Available via license: CC BY-NC-ND 4.0
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Available via license: CC BY-NC-ND 4.0
Content may be subject to copyright.
15 February 1996
ELSESIER
PHYSICS LETTERS B
Physics Letters B 369 (1996) 55-68
Rapidity gaps between jets in photoproduction at HERA
ZEUS Collaboration
M. Derrick, D. Krakauer, S. Magill, D. Mikunas, B. Musgrave, J. Repond, R. Stanek,
R.L. Talaga, H. Zhang
Argonne National Laboratory, Argonne, IL, USA 55
G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, P. Bruni,
G. Cara Romeo, G. Castellini ‘, M. Chiarini, L. Cifarelli2, F. Cindolo, A. Contin,
M. Corradi, I. Gialas 3, P. Giusti, G. Iacobucci, G. Laurenti, G. Levi, A. Margotti,
T. Massam, R. Nania, C. Nemoz, F. Palmonari, A. Polini, G. Sartorelli, R. Timellini,
Y. Zamora Garcia4, A. Zichichi
University and INFN Bologna, Bologna, Italy”
A. Bornheim, J. Crittenden, K. Desch, B. Diekmann5, T. Doeker, M. Eckert, L. Feld,
A. Frey, M. Geerts, M. Grothe, H. Hartmann, K. Heinloth, L. Heinz, E. Hilger,
H.-P Jakob, U.F. Katz, S. Mengel, J. Mollen 6, E. Paul, M. Pfeiffer, Ch. Rembser,
D. Schramm, J. Stamm, R. Wedemeyer
Physikalisches Institut der Universitiit Bonn, Bonn, Gerrnany42
S. Campbell-Robson, A. Cassidy, W.N. Cottingham, N. Dyce, B. Foster, S. George,
M.E. Hayes, G.P. Heath, H.F. Heath, C.J.S. Morgado, J.A. O’Mara, D. Piccioni, D.G. Roff,
R.J. Tapper, R. Yoshida
H.H. Wills Physics Laboratory University of Bristol, Bristol, UK%
R.R. Rau
Brookhaven National Laboratory, Upton, L.I., USAs5
M. Arneodo7, R. Ayad, M. Capua, A. Garfagnini, L. Iannotti, M. Schioppa, G. Susinno
Calabria University, Physics Dept. and INFN, Cosenza, Italy 45
A. Bernstein, A. Caldwell 8, N. Cartiglia, J.A. Parsons, S. Ritzg, F. Sciulli, P.B. Straub,
L. Wai, S. Yang, Q. Zhu
Columbia University, Nevis Labs., Irvington on Hudson, N.E, USAs
P. Borzemski, J. Chwastowski, A. Eskreys, K. Piotrzkowski, M. Zachara, L. Zawiejski
Inst. of Nuclear Physics, Cracow, Poland4g
0370-2693/96/$12.00 @ 1996 Elsevier Science B.V. All rights reserved
SSDIO370-2693(95)01588-4
56 ZEUS Collaboration/Physics Letters B 369 (1996) 55-68
L. Adamczyk, B. Bednarek, K. Jelen, D. Kisielewska, T. Kowalski, M. Przybycied,
E. Rulikowska-Zarebska, L. Suszycki, J. Zajac
Faculty of Physics and Nuclear Techniques, Academy of Mining and Metallurgy, Cracow, Poland4g
A. Kotanski
Jagellonian Univ., Dept. of Physics, Cracow, Poland 5o
L.A.T. Bauerdick, U. Behrens, H. Beier, J.K. Bienlein, C. Coldewey, 0. Deppe, K. Desler,
G. Drews M Flasinski lo, D.J. Gilkinson, C. Glasman, P. Giittlicher, J. GroBe-Knetter,
B. Gutj& .
l1 ‘T Haas, W. Hain, D. Hasell, H. Helling, Y. Iga, K.F. Johnson 12, P. Joos,
M. Kasemann, R. Klanner, W. Koch, L. Kiipke 13, U. K&z, H. Kowalski, J. Labs
A. Ladage, B. Liihr, M. Lowe, D. Luke, J. Mainusch14, 0. Mariczak, T. Monteiro ;5 ,
J.S.T. Ng, S. Nickel , .
l6 D Notz, K. Ohrenberg, M. Roco, M. Rohde, J. Roldan,
U. Schneekloth, W. Schulz, F. Selonke, E. Stiliaris17, B. Surrow, T. Vol3, D. Westphal,
G. Wolf, C. Youngman, W. Zeuner, J.F. Zhou ‘*
Deutsches Elektronen-Synchrotron DESI: Hamburg, Germany
H.J. Grabosch, A. Kharchilava lg, A. Leich, SM. Mari 3, M.C.K. Mattingly 20, A. Meyer,
S. Schlenstedt, N. Wulff
DESY-Zeuthen, Inst. fiir Hochenergiephysik, Zeuthen, Germany
G. Barbagli, E. Gallo, P. Pelfer
University and INFN, Florence, Italy 4i
G. Anzivino, G. Maccarrone, S. De Pasquale, L. Votano
INFN, Iaboratori Nazionali di Frascati, Frascati, Italy4’
A. Bamberger, S. Eisenhardt, A. Freidhof, S. Sbldner-Rembold21, J. Schroeder22,
T. Trefzger
Fakultiitfiir Physik der Universitiit Freiburg i.Br., Freiburg i.Br., Germany42
N.H. Brook, P.J. Bussey, A.T. Doyle, D.H. Saxon, M.L. Utley, A.S. Wilson
Dept. of Physics and Astronomy, University of Glasgow, Glasgow, UK%
A. Dannemann, U. Holm, D. Horstmann, T. Neumann, R. Sinkus, K. Wick
Hamburg University, I. Institute of Exp. Physics, Hamburg, Germany42
E. Baduraz3, B.D. Burow 24, L. Hagge , . l4 E Lohrmann, J. Milewski, M. Nakahata25,
N. Pavel, G. Poelz, W. Schott, F. Zetsche
Hamburg University, II. Institute of Exp. Physics, Hamburg, Germany42
T.C. Bacon, N. Bruemmer, I. Butterworth, V.L. Harris, B.Y.H. Hung, K.R. Long,
D.B. Miller, P.P.O. Morawitz, A. Prinias, J.K. Sedgbeer, A.F. Whitfield
Imperial College London, High Energy Nuclear Physics Group, London, UK2
U. Mallik, E. McCliment, M.Z. Wang, S.M. Wang, J.T. Wu
University of Iowa Physics and Astronomy Dept., Iowa City, USA55
ZEUS Collaboration/Physics Letters B 369 (1996) 55-68
P. Cloth, D. Filges
Forschungszentrum Jtilich, Institut fiir Kernphysik, Jillich, Germany
S.H. An, S.M. Hong, S.W. Nam, S.K. Park, M.H. Suh, S.H. Yon
Korea University, Seoul, South Korea47
R. Imlay, S. Kartik, H.-J. Kim, R.R. McNeil, W. Metcalf, V.K. Nadendla
Louisiana State University, Dept. of Physics and Astronomy, Baton Rouge, LA, USA 55
F. Barreiro 26 G. Cases, J.P. Fernandez, R. Graciani, J.M. Hernarrdez, L. Hervas 26,
L. Labarga ,
26 M. Martinez, J. de1 Peso, J. Puga, J. Terron, J.F. de Troconiz
Univer. Autdnoma Madrid, Depto de Ftsica Tedrica. Madrid, Spain53
G.R. Smith
University of Manitoba, Dept. of Physics, Winnipeg, Manitoba, Canada40
F. Corriveau, D.S. Hanna, J. Hartmann, L.W. Hung, J.N. Lim, C.G. Matthews, P.M. Patel,
L.E. Sinclair, D.G. Stairs, M. St.Laurent, R. Ullmann, G. Zacek
McGill University, Dept. of Physics, Montreal, Quebec, Canada40s41
V. Bashkirov, B.A. Dolgoshein, A. Stifutkin
Moscow Engineering Physics Institute, Moscow, Russia 51
G.L. Bashindzhagyan , . . 27 PF Ermolov, L.K. Gladilin, Yu.A. Golubkov, V.D. Kobrin,
I.A. Korzhavina, V.A. Kuzmin, O.Yu. Lukina, A.S. Proskuryakov, A.A. Savin,
L.M. Shcheglova, A.N. Solomin, N.P Zotov
Moscow State University, Institute of Nuclear Physics, Moscow, Russia s2
M. Botje, F. Chlebana, A. Dake, J. Engelen, M. de Kamps, l? Kooijman, A. Kruse,
H. Tiecke, W. Verkerke, M. Vreeswijk, L. Wiggers, E. de Wolf, R. van Woudenberg2*
NIKHEF and University of Amsterdam, Netherlands48
D. Acosta, B. Bylsma, L.S. Durkin, J. Gilmore, K. Honscheid, C. Li, T.Y. Ling,
K.W. McLean 2g, P. Nylander, I.H. Park, T.A. Romanowski 30, R. Seidlein 31
Ohio State University, Physics Department, Columbus, Ohio, VSA55
D.S. Bailey, A. Byrne ,
32 R.J. Cashmore, A.M. Cooper-Sarkar, R.C.E. Devenish,
N. Harnew, M. Lancaster, L. Lindemann 3, J.D. McFall, C. Nath, V.A. Noyes, A. Quadt,
J.R. Tickner, H. Uij terwaal, R. Walczak, D.S. Waters, F.F. Wilson, T. Yip
Deparmzent of Physics, University of Oxford, Odord, VKs4
G. Abbiendi, A. Bertolin, R. Brugnera, R. Carlin, F. Dal Corso, M. De Giorgi, U. Dosselli,
S. Limentani, M. Morandin, M. Posocco, L. Stance, R. Stroili, C. Voci
Dipartimento di Fisica dell’ Vniversita and INFN, Padova, Italy4i
J. Bulmahn, J.M. Butterworth, R.G. Feild, B.Y. Oh, J.R. Okrasinski 33, J.J. Whitmore
Pennsylvania State University, Dept. of Physics, University Park, PA, USA ”
ZKLJS Collaboration/Physics Letters B 369 (1996) 55-68
G. D’Agostini, G. Marini, A. Nigro, E. Tassi
Dipartimento di Fisica, Univ. ‘La Sapienza’ and INFN, Rome, ltaIy45
J.C. Hart, N.A. McCubbin, K. Prytz, TX Shah, T.L. Short
Ruthegord Appleton Laboratory, Chilton, Didcot, Oxon, UK54
E. Barber-is, T. Dubbs, C. Heusch, M. Van Hook, W. Lockman, J.T. Rahn,
H.F.-W. Sadrozinski, A. Seiden, D.C. Williams
University of California, Sanfa Cruz, CA, USAs
J. Biltzinger, R.J. Seifert, 0. Schwarzer, A.H. Walenta, G. Zech
Fachbereich Physik der Universitiit-Gesamthochschule Siegen, Germany42
H. Abramowicz, G. Briskin, S. Dagan34, C. Handel-Pikielny, A. Levy 27
School of Physics,Tel-Aviv University, Tel Aviv, Israel#
J.I. Fleck, T. Hasegawa, M. Hazumi, T. I&ii, M. Kuze, S. Mine, Y. Nagasawa, M. Nakao,
I. Suzuki, K. Tokushuku, S. Yamada, Y. Yamazaki
Institute for Nuclear Study, University of Tokyo, Tokyo, Japan4
M. Chiba, R. Hamatsu, T. Hirose, K. Homma, S. Kitamura, Y. Nakamitsu, K. Yamauchi
Tokyo Metropolitan University, Dept. of Physics, Tokyo. Japan 46
R. Cirio, M. Costa, M.I. Ferrero, L. Lamberti, S. Maselli, C. Peroni, R. Sacchi, A. Solano,
A. Staiano
Universita di Torino, Dipartimento di Fisica Sperimentale and INFN, Torino, Italy4’
M. Dardo
II Faculty of Sciences, Torino Universiiy and INFN - Alessandria, Italy45
D.C. Bailey, D. Bandyopadhyay, F. Benard, M. Brkic, D.M. Gingrich35, G.F. Hartner,
K.K. Joo, G.M. Levman, J.F. Martin, R.S. Orr, S. Polenz, C.R. Sampson, R.J. Teuscher
University of Toronto, Dept. of Physics, Toronto, Ont., Canada40
C.D. Catterall, T.W. Jones, P.B. Kaziewicz, J.B. Lane, R.L. Saunders, J. Shulman
University College London, Physics and Astronomy Dept., London. UK54
K. Blankenship, B. Lu, L.W. MO
Virginia Polytechnic Inst. and State University, Physics Dept., Blacksburg, VA, USAs
W. Bogusz, K. Charchula, J. Ciborowski, J. Gajewski, G. Grzelak36, M. Kasprzak,
M. Krzyianowski, K. Muchorowski ,
37 R.J. Nowak, J.M. Pawlak, T. Tymieniecka,
A.K. Wroblewski, J.A. Zakrzewski, A.F. Zarnecki
Warsaw University, Institute of Experimental Physics, Warsaw, Poland4’
M. Adamus
Institute for Nuclear Studies, Warsaw, Poland 4g
ZEUS Collaboration/Physics Letters B 369 (1996) 55-68
Y. Eisenberg 34, U. Karshon 34, D. Revel 34, D. Zer-Zion
Weiznmn Institute, Particle Physics Dept., Rehovot, Israe143
59
I. Ali, W.F. Badgett, B. Behrens , .
38 S Dasu, C. Fordham, C. Foudas, A. Goussiou39,
R.J. Loveless, D.D. Reeder, S. Silverstein, W.H. Smith, A. Vaiciulis, M. Wodarczyk
University of Wisconsin, Dept. of Physics, Madison, WI, USA 55
T. Tsurugai
Meiji Gakuin University, Faculty of General Education, Yokohama, Japan
S. Bhadra, M.L. Cardy, C.-P. Fagerstroem, W.R. Frisken, KM. Furutani, M. Khakzad,
W.N. Murray, W.B. Schmidke
York University, Dept. of Physics, North York, Ont., Canadam
Received 1 November 1995
Editor: K. winter
Abstract
Photoproduction events which have two or more jets have been studied in the W,, range 135 GeV < W,, < 280 GeV
with the ZEUS detector at HERA. A class of events is observed with little hadronic activity between the jets. The jets are
separated by pseudorapidity intervals (Ar]) of up to four units and have transverse energies greater than 6 GeV. A gap is
defined as the absence between the jets of particles with transverse energy greater than 300 MeV. The fraction of events
containing a gap is measured as a function of AT. It decreases exponentially as expected for processes in which colour is
exchanged between the jets, up to a value of AT - 3, then reaches a constant value of about 0.1. The excess above the
exponential fall-off can be interpreted as evidence for hard diffractive scattering via a strongly interacting colour singlet
object.
’ Also at IROE Florence, Italy.
* Now at Univ. of Salerno and INFN Napoli, Italy.
3 Supported by EU HCM contract ERB-CHRX-CT93-0376.
4 Supported by Worldlab, Lausanne, Switzerland.
5 Now a self-employed consultant.
6 Now at ELEKLUFT, Bonn.
’ Now also at University of Torino.
* Alexander von Humboldt Fellow.
g Alfred P. Sloan Foundation Fellow.
lo Now at Inst. of Computer Science, Jagellonian Univ., Cracow.
l1 Now at Comma-Soft, Bonn.
‘* Visitor from Florida State University.
I3 Now at Univ. of Mainz.
I4 Now at DESY Computer Center.
I5 Supported by European Community Program PRAXIS XXI.
l6 Now at Dr. Seidel Informationssysteme, Frankfurt/M.
” Now at Inst. of Accelerating Systems & Applications (IASA),
Athens.
l8 Now at Mercer Management Consulting, Munich.
lg Now at Univ. de Strasbourg.
” Now at Andrews University, Barrien Springs, USA.
*’ Now with OPAL Collaboration, Faculty of Physics at Univ. of
Freiburg.
** Now at SAS-Institut GmbH, Heidelberg.
u Now at GSI Darmstadt.
24 Also supported by NSERC.
Z Now at Institute for Cosmic Ray Research, University of Tokyo.
26 Partially supported by CAM.
” Partially supported by DESY.
28 Now at Philips Natlab, Eindhoven, NL.
2g Now at Carleton University, Ottawa, Canada.
3o Now at Department of Energy, Washington.
31 Now at HEP Div., Argonne National Lab., Argonne, IL, USA.
32 Now at Oxford Magnet Technology, Eynsham, Oxon.
33 In part supported by Argonne National Laboratory.
34 Supported by a MINERVA Fellowship.
35 Now at Centre for Subatomic Research, Univ.of Alberta, Canada
and TRIUMF, Vancouver, Canada.
36 Supported by the Polish State Committee for Scientific Re-
search, grant No. 2P03B09308.
37 Supported by the Polish State Committee for Scientific Re-
search, grant No. 2P03B09208.
38 Now at University of Colorado, USA.
3g Now at High Energy Group of State University of New York,
60 ZEUS Collaboration/Physics Letters B 369 (1996) 55-68
1. Introduction
In high energy hadronic collisions, the dominant
mechanism for jet production is described by a hard
scatter between partons in the incoming hadrons via
a quark or gluon propagator. This propagator carries
colour charge. Since colour confinement requires that
the final state contain only colour singlet objects, the
exchange of colour quantum numbers in the hard pro-
cess means that a jet at some later stage generally
exchanges colour with another jet ‘or beam remnant
widely separated from it in rapidity. Such jets are said
to be “colour connected” and this leads to the pro-
duction of particles throughout the rapidity region be-
tween the jets. However, if the hard scattering were
mediated by the exchange of a colour singlet propa-
Stony Brook, NY.
4o Supported by the Natural Sciences and Engineering Research
Council of Canada (NSERC)
41 Supported by the FCAR of Quebec, Canada.
42 Supported by the German Federal Ministry for Education and
Science, Research and Technology (BMBF) , under contract num-
bers 056BN191, 056FR19R 056HH191, 056HH291,056SI791.
43 Supported by the MINERVA Gesellschaft fur Forschung GmbH,
and by the Israel Academy of Science.
44 Supported by the German Israeli Foundation, and by the Israel
Academy of Science.
45 Supported by the Italian National Institute for Nuclear Physics
(INFN).
46 Supported by the Japanese Ministry of Education, Science and
Culture (the Monbusho) and its grants for Scientific Research.
47 Supported by the Korean Ministry of Education and Korea
Science and Engineering Foundation.
48 Supported by the Netherlands Foundation for Research on Mat-
ter (FOM).
4g Supported by the Polish State Committee for Scientific Re-
search, grants No. 115/E_343/SPUB/PO3/109/95, 2PO3B 244
08~02, ~03, pO4 and ~05, and the Foundation for Polish-German
Collaboration (proj. No. 506/92).
5o Supported by the Polish State Committee for Scientific Research
(grant No. 2 PO3B 083 OS).
5* Partially supported by the German Federal Ministry for Educa-
tion and Science, Research and Technology (BMBF)
52 Supported by the German Federal Ministry for Education and
Science, Research and Technology (BMBF), and the Fund of Fun-
damental Research of Russian Ministry of Science and Education
and by INTAS-Grant No. 93-63.
53 Supported by the Spanish Ministry of Education and Science
through funds provided by CICYT.
54 Supported by the Particle Physics and Astronomy Research
Council.
55 Supported by the US Department of Energy.
56 Supported by the US National Science Foundation.
gator in the t-channel, each jet would be colour con-
nected only to the beam remnant closest in rapidity
and the rapidity region between the jets would con-
tain few final-state particles [ 11. The colour singlet
propagator could be an electroweak gauge boson or a
strongly interacting object, and the soft gluon emission
pattern produced in each case is similar [ 21. However
the rates could be very different. In order to determine
the rate of colour singlet exchange processes it has
been proposed [ 31 to study the multiplicity distribu-
tion in pseudorapidity (7) and azimuth (9) of the
final state particles in dijet events, and to count events
with an absence of particles (i.e. with a rapidity gap)
between the two jets.
DO [ 41 and CDF [ 51 have reported the results of
searches in pp collisions at fi = 1.8 TeV for di-
jet events containing a rapidity gap between the two
highest transverse energy ( EFt) jets. Both collabora-
tions see an excess of gap events over the expectations
from colour exchange processes. DO report an excess
of 0.0107 f O.OOlO( stat) $$i:“,( sys.), whereas CDF
measure the fraction to be 0.0086 f0.0012. We report
here the results of a similar search in yp interactions
obtained from e+p collisions at HERA.
In leading order, two processes are responsible for
jet production in yp interactions at HERA. In the first
case, the direct contribution, the photon interacts di-
rectly with a parton in the proton. In the second case,
the resolved contribution, the photon first fluctuates
into a hadronic state which acts as a source of partons
which then scatter off partons in the proton. Fig. la
shows schematically an example of colour singlet ex-
change in resolved photoproduction in which a par-
ton in the photon scatters from a parton in the proton,
via t-channel exchange of a colour singlet object. An
example of the more common colour non-singlet ex-
change mechanism is shown in Fig. lb. For high EFt
dijet production, the magnitude of the square of the
four-momentum ( 1 t 1) transferred by the colour singlet
object is large. Thus it is possible to calculate in per-
turbative QCD the cross section for the exchange of
a strongly interacting colour singlet object [3,6-81.
For instance, the ratio of the two-gluon colour singlet
exchange cross section to the single gluon exchange
s7 7 = - ln( tan $ ) where 4 is the polar angle with respect to the
z axis, which in the ZEUS coordinate system is defined to be the
proton direction.
ZEUS Collaboration/Physics Letters B 369 (1996) 55-68 61
(a) (b)
-3
(cl
Fig. 1. Resolved photoproduction via (a) colour singlet exchange
and (b) colour non-singlet exchange. The rapidity gap event mor-
phology is shown in (c) where black dots represent final slate
hadrons and the boundary illustrates the limit of the ZEUS accep-
tance. Two jets of radius R are shown, which are back to back
in azimuth and separated by a pseudorapidity interval Av. An ex-
pectation for the behaviour of the gap-fraction is shown in (d)
(solid line). The non-singlet contribution is shown as the dotted
line and the colour singlet contribution as the dashed line.
cross section has been estimated to be about 0.1 [ 31.
Studies of rapidity gaps at high 1 t/ (“hard diffractive
scattering”) are complementary to studies of diffrac-
tive hard scattering where the rapidity gap is between
a colourless beam remnant, produced with low four-
momentum transfer with respect to one of the beam
particles, and hadronic activity in the central detec-
tor [9].
The event morphology for the process of Fig. la is
illustrated in Fig. lc. There are two jets in the final
state, shown as circles in (v, 50) space. Here A~,J is
defined as the distance in 7 between the centres of the
two jet cones. For the colour singlet exchange process
of Fig. la, radiation into the region (labelled “gap”)
between the jet cones is suppressed, giving rise to
the rapidity gap signature. Multiplicity fluctuations in
colour non-singlet exchange events can also produce
gaps between jets. In order to disentangle the different
mechanisms for gap production it is useful to introduce
the concept of the “gap-fraction”.
The gap-fraction, f( AT), is defined as the ratio of
the number of dijet events at this AT which have a ra-
pidity gap between the jets to the total number of dijet
events at this A?. For colour non-singlet exchange, the
gap-fraction is expected to fall exponentially with in-
creasing Aq. This exponential behaviour can be taken
as a definition of non-diffractive processes [ 31. The
expectation follows from the assumption that the prob-
ability density for radiation of a particle is constant
across the rapidity interval between the jets and it is
consistent with the results of analytic QCD calcula-
tions [ 71, and with Monte Carlo simulation (see sub-
sequent sections). For colour singlet exchange, the
gap-fraction is not expected to depend strongly upon
Av [3,7]. Therefore, at sufficiently large A??, such
a colour singlet contribution will dominate the be-
haviour of the gap-fraction. The situation is illustrated
in Fig. Id, where the colour non-singlet contribution
is shown as an exponential fall-off, and the colour sin-
glet contribution is shown as independent of AT.
In this paper the gap-fraction is studied for a sam-
ple of photoproduction events with two jets of J!$? >
6 GeV. The events are obtained from an integrated lu-
minosity of 2.6 pb-’ of efp collisions measured by
the ZEUS detector and have yp centre-of-mass ener-
gies in the range 135 GeV < W,, < 280 GeV. Dijet
cross sections are measured as a function of Ar] for
events with a gap and for events with no gap require-
ment.
2. Experimental setup
Details of the ZEUS detector have been described
elsewhere [ lo]. The primary components used in this
analysis are the central calorimeter and the central
tracking detectors. The uranium-scintillator calorime-
ter [ 1 l] covers about 99.7% of the total solid angle
and is subdivided into electromagnetic and hadronic
sections with respective typical cell sizes, of 5 x 20 cm2
(10 x 20 cm* in the rear calorimeter, i.e. the positron
direction) and 20 x 20 cm2. The central tracking sys-
tem consists of a vertex detector [ 121 and a central
tracking chamber [ 13 ] enclosed in a 1.43 T solenoidal
magnetic field.
A photon lead-scintillator calorimeter is used
to measure the luminosity via the positron-proton
Bremsstrahlung process. This calorimeter is installed
inside the HERA tunnel and subtends a small angle in
the positron beam direction from the interaction ver-
62 ZEUS Collaboration/Physics Letters B 369 (1996) 55-68
tex [ 141. Low angle scattered positrons are detected
in a similar lead-scintillator calorimeter.
In 1994 HERA provided 820 GeV protons and
27.5 GeV positrons colliding in 153 bunches. Addi-
tional unpaired positron and proton bunches circulated
to allow monitoring of background from beam-gas
interactions.
3. Data selection
The ZEUS data acquisition uses a three level trig-
ger system. At the first level events are selected
which were triggered on a coincidence of a regional
or transverse energy sum in the calorimeter with a
track coming from the interaction region measured
in the central tracking chamber. At the second level
a cut was made on the total transverse energy, and
cuts on calorimeter energies and timing were used to
suppress events caused by interactions between the
proton beam and residual gas in the beam pipe [ 151.
At the third level, tracking cuts were made to reject
events arising from proton beam-gas interactions and
cosmic ray events. Also at the third level, jets were
found from the calorimeter cell energies and positions
using a fast cone algorithm and events were required
to have at least two jets.
Charged current events are rejected by a cut on
the missing transverse momentum measured in the
calorimeter. Events with a scattered positron candidate
in the calorimeter are rejected. This restricts the range
of the photon virtuality to P2 < 4 GeV2, and results
in amedian P2 of ~10~~ GeV2. Acut of 0.15 5 y <
0.7 is applied on the fraction of the positron’s momen-
tum which is carried by the photon, where y is recon-
structed using the Jacquet-Blonde1 method [ 161. This
cut restricts the yp centre-of-mass energies to lie in
the range 135 GeV < W,, < 280 GeV.
To select the final jet sample, a cone algorithm [ 171
is applied to the calorimeter cells. Cells within a ra-
dius R = ,/m vcel, + tpcell of 1.0 from the jet centre
are included in the jet where Svce” amd sp” repre-
sent respectively the difference in pseudorapidity and
azimuthal angle (in radians) between the centre of
the cell and the jet axis. Events are then required to
have at, least two jets found in the uranium calorimeter
with EFk > 5 GeV and vjet < 2.5. In addition the two
highest transverse energy jets 58 were required to have
AT > 2 (i.e. cones not overlapping in v) and boost
I(71 + ~2) l/2 = 171 < 0.75. These conditions con-
strain the jets to lie within the kinematic region where
the detector and event simulations are best understood.
To identify gap events, the particle multiplicity is
determined by grouping calorimeter cells into ‘“is-
lands”. This is done by assigning to every cell a
pointer to its highest energy neighbour. A cell which
has no highest energy neighbour is a local maximum.
An island is formed for each local maximum which
includes all of the cells that point to it. The events with
IZO islands of transverse energy E$land > 250 MeV
and 7 between the edges of the jet cones (as defined
by the cone radius R) are called gap events.
A total of 8393 dijet events were selected, of which
3186 are gap events. The non-e+p collision back-
ground was estimated using the number of events as-
sociated with unpaired bunch crossings. The beam
gas background was found to be less than 0.1%. The
cosmic ray contamination is estimated to be about
0.1%. For those events in which the low angle scat-
tered positron is detected in lead-scintillator calorime-
ter, P2 < 0.02 GeV2. The fraction of these events is
around 20%, in agreement with the Monte Carlo ex-
pectation. The 43 gap events which have Aq > 3.5
were also scanned visually to search for contamina-
tion from events where the energy deposits of the scat-
tered positron or a prompt photon might mimic a jet.
No such events were found.
4. Results
In Section 4.1 we present results obtained from
ZEUS data which are not corrected for detector ef-
fects, by comparing the data to Monte Carlo gener-
ated events which have been passed through a detailed
simulation of the ZEUS detector and selection criteria.
The PYTHIA [ 181 Monte Carlo program was used
with the minimumpT of the hard scatter set to 2.5 GeV.
The GRV [ 191 parton distributions were used for the
photon and the MRSA [ 201 parton distributions were
58 In [7] the jets are ordered in pseudorapidity rather than trans-
verse energy and the two jets at lowest and highest pseudorapidity
are used in the calculation. When the uncorrected gap-fraction is
made with this selection, it is about 0.01 lower.
ZEUS Collaboration/Physics Letters B 369 (1996) 55-68 63
used for the proton. Two Monte Carlo event sam-
ples were generated. For the first sample (“PYTHIA
non-singlet”), resolved and direct photon interactions
were generated separately and combined according to
the cross sections determined by PYTHIA. No elec-
troweak exchange (quark quark scattering via y/Z0
or W* exchange) events were included. For the sec-
ond sample (“PYTHIA mixed”), 10% of electroweak
exchange events were included. This fraction is two
orders of magnitude higher than the level actually ex-
pected from the cross section for these events and is
chosen in order to mimic the effect of strongly in-
teracting colour singlet exchange processes which are
not included in PYTHIA.
In Section 4.2 we present the ZEUS data after cor-
rections for all detector acceptance and resolution ef-
fects. These hadron-level measurements are then com-
pared to model predictions, and to the expectation of
an exponential suppression of gap production for non-
diffractive processes.
4. I. Uncorrected results
The energy flow l/NdE~“/d2i~ce11 with respect to
the jet axis for cells within one radian in 40 of the jet
axis is shown in Fig. 2a for the two highest transverse
energy jets of each event. PYTHIA mixed events are
shown as the solid line. Here and throughout Fig. 2
the data are shown as black dots and the errors shown
are statistical only. This jet profile shows highly colli-
mated jets in the data and a pedestal of less than 1 GeV
of transverse energy per unit pseudorapidity outside
the jet cone radius of 1 .O. The pedestal transverse en-
ergy is higher toward the proton direction. The super-
position of profiles of one jet at high 77j”t and one at
low vjet leads to the bump at 6r]ce11 N 1.5, due to the
forward edge of the calorimeter. The profiles for the
PYTHIA non-singlet sample are not shown, as they
are similar to those of the mixed sample. The PYTHIA
events generally describe the data well, although they
are slightly more collimated and underestimate the
forward jet pedestal. This small discrepancy may be
related to secondary interactions between the photon
remnant and the proton remnant, which are not sim-
ulated in these PYTHIA samples. Including any kind
of multiple interactions in the simulation increases the
energy flow and particle multiplicity [21] and thus
can only decrease the number of gaps predicted by the
Monte Carlo program.
The distribution of the total number of events (with-
out any demand on the presence or absence of a gap)
as a function of A7 is shown in Fig. 2b. It decreases
with increasing Av, extending out to AT N 4. The
PYTHIA distributions are normalized to the number
of events in the data. Both PYTHIA samples provide
an adequate description of this distribution although
the total number of events seen at large Ar] is slightly
underestimated.
The distribution of the gap events as a function of
A7 is plotted in Fig. 2c where the normalisation for
the PYTHIA distribution is the same as in Fig. 2b.
The number of events in the data exhibiting a gap falls
steeply with AT. However the expectation from the
PYTHIA non-singlet sample falls more steeply than
the data, significantly underestimating the number of
gap events at large AT. The PYTHIA sample with a
mixture of 10% of electroweak boson exchange can
account for the number of gap events in the data at
large AT. However this sample significantly overesti-
mates the number of gap events at low AT. As men-
tioned previously, including secondary interactions in
the simulation could reduce the predicted number of
gap events and possibly account for this discrepancy.
By taking the ratio of Fig. 2c to Fig. 2b, the gap-
fraction shown in Fig. 2d is obtained. The gap-fraction
falls exponentially out to AT N 3.2. Thereafter it
levels off at a value of roughly 0.08. The PYTHIA
non-singlet sample fails to describe the flat region in
the data, falling approximately exponentially over the
whole measured range of AT. This sample also over-
estimates the fraction of gap events at low Av. The
PYTHIA mixed sample can describe the flat region
of the data but again overestimates the gap-fraction
at low AQ. The gap-fraction for the electroweak ex-
change events alone exceeds 0.4 over the full Av range
(not shown).
The uncorrected data exhibit a flat region at large
AT consistent with a colour singlet contribution of
around 10%. Detector effects are expected to largely
cancel in the gap-fraction. In the next section we find
that this is indeed the case and provide quantitative
estimates of both the discrepancy between PYTHIA
and the data and of the significance of the deviation of
the measured gap-fraction from an exponential fall.
64 ZEUS Collaboration/Physics Letters B 369 (1996) 55-68
ZEUS 1994
(a) (b>
10-21, , , / , , , , , , , , , , , , , / /
Fig. 2. Uncorrected data compared with the predictions from PYTHIA events which have been passed through a detailed simulation of
the ZEUS detector and of the sample selection criteria. The errors shown are statistical only. The transverse energy flow with respect to
the jet axis is shown in (a) where the data are shown as black dots and the PYTHIA non-singlet sample is shown as a solid line. In (b),
(c) and (d) the data are again shown as black dots. The PYTHIA non-singlet sample is shown as open circles and the PYTHIA mixed
sample (which contains 10% of colour singlet exchangeevents) is shown as stars. The number of events versus An is shown in (b). The
number of gap events versus AT is shown in (c) and the gap-fraction is shown in (d). In (d) the points are drawn at the mean AT of
the inclusive distribution in the corresponding bin.
4.2. Corrected results
In order to investigate whether the observed flat
region in the gap-fraction might be a detector effect,
the PYTHIA mixed sample has been used to correct
the data for all detector effects, including acceptance,
smearing and the shift in the measurement of energies.
Cross sections are determined and the gap-fraction is
measured in four bins of AT in the range 2 < AT < 4.
The cross section da/dAv is measured for dijet
photoproduction, ep -+ eyp + eX, where X con-
tains at least two jets of final state particles. The cross
section is measured in the range 0.2 < y < 0.85 for
photon virtualities P2 < 4 GeV2. The two jets are de-
fined by a cone algorithm with a cone radius of 1 .O in
(v, q) and satisfy EFt > 6 GeV, p < 2.5. The two
jets of highest Ep satisfy A7 > 2 and Ifll < 0.75. The
rear vlet distribution falls to zero at fl N -2, well
within the rear detector acceptance. Therefore no ex-
plicit rear pseudorapidity cut is made. The gap cross
section, dagap/dAv, is measured, in the same kine-
matic range, for events with no final state particles with
transverse energy EFzhcle > 300 MeV between the jet
cones. The corrected gap-fraction f(Aq) is then ob-
ZEUS Collaboration/Physics Letters B 369 (1996) 55-68 65
mined from the ratio of da,,/dAq to du/dAT.
The efficiency of the data selection described in Sec-
tion 3 for finding events in this kinematic region was
estimated using the Monte Carlo samples. The com-
bined efficiency of the online triggers is at least 80%
in every bin of AT. The efficiency of the offline selec-
tion is about 50% leading to a combined efficiency of
the online and offline selection criteria of about 40%.
The low efficiency of the offline selection is due to the
finite detector resolution of the jet energy and angular
variables, and the steeply falling EFt spectrum. The
shifts and resolutions of these variables are consistent
with those obtained in extensive studies of the 1993
dijet sample [ 221. The Efarticle resolution is 27% with
a shift of -14%. The $articre resolution is 0.01 with
negligible shift.
The final correction factors for the inclusive cross
section are smoothly varying between 1.6 in the low-
est AT bin and 1.4 in the highest AT bin. The cor-
rection factors for the gap cross section are between
1.5 and 1.8. The ratios of these correction factors form
effective correction factors for the gap-fraction which
are all within 27% of unity.
The systematic uncertainties have been estimated
by varying the cuts made on the reconstructed quan-
tities. The island algorithm for counting particles was
replaced by an algorithm which clusters cells based
on proximity in (7,~) space. Also the results of the
bin-by-bin correction were checked using an unfolding
procedure [ 231. The correction was also performed by
using the PYTHIA non-singlet sample and by leaving
out the leading order direct contribution. The uncer-
tainty due to the parton distribution was included. The
uncertainty due to a 3.3% systematic error in the lu-
minosity measurement was included. Finally the sys-
tematic uncertainty arising from a 5% uncertainty in
the mean energies measured by the calorimeter was
estimated. This represents the largest uncertainty in
the two cross sections but cancels in the gap-fraction.
The largest systematic uncertainty which remains in
the gap-fraction comes from the variation of the Z$tand
cut from 200 to 300 MeV. The combined effect of
these uncertainties is included in the outer error bars
in Fig. 3.
The inclusive and gap cross sections and the cor-
rected gap-fraction as a function of AT are presented
in Figs. 3a to c respectively (black dots) and com-
pared with the expectations from the PYTHIA non-
Table 1
du/dAq for ep -+ eyp -+ eX in the kinematic range 0.2 < .v <
0.g and P* < 4 GeV* and where X contains two or more jets of
EFL > 6 GeV, $* < 2.5, 141 < 0.75 and A77 > 2.
A71 da/dAv
(nb)
Statistical
uncertainty (nb)
Systematic
uncertainty (nb)
2.25 4.93 0.24 +0.83
-0.68
2.75 3.06 0.15 +0.54
-0.52
3.25 1.67 0.07 +0.31
-0.19
3.75 0.54 0.03 +0.08
-0.03
Table 2
d&P/dA~ for ep -+ eyp ---t eX in the kinematic range 0.2 <
y < 0.8 ,and P* < 4 GeV* and where X contains two or more
jets of IIF’ > 6 GeV, @r < 2.5, 111 < 0.75 and AT > 2 with no
final state particles of Epruc’e > 300 MeV between the jets.
du@P jdAq
(nb)
Statistical
uncertainty (nb)
Systematic
uncertainty (nb)
2.25 2.85 0.17 +0.45
-0.45
2.75 0.66 0.06 +0.11
-0.15
3.25 0.16 0.02 +0.03
-0.04
3.75 0.06 0.01 to.01
-0.01
singlet exchange cross sections (open circles). For the
data, the inner error bars show the statistical errors
and the outer error bars display the systematic uncer-
tainties, added in quadrature. The cross section points
are plotted at the centres of the bins. The gap-fraction
points are plotted at the mean AT values of the inclu-
sive cross section. Numerical values for the inclusive
cross section, the gap cross section and the corrected
gap-fraction are provided in Tables 1,2 and 3, respec-
tively.
The inclusive cross section is around 5 nb per unit
AT at AT = 2, falling to about 0.5 nb for AT > 3.5. The
gap cross section is around 3 nb per unit AT at AT = 2
and falls to about 0.06 nb for AT between 3.5 and
4. The overall normalization of PYTHIA agrees with
the data within the errors. PYTHIA also describes the
shape of the inclusive cross section. However it fails
to describe the gap cross section, falling too steeply
with AT and disagreeing significantly in the last bin.
The corrected gap-fraction falls exponentially in the
first three bins but the height of the fourth bin is con-
sistent with the height of the third. The height of the
66 ZEUS Collaboration/Physics Letters B 369 (1996) S-68
ZEUS 1994
77 Fro 7
i
__ 2 2.5 3 3.5 4 2 2.5 3 3.5 4
(a) A?7 0) A77
-2
10 , , , , , , , , , / , , , , /‘,(/ ,
'.. 1
2 2.5 3 3.5 4 2 2.5 3 3.5 4
Cc) Arl (4 AT
Fig. 3. ZEUS data (black circles) corrected for detector effects. The inner error bars represent the statistical errors from the data and
Monte Carlo samples, and the outer error bars include the systematic uncertainty, added in quadrature. In (a), (b) and (c) the PYTHIA
prediction for non-singlet exchange events is shown as open circles. The inclusive cross section is shown in (a). The cross section for gap
events is shown in (b) and the gap-fraction is shown in (c) . The gap-fraction is redisplayed in (d) and compared with the result of a fit to
an exponential plus a constant. In (c) and (d) the points are drawn at the mean Ar] of the inclusive distribution in the corresponding bin.
Table 3
The gap-fraction, f( Av), for ep - eyp -+ eX in the kinematic
range 0.2 < y < 0.8 and P* < 4 GeVZ and where X contains two
or more jets of EFt > 6 GeV, yjet < 2.5, IfI < 0.75 and Ar) > 2.
also consistent with the flat region at large AT seen in
the uncorrected gap-fraction and inconsistent with the
expectation from PYTHIA.
f(b) statistical
uncertainty
Systematic
uncertainty 5. Discussion
2.23 0.58 0.04
2.73 0.22 0.02
3.22 0.10 0.01
3.70 0.11 0.02
+o.o4
-0.02
+0.02
-0.02
+0.01
-0.02
+0.01
-0.02
Two methods have been used to estimate the signif-
icance of the excess of the gap-fraction over the ex-
pectation from multiplicity fluctuations in non-singlet
exchange.
fourth bin is 0.11 k 0.02( stat.):t,z\( sys.), which is
The first method is to take the difference between
the data and the PYTHIA non-singlet gap-fractions,
shown in Fig. 3c. An excess of 0.07 f 0.03 is ob-
ZEUS Collaboration/Physics Letters B 369 (1996) 55-68 67
mined, based entirely on the Iast bin. However this
is a model-dependent estimate. For instance replac-
ing the Lund symmetric fragmentation function by
the Field-Feynman fragmentation function yields a
lower predicted gap-fraction and a larger excess. Intro-
ducing multiple interactions into PYTHIA also low-
ers the fraction of gap events expected. On the other
hand, lowering car (which controls the hadron mo-
mentum distribution transverse to the parent parton)
in the Monte Carlo simulation from the default value
of 0.36 GeV to 0.25 GeV produces a gap-fraction that
is very like the data. It has a height in the fourth bin of
0.07 f 0.02 and therefore if one believes this model,
there is no significant excess. However this option
yields jet profiles which are narrower than the default
PYTHIA profiles, which are already slightly narrower
than the data.
The second way to estimate the excess of the gap-
fraction over that expected from purely non-singlet
exchange does not rely on comparisons to Monte
Carlo predictions. In Fig. 3d the gap-fraction is shown
again and compared with the result of a two-parameter
(@) X*-fit to the expression
$t(~,RArl) =C(a,P)e”*?l +P
where C (a, /?) is the normalization coefficient con-
straining ffi’ ( CY, p; AT) to 1.0 at Aq = 2. The result
of this fit is shown as the solid curve in Fig 3d, and
the exponential (dotted line) and constant (dashed
line) terms are also shown. The quality of this fit,
as indicated by the x2 value of 1.2 for the two de-
grees of freedom is superior to that of a fit to an ex-
ponential alone which yields x2 = 9. The fit param-
eters are cr = -2.7 + 0.3(stat.) fO.l(sys.) and p =
0.07 f 0.02( stat.) :$,$( sys.) . The parameter p gives
an estimate of the gap-fraction for colour singlet pro-
cesses. This method uses the full information of the
four measured data points and is not dependent on
the details of the Monte Carlo fragmentation model.
However, the assumption that the colour singlet gap-
fraction is constant with Av is only one of many pos-
sibilities.
Both the comparison with the default PYTHIA non-
singlet prediction and the fit to an exponential form
give an excess of about 0.07 in the gap-fraction over
the expectation from colour non-singlet exchange.
The excess in the gap-fraction over the expectation
from non-singlet exchange may be interpreted as ev-
idence for the exchange of a colour singlet object. In
fact the fraction of events due to colour singlet ex-
change, f(Ar]), may be even higher than the mea-
sured excess. As previously mentioned, secondary in-
teractions of the photon and proton remnant jets could
fill in the gap. A survival probability, P, has been
defined [3] which represents the probability that a
sA=ondary interaction does not occur. Then f(Av) =
f(Av) . P. Estimates of the survival probability for
pp collisions at the Tevatron range from about 5%
to 30% [ 3,24,25]. The survival probability at HERA
could be higher due to the lower centre-of-mass en-
ergy, the fact that one remnant jet comes from a pho-
ton rather than a proton and the fact that the mean
fraction of the photon energy participating in the jet
production in these events is high59 . Therefore the
ZEUS result of N 0.07 and the DO and CDF results
of 0.0107 + O.OOlO(stat.)?$$$(sys.) and 0.0086 f
0.0012 could arise from the same underlying process.
The magnitude of the squared four-momentum
transfer across the rapidity gap as calculated from the
jets is large (It] > (EF’)*). Thus the colour singlet
exchange is unambiguously “hard’.
The PYTHIA generator predicts that the ratio of
the electroweak (aEW) to QCD (crQcD) exchange
cross sections in this kinematic range is aEW/aQCD <
7 . low4 (compatible with the estimation (a/a,)*).
Therefore quark quark scattering via y/Z0 and W*
exchange cannot explain the height of the flat re-
gion in the gap-fraction. On the other hand, using the
simple two-gluon model for pomeron exchange gives
p( Av) - 0.1 [ 31. Thus pomeron exchange could ac-
count for the data.
,In summary, dijet photoproduction events with
EF’ > 6 GeV contain an excess of events with a
rapidity gap between the two jets over the expec-
tations of colour exchange processes. This excess
is observed as a flat region in the gap-fraction at
large rapidity separation (Av= 3.7) at a level of
0.1 1 & 0.02( stat.) $$i( sys.). ft can be interpreted as
5g The average fraction of the photon energy participating in the
production of the two jets [22] is 0.7 for these events. Never-
theless, according to the PYTHL4 simulation the dominant con-
tribution in this kinematic regime is from leading order resolved
events.
68 ZEUS Collaboration/Physics Letters B 369 (1996) 55-68
evidence of hard diffractive scattering via a strongly [lo] ZEUS Collaboration, The ZEUS detector, Status Report
interacting colour singlet object. (1993).
Acknowledgements
We thank the DESY Directorate for their strong
support and encouragement and the HERA machine
group for providing colliding beams. We acknowledge
the assistance of the DESY computing and networking
staff. It is also a pleasure to thank V. Del Duca for
useful discussions.
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an ep facility for Europe, Hamburg, ed. U. Amaldi, (DESY
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