expected to be
produced predominantly via the photon-gluon fusion (PGF) process which couples the virtual
photon to a gluon of the proton. The leading order (LO) diagram is shown in Fig. 1a.
Recently, analytic calculations of the DIS charm cross section from photon-gluon coupling have
become available [1], which relate the DIS charm cross section to the gluon distribution in the
proton using next-to-leading order (NLO) QCD.
Measurements of deep inelastic neutral current scattering at HERA have demonstrated a rapid
rise of the proton structure function F2as Bjorken-xdecreases below 10−2[2]. A QCD analysis
of these data has connected this rise to an increase of the gluon momentum density in the
proton with a dependence x−λwith λ= 0.35 +0.04
−0.10 at Q2= 7 GeV2[3, 4]. At HERA, the
DIS charm cross section is sensitive to the gluon distribution of the proton at low fractional
momentum (xg∼10−3) of the proton. Comparison of the DIS charm cross section with NLO
QCD calculations allows an independent check of the increase of the gluon momentum density,
testing the consistency of the QCD calculations.
The other processes, apart from PGF, that contribute to open charm production in DIS are:
diffractive heavy flavour production [5], scattering of the virtual photon off the charm sea quark
[6], charmed hadron production from b¯
b[7] and production of c¯cin fragmentation [8]. These
processes, however, are expected to have much smaller cross sections than PGF. The possible
contribution from intrinsic charm [9, 6] is outside the acceptance of the main detector.
ZEUS [10, 11] and H1 [12] have reported on D∗production1by quasi-real photons at HERA
(i.e. in the photoproduction regime). In [10] first D∗signals in the DIS regime were also shown.
In this paper a detailed study of the D∗produced in DIS events is presented. D∗are investigated
in the decay channel
D∗+→D0π+
s→K−π+π+
s(+ c.c.) (1)
using a procedure first proposed in [13], where π+
sstands for the ‘soft pion’.
The e+pcross section for inclusive D∗± production and differential cross sections as functions
of pT(D∗), η(D∗), Wand Q2are presented. The measurements are compared with LO and
NLO QCD analytic calculations [1] based on the photon-gluon fusion production mechanism.
By extrapolation, the charm contribution to the proton structure function F2,Fc¯c
2(x, Q2), is
estimated and compared with the NLO QCD analytic calculations. A similar analysis has been
recently presented by the H1 Collaboration [14].
2 Experimental setup
The data were collected at the positron-proton collider HERA using the ZEUS detector during
the 1994 running period. HERA collided 27.5 GeV positrons with 820 GeV protons yielding a
1In this paper charge conjugate modes are always implied. D∗always refers to both D∗+and the charge
conjugate mode (c.c.) D∗−.
1
center-of-mass energy of 300GeV. 153 bunches were filled for each beam, and, in addition, 15
positron and 17 proton bunches were left unpaired for background studies. The r.m.s. of the
vertex position distribution in the Zdirection2is 12 cm. The data used in this analysis come
from an integrated luminosity of 2.95 pb−1.
ZEUS is a multipurpose detector which has been described in detail elsewhere [15]. The key
component for this analysis is the central tracking detector (CTD) which operates in a magnetic
field of 1.43 T provided by a thin superconducting solenoid. The CTD is a cylindrical drift
chamber consisting of 72 layers organized into 9 “superlayers” covering the polar angular region
15◦< θ < 164◦[16]. Five of the superlayers have wires parallel (axial) to the beam axis and
four have wires inclined at a small angle to give a stereo view. The spatial resolution in the
drift direction is 190 µm. The interaction vertex is measured with a resolution of 0.4 cm in the
Zdirection and 0.1 cm in the XY plane. The momentum resolution for tracks traversing all 9
superlayers is σ(pT)/pT= 0.005 pTL0.016 (pTin GeV).
The solenoid is surrounded by a high resolution uranium-scintillator calorimeter (CAL) de-
scribed elsewhere [17]. The position of positrons scattered close to the positron beam direction
is determined by a scintillator strip detector (SRTD) [18]. The luminosity is measured via the
Bethe-Heitler process, ep →epγ, where the photon is tagged using a lead-scintillator calorime-
ter [19] located at Z=−107 m in the HERA tunnel.
3 HERA kinematics
The kinematics of deep inelastic scattering processes at HERA, e+(k) + p(P)→e+(k′) + X,
where Xis the hadronic final state, can be described by the Lorentz invariant variables Q2,xand
y. Here −Q2is the square of the four-momentum transfer at the lepton vertex, xis the Bjorken
variable and yis the fractional energy transfer between the positron and the proton in the
proton rest frame. In the absence of QED radiation, Q2=−q2=−(k−k′)2,and x=Q2
2P·q,
where kand Pare the four-momenta of the incoming particles and k′is the four-momentum of
the scattered positron. The variables are related by Q2=sxy, where sis the squared invariant
mass of the ep system. Since the ZEUS detector is nearly hermetic, for neutral current DIS
Q2,xand ycan be calculated from the kinematic variables of the scattered positron, from the
hadronic final state variables, or from a combination of both. In this paper we use the double
angle method (DA) [20] to calculate the Q2,x,yvariables. The center-of-mass energy of the
virtual photon-proton system (γ∗p), W, is determined using W2
DA =m2
p+Q2
DA(1
xDA −1), mp
being the proton mass. The variable y, determined using the energy E′
eand angle θ′
eof the
scattered positron, ye= 1 −E′
e
2Ee(1 −cos θ′
e), is used for background suppression. The use of
the positron information alone to calculate Q2,xand yis used in the study of the systematic
uncertainties.
Using the calorimeter information the quantity δ=Pi(Ei−pZi) is measured, where Eiis
the energy and pZithe longitudinal momentum assigned to the calorimeter cell i. For perfect
detector resolution and acceptance, δ= 55 GeV for DIS events while for photoproduction
2The ZEUS coordinate system is defined as right-handed with the Zaxis pointing in the proton beam
direction and the Xaxis horizontal pointing towards the center of HERA. The polar angle θis defined with
respect to the positive Zdirection.
2
events and events with hard initial state radiation, where the scattered positron or the radiated
photon escapes down the beam pipe, δpeaks at lower values.
4 DIS event selection
4.1 Trigger selection
The trigger selection is almost identical to that used for the measurement of the structure
function F2[21]. Events are filtered online by a three level trigger system [22]. At the first
level DIS events are selected by requiring a minimum energy deposition in the electromagnetic
section of the CAL. The threshold depends on the position in the CAL and varies between 3.4
and 4.8 GeV. At the second level trigger (SLT), beam-gas background is further reduced using
the measured times of energy deposits and the summed energies from the calorimeter. The
events are accepted if δcalculated at the SLT level using the nominal vertex position satisfies
δSLT >24 GeV −2Eγ,where Eγis the energy deposit measured in the luminosity photon
calorimeter.
The full event information is available at the third level trigger (TLT). Tighter timing cuts as
well as algorithms to remove beam halo muons and cosmic muons are applied. The quantity
δT LT is determined in the same manner as for δS LT . The events are required to have δT LT >
25 GeV −2Eγ. Finally, events are accepted as DIS candidates if a scattered positron candidate
of energy greater than 4 GeV is found. For events with the scattered positron detected in the
calorimeter, the trigger acceptance is essentially independent of the DIS hadronic final state.
It is greater than 90% for Q2≈5 GeV2(lower limit for the DIS sample in this analysis) and
increases to 99 % for Q2>10 GeV2as determined from MC simulation.
4.2 Offline event selection
The selection of DIS events is similar to that described in our earlier publication [21]. The
characteristic signature of a DIS event is the scattered positron detected in the uranium scin-
tillator calorimeter. The positron identification algorithm is based on a neural network using
information from the CAL and is described elsewhere [23]. The efficiency for finding the scat-
tered positron is sensitive to details of the shower evolution, in particular to energy loss in the
material between the interaction point and the calorimeter. The efficiency of the identification
algorithm when the scattered positron has an energy of 8 GeV is 50%, rising to 99 % for energies
above 15 GeV. For the present data, 89 % of the events have an e+energy greater than 15 GeV.
The impact position of the positrons is determined by either the position reconstructed by the
CAL or by the SRTD if inside its fiducial volume. The resolution of the impact position is
about 1 cm for the CAL and 0.3 cm for the SRTD. The positron traverses varying amounts of
material in the detector before entering the CAL, which causes a variable energy loss. This en-
ergy loss is corrected on an event by event basis, as explained in [18]. The scale uncertainty on
the energy of the scattered positron after these corrections is 2% at 10 GeV linearly decreasing
to 1% at 27.5 GeV.
3
The following criteria are used to select DIS events: the presence of a scattered positron candi-
date with a corrected energy E′
e>8 GeV, and δ > 35 GeV to remove photoproduction events
and to suppress events with hard initial state radiation. After applying these criteria, 4.3·105
events are retained. For this analysis the region 5< Q2<100 GeV2and ye<0.7 is selected
which contains 3.7·105events. A subsample of DIS events in the (x, Q2) plane is plotted in
Fig. 1b. The lines of constant yand Q2delimiting the region chosen for this study are shown.
The photoproduction contamination is less than 2% and the beam gas background is negligible.
4.3 Event selection efficiency
The efficiencies of the event selection and the D∗reconstruction are determined using a GEANT
[24] based Monte Carlo (MC) simulation program which incorporates the knowledge of the
detector and the trigger. For this analysis two different types of event generators were used:
neutral current (NC) DIS generators, discussed below, for event selection efficiency calculations
and heavy flavour generators, discussed in section 5.2, for D∗reconstruction efficiency estimates.
Neutral current DIS events with Q2>4 GeV2were generated using the HERACLES 4.4
program [25] which incorporates first order electroweak corrections. It was interfaced using
DJANGO 6.1-2 [26] to either LEPTO 6.1-3 [27] or ARIADNE 4.03 [28] for the simulation of
QCD cascades. The calculation of the zeroth and first order matrix elements plus the parton
shower option (MEPS) was used in LEPTO. The latter includes coherence effects in the final
state cascade via angular ordering of successive parton emissions. In ARIADNE, the colour-
dipole model including the photon-gluon fusion process (CDMBGF) was used. In this model
coherence effects were implicitly included in the formalism of the parton cascade. The Lund
string fragmentation [29], as implemented in JETSET 7.4 [30], was used for the hadronisation
phase.
The GRV-HO [31] parameterisation was used for the MEPS data set. For the CDMBGF event
sample the MRSD−′[32] parton density parameterisation for the proton was used. These
parameterisations describe the HERA measurements of the proton structure function F2[4, 21,
2] reasonably well.
Monte Carlo samples of DIS events containing D∗mesons (all decay modes) corresponding to
5.5 pb−1were generated. The CDMBGF sample was used for cross section determinations and
the MEPS sample for systematic studies. The efficiency for selecting DIS events where a D∗
has been produced is determined as a function of Q2and y. For the kinematic region selected,
an average event selection efficiency of 75% is found, increasing from about 60 % to 90 % as Q2
increases from 5 to 100 GeV2and from about 65 % to 85 % as yincreases from 0 to 0.7.
5D∗Reconstruction
The tracks of charged particles are reconstructed using the CTD. The single hit efficiency of
the chamber is greater than 95%. The efficiency for assigning hits to tracks depends on several
factors, for example the pTof the track and the number of nearby charged particles. In addi-
tion the 45◦inclination of the drift cells, which compensates for the Lorentz angle, introduces
4
some asymmetry in the chamber response for positive and negative particles, particularly at
low pT. The reconstructed tracks used are required to have more than 20 hits, a transverse
momentum pT>0.125 GeV and a polar angle between 20◦< θ < 160◦. In terms of pseudora-
pidity, η=−log(tan(θ/2)), this angle corresponds to |η|<1.75. This is the region where the
tracking detector response and systematics are sufficiently understood. For those tracks with
pT>0.125 GeV and |η|<1.75 the track reconstruction efficiency is greater than 94%.
5.1 D∗Identification
D∗production is investigated in the decay channel (1). The tight kinematic constraint on the
D∗+→D0π+
sdecay limits the momentum of the decay products to just 40 MeV in the D∗rest
frame. This fact allows one to measure the mass difference M(D∗)−M(D0) more accurately
than the measurement of the D∗mass itself. In practice it leads to a prominent signal in the
∆M=M(Kππs)−M(Kπ) distribution, in an otherwise highly suppressed region of phase
space.
The D∗reconstruction procedure consists of two steps. First a D0pre-candidate is formed by
taking all combinations of pairs of oppositely charged tracks and assuming each track in turn to
correspond to a kaon or a pion. If the Kπ invariant mass of the track combination lies between
1.4 and 2.5 GeV, the track pair is considered to be a D0pre-candidate. In the second step this
D0pre-candidate is combined with a third track, which has the sign of charge opposite to that
of the kaon candidate. The third track is assumed to be a pion (the so called soft pion, πs). If
the mass difference, ∆M, is below 180 MeV, the three tracks form a D∗pre-candidate.
To reduce the combinatorial background and restrict the analysis to a kinematic region where
the detector reconstruction efficiency is acceptable, the following requirements are applied:
•The spatial resolution of the CTD does not allow the D0decay vertex to be distinguished
from the primary vertex. Therefore, we require all tracks to be associated with the
reconstructed primary vertex of the event.
•The kaon and pion candidates from the D0decay must have transverse momenta greater
than 0.4 GeV. This reduces the combinatorial background in the D0reconstruction step.
•D∗pre-candidates must have transverse momenta in the range 1.3 < pT<9.0 GeV and
directions of flight away from the beams, |η|<1.5. The lower pTlimit is due to the very
small acceptance for D∗with pTbelow 1.3 GeV which results from the pT(πs)>0.125 GeV
cut. The higher pTlimit is due to the lack of statistics. Note that these cuts limit the
actual acceptance to y > 0.015.
The analysis is restricted to those combinations (called D∗candidates) which pass the above
requirements, have a Kπ invariant mass in the range 1.8 < M (Kπ)<1.92 GeV and have a
mass difference in the range 143 <∆M < 148 MeV. Fig. 1b shows the distribution of all DIS
events which have a D∗candidate in the (x, Q2) plane.
Figs. 2a,b show the resulting ∆M(M(Kπ)) spectrum for those pre-candidates with M(Kπ)
(∆M) inside the corresponding signal region. Clear signals above the combinatorial background
5
are seen around the nominal M(D∗)−M(D0) and M(D0) values. The ∆Mspectrum is fitted
with the maximum likelihood method by a Gaussian shaped signal plus a background of the
form dN/d∆M=a(∆ M−mπ)bin the 140-180 MeV mass region where aand bare free
parameters. This fit yields M(D∗)−M(D0) = 145.44 ±0.09 MeV, in good agreement with the
PDG [33] value of 145.42 ±0.05 MeV. The width of the signal is 0.65 ±0.10 MeV. The excess
of events seen in Fig. 2b in the range of masses between 1.5 and 1.7 GeV is mainly due to the
decay D0→K−π+π0, where the π0is not measured. A fit of the M(Kπ) spectrum in the mass
range 1.7 < M (Kπ)<2.5 GeV to an exponential curve plus a Gaussian gives a value for the
D0mass of 1858 ±3 MeV, slightly below the PDG[33] value of 1864.6 ±0.5 MeV. The width
of the signal is 19 ±3 MeV. The fit result is shown by the solid curve in Fig. 2b. The signal
region in ∆Mextends from 143 to 148 MeV. The background under the signal is estimated in
two independent ways:
•counting the number of combinations which use pairs of tracks with the same charge
for the D0pre-candidate and fulfill the above requirements (wrong charge combinations,
shown in Fig. 2a as the solid histogram);
•using a control region 2.0 < M(Kπ)<2.5 GeV instead of the signal region 1.80 <
M(Kπ)<1.92 GeV where the events from the control region have been normalized to
the number of events with 155 <∆M < 180 MeV.
The number of background candidates is taken as the weighted mean of the two estimates. The
number of reconstructed D∗candidates is taken to be the number of entries in the signal region
minus the number of background candidates. For 5 < Q2<100 GeV2,y < 0.7 in the restricted
kinematic region 1.3< pT(D∗)<9.0 GeV and |η(D∗)|<1.5 the result is 122 ±17 D∗mesons
above a background of 95 ±8. The ∆Mand D0mean value and width of the signals predicted by
the NC DIS MC sample agree well with the data. The same is true for the background shape
and the signal to background ratio. For the measurement of the pT(D∗), η(D∗), Q2and W
dependence, the data sample in each variable has been divided in three bins. The number of
D∗obtained in each bin is determined following the same procedure described above.
5.2 D∗reconstruction efficiency
The D∗reconstruction efficiency is determined using MC simulations. DIS events with c¯cpro-
duction by photon-gluon fusion were generated using two different MC models: AROMA 2.1 [34]
and HERWIG 5.8 [35]. AROMA is a MC model for heavy flavour production. It is based on the
following ingredients: (i) the complete matrix elements in LO for the PGF process γ∗g→c¯c
(taking into account the mass of the charm quark and the full electroweak structure of the
interaction), (ii) gluon emission from the c¯csystem in a parton shower approach, (iii) initial
state parton showers, (iv) hadronisation with the Lund string model [29] as implemented in
JETSET 7.4 [30] and heavy flavour decay. In addition bremsstrahlung emission of gluons (some
of which may split perturbatively into two gluons or q¯qpairs) is also included.
HERWIG is a general purpose QCD MC event generator for high energy hadronic processes.
Here it is used for simulating γ∗g→c¯cin LO. In addition, leading-logarithm parton show-
ers were included in the simulation. Fragmentation into hadrons is modeled with a cluster
fragmentation model which takes into account the charm quark mass.
6
The parameters of the MC programs were set to their default values. In particular, the charm
quark mass (mc) was set to 1.35 GeV for AROMA and 1.8 GeV for HERWIG. For the parton
densities of the proton the parameterisations of MRSD−′[32] and MRSA [36] were considered.
Those events containing at least one charged D∗, decaying into D0π+with subsequent decay
D0→K−π+, were processed through the standard ZEUS detector and trigger simulation
programs and through the event reconstruction package. Approximately 5000 MC events from
the samples generated and processed in this way, passed all the selection criteria. Both MC
reproduced equally well the shapes of the uncorrected data distributions relevant for this study.
These events were used to determine the D∗reconstruction efficiency as a function of pT(D∗) and
η(D∗). For each D∗candidate, the efficiency in a given (pT(D∗), η(D∗)) bin is the ratio of the
number of reconstructed D∗in the bin to the number of generated D∗in the bin. Both
AROMA and HERWIG MC samples give compatible results and were combined for the final
efficiency determination. The η(D∗) resolution is less than 0.01 in units of pseudorapidity.
The pT(D∗) resolution is about 2% of the bin width chosen for the figures; none of these
show systematic shifts. The D∗reconstruction efficiency in the kinematic range considered
varies between 20% for low pT(D∗) and 70% for pT(D∗)≥5.8 GeV. The efficiency varies in η
from 30% near |η|= 1.5 to 50% for D∗moving transversely to the beam direction (η= 0).
The average D∗reconstruction efficiency is about 38%. As described in section 4.3, the event
selection efficiency is determined using the HERACLES MC. The convolution of the event
selection efficiency and the D∗reconstruction efficiency gives an overall detection efficiency of
approximately 30% for D∗decaying via (1).
6 Results
6.1 D∗fractional momentum distribution
To investigate the charm production mechanism, the distribution of the fractional momentum of
the D∗in the γ∗psystem (xD∗=2|~p ∗
D∗|
W) is studied. The PGF process produces a c¯cpair that, in
the γ∗psystem, recoils against the proton remnant. In contrast, in the flavor-excitation process,
a single cquark from the proton sea is scattered off the proton, flying, in the γ∗psystem, in a
direction opposite to the proton remnant. Since, in general, the D∗carries a large fraction of
the momentum of the parent charm quark [33], clear differences are expected between the xD∗
distributions from the two production mechanisms. The distribution is expected to be centered
at xD∗<0.5 for PGF and peaked at high xD∗values for direct production. Figure 1c shows
the normalized xD∗differential distribution measured in the data and the AROMA prediction
(based on PGF). For comparison we also show the prediction by LEPTO6.1 with only direct
production from charm. Both MC use the proton density of MRSD−′[32]. The shape of our
data distribution is compatible with PGF, in accord with the H1 result[14].
6.2 Cross sections in a restricted pT(D∗) and η(D∗) kinematic region
Differential cross sections as well as cross sections integrated over the kinematic region 1.3<
pT(D∗)<9.0 GeV and |η(D∗)|<1.5 are presented in this section. The e+p→e+D∗± X
7
differential cross sections are corrected for the efficiencies of the selection criteria as well as for
the branching ratios B(D∗+→D0π+
s)×B(D0→K−π+) = 0.0262 ±0.0010 [33].
Fig. 3 shows the differential e+p→e+D∗±Xcross section as a function of a) pT(D∗), b) η(D∗),
c) Wand d) Q2in the kinematic region defined above. The inner error bars are the statistical
errors and the outer ones show the statistical and systematic errors added in quadrature. The
data points in the pT(D∗) and Q2distributions are shown at the positions of the average values
of an exponential and polynomial fit, respectively, for a given bin.
The D∗± differential cross section dσ/dpT2(D∗) exhibits an exponential falloff in pT(D∗) and
dσ/dη(D∗) is approximately flat. The shape of the Wdependence of the cross section predicted
for photon-gluon fusion at low Wis determined by the pT(D∗)>1.3 GeV and the |η(D∗)|<1.5
cuts. The falloff at large Wis mainly due to the virtual photon flux. The D∗± cross section
drops steeply as Q2increases.
The systematic uncertainties arise from several effects. The main uncertainties coming from
DIS event selection and D∗reconstruction are: the transverse momentum cut of the soft pion
(7%), the range used for the K π invariant mass (5%) and the positron energy cut (5%). The
differences in acceptances evaluated from different MC samples correspond to an uncertainty
in the integrated e+p→e+D∗±Xcross section at the 7% level. The QED radiative effects
contribute 3 % to the systematic error. The systematic error attributed to the branching ratios
is 4%. The normalisation uncertainty due to the determination of the luminosity and trigger
efficiency is 2%. No significant contribution to the systematic error from the primary vertex
requirement was found. Adding the contributions in quadrature, a total systematic error of 15%
is obtained in the determination of the integrated cross section. For the differential distributions
the systematic errors are estimated bin by bin.
The results of the NLO analytic calculation of [1] using the GRV[37] NLO gluon density,
xg(x, Q2), of the proton is shown as a band in Fig. 3, where the upper (lower) limit corresponds
to a charm quark mass of 1.35 (1.7) GeV. The calculation is based exclusively on the PGF
process and no charm produced from the proton sea is considered. The GRV NLO parton
density gives a reasonable description of the proton structure function F2measured at HERA
[4, 21] and treats the charm quarks as massive particles, consistent with the ansatz of [1].
Charm quarks were hadronized according to the Peterson fragmentation function [38] with
ǫc= 0.035 ±0.009 [39] and the probability for a charm quark to fragment into D∗,P(c→D∗),
was assumed to be 0.26 ±0.02 [39]. Both the renormalisation and the factorisation scales were
chosen to be µ=qQ2+ 4 m2
c. In this calculation the largest uncertainty is related to that of
the charm quark mass. The predicted differential cross section for D∗± production as a function
of a) pT(D∗) and b) η(D∗) is shown in Fig. 3 outside the restricted kinematic region selected for
the data. The NLO analytic calculation reproduces well the shapes of the pT(D∗), Wand Q2
distributions and is consistent with the η(D∗) dependence in the restricted kinematic region.
Below, unless explicitly stated otherwise, the LO and NLO analytic calculations are those from
[1] and use respectively the LO and NLO gluon density parameterisations of GRV [37] and a
charm quark mass mc= 1.5 GeV.
In Fig. 3 the data are also compared with the LO analytic calculation, shown as histograms.
The LO calculation performed with the MC programs used in the acceptance calculation and
described in section 5.2 agrees well with the LO analytic calculation. Note that the MC
8
calculation includes parton showers in addition to the LO matrix elements. The LO MC and
LO analytic calculations describe the shapes of the pT(D∗) , Q2and Wdependences equally
well.
Integrated over the region 5 < Q2<100 GeV2and y < 0.7, the e+p→e+D∗±Xcross section
for D∗± in the restricted kinematic range 1.3< pT(D∗)<9.0 GeV and |η(D∗)|<1.5 is
σpT,η (e+p→e+D∗± X) = 5.3±1.0±0.8 nb
where the first error is statistical and the second error is systematic. The e+p→e+D∗±X
cross section represents about 4 % of the DIS cross section (calculated from structure functions
determined by fits to global data [32]) in the same Q2and yranges. Table 1 compares the
measured cross section with the LO and NLO predictions. The integrated cross sections pre-
dicted in NLO is in reasonable agreement with the data. The LO MC and analytic calculation
agree with the data, although their sensitivity to the parameters involved in the program (mc,
xg(x, Q2), ǫcand µ) is higher than in the case of NLO calculations.
6.3 Integrated charm cross section and Fc¯c
2
The DIS inclusive cross section for charm production, e+p→e+c¯c X,σc¯c, can be expressed in
terms of Fc¯c
2using:
d2σc¯c
dxdQ2=2πα2
Q4x[(1 + (1 −y)2)Fc¯c
2(x, Q2)−y2Fc¯c
L(x, Q2)] (2)
In the standard model, the contribution from Z-boson exchange is expected to be small in the
(Q2, x) range of the present analysis and therefore the Fc¯c
3contribution is neglected. The Fc¯c
L
contribution has been estimated [40] to be smaller than 2% and, therefore, no correction has
been applied.
In order to estimate σc¯cand to evaluate Fc¯c
2as a function of Q2and y, the measurements must
be extrapolated to the full pT(D∗) and η(D∗) range. σc¯cis obtained from the integrated D∗
cross section using:
σ(e+p→e+c¯c X) = 1
2·σ(e+p→e+D∗X)
P(c→D∗).(3)
According to the extrapolation with the NLO calculation, about 50 % (at Q2= 45 GeV2) to
65 % (at Q2= 7 GeV2) of the D∗production is found to be outside the restricted kinematic
region. Using the LO analytic calculation for the extrapolation results in similar cross sections.
For the determination of the integrated cross section the data are divided into two Q2in-
tervals, namely 5 < Q2<10 GeV2and 10 < Q2<100 GeV2. The extrapolated accep-
tance is calculated using the NLO prediction of GRV[37] with mc=1.5 GeV, µ=qQ2+ 4m2
c,
P(c→D∗+) = 0.26 ±0.02 and ǫc= 0.035 ±0.009 [39]. The integrated charm cross sec-
tion, σ(e+p→e+c¯c X), for y < 0.7, is 13.5±5.2±1.8+1.6
−1.2nb for 5< Q2<10 GeV2, and
12.5±3.1±1.8+1.5
−1.1nb for 10 < Q2<100 GeV2. The first error gives the statistical, the sec-
ond the experimental systematic uncertainty and the third the model dependent uncertainty.
The latter is studied by varying the relevant parameters of the model used for extrapolation,
9
namely mc(from 1.35 to 1.7 GeV), µ(from 2mcto 2 qQ2+m2
c), ǫc(from 0.025 to 0.045), and
xg(x, Q2) (GRV [37], MRSG [41], CTEQ3 [42], MRSA′[36]) 3and also includes the error in the
probability for c→D∗. An uncertainty between −12 % and +9 % is obtained for both Q2
ranges.
The integrated charm cross sections as well as the corresponding predictions from the LO
and NLO analytic calculations are also listed in Table 1. The NLO predictions are about one
standard deviation smaller than the measured value in the first Q2interval (from 5 to 10 GeV2)
but in agreement in the second Q2interval (from 10 to 100 GeV2). The charm cross section
obtained for 10 < Q2<100 GeV2is in reasonable agreement with the recent H1 measurement
in the same Q2region.
σpT,η [nb] σ1[nb] σ2[nb]
Data 5.3 ±1.0 ±0.8 13.5±5.2±1.8+1.6
−1.212.5±3.1±1.8+1.5
−1.1
AROMA 4.57 12.6 14.2
LO Analytic calculation 4.79 11.0 12.4
NLO Analytic calculation 4.15 9.4 11.1
Table 1: The measured e+p→e+D∗±Xcross section for 5 < Q2<100 GeV2,y < 0.7 in the
restricted kinematic region 1.3< pT(D∗)<9.0 GeV and |η(D∗)|<1.5 (σpT,η); the extrapolated
e+p→e+c¯cX cross section for y < 0.7 and for 5 < Q2<10 GeV2(σ1) and 10 < Q2<100 GeV2(σ2).
The measured cross sections are compared with the NLO analytic calculation for the GRV NLO gluon
density as well as with the LO MC and LO analytic calculations. The statistical uncertainties of the
NLO analytic calculation have about 0.01 the size of the uncertainties of the data points.
Fig. 4 shows the resulting Fc¯c
2values as a function of xfor the different Q2and ybins as defined
in Fig. 1b. The H1 measurements [14] are also shown; they were taken in the same Q2bins.
The two sets of data are in good agreement. Also included in Fig. 4 are the data from the
EMC fixed target experiment [43] which were measured at xvalues between 0.02 and 0.3. The
charm contribution, Fc¯c
2, to the proton structure function is seen to rise by about one order of
magnitude from the high xregion covered by the fixed target experiments to the low xregion
measured by H1 and ZEUS. The NLO analytic calculation for Fc¯c
2is shown in Fig. 4 as a band
(the upper (lower) limit corresponds to a charm quark mass of 1.35 (1.7) GeV). In the region of
our measurements, the use of CTEQ4F3 [44] or MRRS [45] gluon densities give results which
are within 5 % of those using GRV. In the present Q2range, Fc¯c
2scales roughly with the input
gluon density in NLO perturbative QCD. The measured rise of Fc¯c
2from the high xto the low
xregion is reasonably described by NLO perturbative QCD.
Comparing with our measurement of F2(x, Q2) [21], we observe that the ratio Fc¯c
2/F2is about
25% for the entire (Q2, x) range of the present analysis.
3Note that while GRV (and also CTEQ4F3 and MRRS, see below) follow the charm quark treatment of [1]
based on PGF, the CTEQ3 and MRS distributions treat charmed quarks as massless quarks with a parametrized
density contributing above a certain threshold in Q2and therefore are not completely consistent with the
program of [1].
10
7 Summary and conclusions
We have measured the D∗differential and integrated e+p→e+D∗±Xcross sections for deep
inelastic scattering at √s= 300 GeV with 5 < Q2<100 GeV2and y < 0.7 in the restricted
kinematic region of 1.3< pT(D∗)<9.0 GeV and |η(D∗)|<1.5. The integrated D∗± cross
section is measured to be 5.3 ±1.0 (stat.) ±0.8 (syst.) nb.
The shape of the D∗fractional momentum distribution in the γ∗prest system, xD∗, shows that
the PGF mechanism prediction agrees well with the data for DIS charm production in this
kinematic range.
A QCD analytic calculation in NLO with the NLO GRV gluon density reproduces the shapes
of the pT(D∗)Wand Q2distributions and is consistent with the η(D∗) dependence in the
restricted kinematic region. The predicted cross sections are in reasonable agreement with the
data. We have used QCD calculations to extrapolate the D∗± cross section measured in the
restricted pT(D∗), η(D∗) region to the full region and estimated the integrated charm cross
section and the charm contribution Fc¯c
2to the proton structure function F2. When compared
to the fixed target measurements (performed at large x)Fc¯c
2is found to rise as xdecreases. The
rise is described by NLO QCD calculations when using a gluon density consistent with that
extracted from the scaling violations in the proton structure function F2measured at HERA.
Such a gluon density distribution is also compatible with our previous measurements of D∗in
photoproduction.
Acknowledgements
The strong support and encouragement by the DESY Directorate have been invaluable. The
experiment was made possible by the inventiveness and diligent efforts of the HERA machine
group.
The design, construction and installation of the ZEUS detector have been made possible by the
ingenuity and dedicated efforts of many people from the home institutes who are not listed here.
Their contributions are acknowledged with great appreciation. We acknowledge the support
of the DESY computing and network services. We would like to thank J. Smith for valuable
discussions and S. Riemersma for helpful comments and for providing us with the NLO analytic
Fc¯c
2calculation.
We warmly acknowledge B. Harris for the close collaboration during the last stage of the analysis
and for providing us with the LO and NLO analytic charm cross section calculations.
References
[1] B. W. Harris, talk A05056, presented at the American Physical Society, Division of Parti-
cles and Fields 1996 Meeting, Minneapolis, Minnesota, 10-15 August 1996, to appear in
proceedings (hep-ph/9608379); B. W. Harris and J. Smith, Nucl. Phys. B452 (1995) 109;
B. W. Harris and J. Smith, Phys. Lett. B353 (1995) 535.
11
[2] ZEUS Collaboration, M. Derrick et al., Phys.Lett. B316 (1993) 412; H1 Collaboration,
I. Abt et al., Nucl. Phys. B407 (1993) 515.
[3] ZEUS Collaboration, M. Derrick et al., Phys. Lett. B345 (1995) 576.
[4] H1 Collaboration, S. Aid et al., Nucl. Phys. B470 (1996) 3.
[5] P.Bruni, G. Ingelman and A. Solano, Proc. of the HERA Workshop, DESY, Hamburg
Vol.1 DESY (1992), 363.
[6] G.Ingelman, L. J¨onsson and M.Nyberg, Phys. Rev. D47 (1993) 4872.
[7] A. Ali et al., Proc. of the HERA Workshop, DESY, Hamburg, Vol. 1 (1988),393; A. Ali
et al., Proc. of the HERA Workshop, DESY, Hamburg Vol.2 (1992), 667.
[8] M. L. Mangano and P. Nason , Phys. Lett. B285 (1992) 160; M. H. Seymour , Z. Phys. C63
(1994) 99; Nucl. Phys. B436 (1995) 16.
[9] S. J. Brodsky et al., Phys. Lett. B93 (1980) 451; S. J. Brodsky et al., Nucl. Phys. B369
(1992) 519 and references therein.
[10] ZEUS Collaboration, M. Derrick et al., Phys. Lett. B349 (1995) 225.
[11] ZEUS Collaboration, J. Breitweg et al., DESY-97-026, accepted for publication in Phys.
Lett. B.
[12] H1 Collaboration, S. Aid et al., Nucl. Phys. B472 (1996) 32.
[13] S. Nussinov, Phys. Rev. Lett. 35 (1976) 1672.
[14] H1 Collaboration, C. Adloff et al., Z. Phys. C72 (1996) 593.
[15] ZEUS Collaboration, M. Derrick et al., The ZEUS Detector, Status Report 1993, DESY
1993.
[16] N. Harnew et al., Nucl. Instr. and Meth. A279 (1989) 290; B. Foster et al., Nucl. Phys. B
(Proc. Suppl.) 32 (1993) 181; B. Foster et al., Nucl. Instr. and Meth. A338 (1994) 254.
[17] A. Andresen et al., Nucl. Instr. and Meth. A309 (1991) 101; A. Bernstein et al., Nucl. Instr.
and Meth. A336 (1993) 23; A. Caldwell et al., Nucl. Instr. and Meth. A321 (1992) 356.
[18] ZEUS Collaboration, M. Derrick et al., Z. Phys. C69 (1996) 607.
[19] J. Andruszk´ow et al., DESY 92-066.
[20] S. Bentvelsen, J. Engelen and P. Kooijman, Proc. of the HERA Workshop, DESY, Ham-
burg Vol.1 DESY (1992) 23.
[21] ZEUS Collaboration, M. Derrick et al., Z. Phys. C72 (1996) 399.
[22] W.H. Smith et al., Nucl. Instr. Meth. A 355 (1995) 278.
[23] H. Abramowicz, A. Caldwell and R. Sinkus, Nucl. Inst. Meth. A365 (1995) 508.
12
[24] R. Brun et al., GEANT3.13, CERN DD/EE/84-1 (1987).
[25] A. Kwiatkowski, H. Spiesberger and H.-J. M¨ohring, Proc. of the HERA Workshop, DESY,
Hamburg Vol.3 DESY (1992) 1294.
[26] K. Charchu la, G. Schuler and H. Spiesberger, CERN-TH. 7133/94.
[27] G. Ingelman, Proc. of the HERA Workshop, DESY, Hamburg Vol.3 DESY (1992) 1366;
G. Ingelman, J.Rathsman and A. Edin, LEPTO version 6.3, DESY 95 - August 1995,
ISSN 0418-9833.
[28] L. L¨onnblad, Comput. Phys. Comm. 71 (1992) 15; L. L¨onnblad, Z. Phys. C 65 (1995) 285.
[29] B. Andersson et al., Phys. Rep. 97 (1983) 31.
[30] T. Sj¨ostrand, CERN-TH.7112/93, Dec. 1993.
[31] M. Gl¨uck, E. Reya and A. Vogt, Phys. Lett. B306 (1993) 391.
[32] A.D. Martin, W.J. Stirling and R.G. Roberts, Phys. Lett. B306 (1993) 145;
Phys. Lett. B309 (1993) 492 (erratum).
[33] Particle Data Group, R.M. Barnett et al., Phys. Rev. D54 (1996) 1.
[34] G.Ingelman, J.Rathsman and G.A.Schuler, AROMA 2.1, DESY 95 - June 1995, ISSN
0418-9833.
[35] G. Marchesini et al., Comput. Phys. Comm. 67 (1992) 465.
[36] A.D. Martin, W.J. Stirling and R.G. Roberts, Phys. Rev. D50 (1994) 6734.
[37] M. Gl¨uck, E. Reya and A. Vogt, Z. Phys. C67 (1995) 433.
[38] C. Peterson et al., Phys. Rev. D27 (1983) 105.
[39] OPAL Collaboration, R. Akers et al., Z. Phys. C67 (1995) 27.
[40] E. Laenen, S. Riemersma, J. Smith, and W.L. Van Neerven, Nucl. Phys. B392 (1993) 162;
S. Riemersma, J. Smith and W.L. Van Neerven, Phys. Lett. B347 (1995) 147; E. Laenen et
al., Proc. of the Workshop ‘Future Physics at HERA’, DESY, Hamburg Vol.1 (1995/96)
393; K. Daum et al., Proc. of the Workshop ‘Future Physics at HERA’, DESY, Hamburg
Vol.1 (1995/96) 89.
[41] A.D. Martin, W.J. Stirling and R.G. Roberts, Phys. Lett. B354 (1995) 155.
[42] H.L. Lai et al., Phys. Rev. D51 (1995) 4763.
[43] J. J. Aubert et al. , Nucl. Phys. B213 (1983) 31.
[44] H. L. Lai and W. K. Tung, hep-ph/9701256.
[45] A.D.Martin, R.G.Roberts ,M.G.Ryskin, W.J.Stirling, hep-ph/9612449.
13
c
c
_
e+
e+
P
γ*
g
a)
10
10 2
10 -4 10 -3 10 -2 10 -1
y = 0.7
x
Q2 [GeV2]
b)
0
0.5
1
1.5
2
2.5
3
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
1/σ dσ(D*±)/dxD*
•
----
DATA (ZEUS 1994)
PGF(AROMA)
QPM (LEPTO 6.1)
c)
xD*
Figure 1: (a) LO diagram for photon-gluon fusion (PGF). (b) The (x, Q2) plane with the Q2,
yregion and bins chosen for the Fc¯c
2analysis. Large dots correspond to the D∗candidates;
small dots correspond to a subsample of DIS events. The second constant yline corresponds to
y= 0.2. (c) Normalized e+p→e+D∗±Xcross section for 5< Q2<100 GeV2,y < 0.7in the
restricted kinematic region 1.3< pT(D∗)<9.0GeV and |η(D∗)|<1.5as a function of xD∗.
The inner error bars show the statistical errors and the outer ones the statistical and systematic
errors added in quadrature. The horizontal bars represent the bin widths. The prediction for
PGF as calculated with AROMA (solid histogram) and the charm sea contribution as calculated
with LEPTO 6.1 selecting QPM events (dashed histogram) are also shown (see text).
14
0
25
50
75
100
0.14 0.15 0.16 0.17 0.18
∆M∆M∆M
•
¬
DATA (ZEUS 1994)
Bck. wrong charge
Fit
a)
[GeV]
No. of candidates per 1 MeV
0
20
40
60
1.5 1.75 2 2.25 2.5
M(Kπ)M(Kπ)M(Kπ)
b)
[GeV]
No. of candidates per 20 MeV
•
¬
DATA (ZEUS 1994)
Bck. wrong charge
Fit
Figure 2: (a) ∆Mmass distribution for Kπ combinations in the M(Kπ)signal re-
gion (1.80 < M(Kπ)<1.92 GeV), full dots, and for the wrong charge combinations,
solid histogram. (b) M(Kπ)mass distribution for the Kππscombinations in the ∆M=
M(Kππs)−M(Kπ)signal region (143 <∆M < 148 MeV), full dots, and for the wrong
charge combinations, solid histogram. The solid lines in both figures show the result from the
fits (see text for details).
15
10 -3
10 -2
10 -1
1
0 2 4 6 8 10
pT(D*)pT(D*)pT(D*) [GeV]
dσ(D*±)/dpT
2 [nb/GeV2]
(a)
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
-2 0 2
η(D*) η(D*)
dσ(D*±)/dη [nb]
•
DATA (ZEUS 1994)
LO GRV (mc=1.5 GeV)
NLO GRV (mc=1.35-1.7 GeV)
(b)
0
0.01
0.02
0.03
0.04
50 100 150 200 250
WW [GeV]
dσ(D*±)/dW [nb/GeV]
WWWW
(c)
10 -3
10 -2
10 -1
1
10 102
Q2[GeV2]
dσ(D*±)/dQ2 [nb/GeV2]
Q2
Q2
(d)
Figure 3: Differential e+p→e+D∗±Xcross sections for 5< Q2<100GeV2,y < 0.7in the
restricted kinematic region 1.3< pT(D∗)<9.0GeV and |η(D∗)|<1.5as a function of pT(D∗)
(a), η(D∗)(b), W(c) and Q2(d). The inner error bars show the statistical errors and the outer
ones correspond to the statistical and systematic errors added in quadrature. The horizontal
bars represent the bin widths. The NLO QCD prediction for different charm quark masses is
shown by the band (see text). The LO prediction for the GRV(LO) gluon density is shown by
the histogram (see text). The predicted cross sections in (a) and (b) are shown without the
pT(D∗) and η(D∗)cuts respectively.
16
0
0.1
0.2
0.3
0.4
F2
cc
_
|
Q2=7 GeV2
● ZEUS D* 1994 H1 D0 H1 D*+ EMC
NLO GRV (mc=1.35-1.7 GeV)
Q2=12 GeV2
0
0.2
0.4
0.6
10 -4 10 -3 10 -2 10 -1
Q2=25 GeV2
10 -4 10 -3 10 -2 10 -1
x
Q2=45 GeV2
Figure 4: The charm contribution, Fc¯c
2, to the proton structure function F2as derived from
the inclusive D∗(ZEUS and H1) and D0(H1) production compared with the NLO QCD predic-
tions based on the GRV parton distribution using different charm quark masses for Q2=7, 12,
25 and 45 GeV2The upper (lower) limit of the band corresponds to a charm quark mass of
1.35 (1.7) GeV(see text). The results from the EMC collaboration are shown as crosses. For
the ZEUS data, the inner error bars show the statistical errors and the outer ones correspond
to the statistical and systematic errors added in quadrature. The error bars from H1 show the
statistical and systematic errors added in quadrature. The error bars for EMC are within the
symbol.
17
