ELSEVIER

14 July 1994

Physics Letters B 332 (1994) 228-243

PHYSICS LETTERS B

Observation of jet production in deep inelastic scattering with a large rapidity gap at HERA

ZEUS Collaboration

M. Derrick a, D. Krakauer a, S. Magill a, B. Musgrave a, J. Repond a, j. Schlereth a, R. Stanek a, R.L. Talaga a, J. Thron ", F. Arzarello b, R. Ayad b,l, G. Bari b, M. Basile b, L. Bellagamba b,

D. Boscherini b, A. Bruni b, G. Bruni b, E Bruni b, G. Cara Romeo b, G. Castellini b,2, M. Chiarini b, L. Cifarelli b'3, F. Cindolo b, F. Ciralli b, A. Contin b, S. D'Auria b, C. Del Papa b,

F. Frasconi b, R Giusti b, G. Iacobucci b, G. Laurenti b, G. Levi b, G. Maccarrone b, A. Margotti b, T. Massam b, R. Nania b, C. Nemoz b, F. Palmonari b, G. Sartorelli b, R. Timellini b,

Y. Zamora Garcia b,1 , A. Zichichi b, A. Bargende c, j. Crittenden c, K. Desch c, B. Diekmann c, T. Doeker c, L. Feld c, A. Frey c, M. Geerts c, G. Geitz c, H. Hartmann c, D. Haun c, K. Heinloth c,

E. Hilger c, H.-R Jakob c, U.F. Katz c, S. Kramarczyk c.4, A. Mass c, S. Mengel c, j. Mollen c, E. Paul c, Ch. Rembser c, R. Schattevoy c, J.-L. Schneider c,5, D. Schramm c, J. Stamm c,

R. Wedemeyer c, S. Campbell-Robson d, A. Cassidy d, N. Dyce a, B. Foster d, S. George d, R. Gilmore d, G.R Heath d, H.F. Heath d, T.J. Llewellyn a, C.J.S. Morgado d, D.J.R Norman d,

J.A. O'Mara a, R.J. Tapper d, S.S. Wilson a, R. Yoshida d, R.R. Rau e, M. Arneodo f, M. Schioppa f, G. Susinno f, A. Bernstein g, A. Caldwell g, I. Gialas g, J.A. Parsons g, S. Ritz g, F. Sciulli g, EB. Straub g, L. Wai g, S. Yang g, R Borzemski h, j. Chwastowski h, A. Eskreys h, K. Piotrzkowski h, M. Zachara h, L. Zawiejski h, L. Adamczyk i, B. Bednarek', K. Eskreys ',

K. Jelefi ', D. Kisielewska l, T. Kowalski l, E. Rulikowska-Zar~bska l, L. Suszycki ', J. Zaj~c i, T. K~dzierskiJ, A. Kotafiski j, M. Przybyciefi j, L.A.T. Bauerdick k, U. Behrens k, J.K. Bienlein k,

S. B6ttcher k, C. Coldewey k, G. Drews k, M. Flasifiski k,6, I. Fleck k, D.J. Gilkinson k, R G6tflicher k, B. Gutjahr k, T. Haas k, L. Hagge k, W. Hain k, D. Hasell k, H. HeBling k,

H. Hultschig k, R Joos k, M. Kasemann k, R. Klanner k, W. Koch k, L. K6pke k, U. K6tz k, H. Kowalski k, W. Kr6ger k, j. Krtiger k,5, j. Labs k, A. Ladage k, B. L6hr k, M. L6we k, D. Ltike k,

J. Mainusch k, O. Maficzak k, J.S.T. Ng k, S. Nickel k, D. Notz k, K. Ohrenberg k, M. Rohde k, J. RoldLn k,7, U. Schneekloth k, j. Schroeder k, W. Schulz k, F. Selonke k, E. Stiliaris k,7, T. Tsurugai k, W. Vogel k.8, D. Westphal k, G. Wolf k, C. Youngman k, H.J. Grabosch e,

A. Leich e, A. Meyer e, C. Rethfeldt e, S. Schlenstedt e, G. Barbagli m, M. Nuti m, p. Pelfer m, G. Anzivino n, S. De Pasquale n, S. Qian n, L. Votano n, A. Bamberger °, A. Freidhof °,

T. Poser 0,9, S. S61dner-Rembold °, G. Theisen o, T. Trefzger °, N.H. Brook P, RJ. Bussey P,

0370-2693/94/$07.00 (~) 1994 Elsevier Science B.V All rights reserved SSD1037 0 - 2 6 9 3 ( 94 ) 0 0 6 7 8 - Z

ZEUS Collaboratton / Phystcs Letters B 332 (1994) 228-243 229

A.T. Doyle P, J.R. Forbes P, V.A. Jamieson P, C. Raine P, D.H. Saxon P, M. Stavrianakou P, A.S. Wilson P, A. Dannemann q, U. Holmq, D. Horstmann q, H. Kammerlocher q,9, B. Krebs q,10, T. Neumann q, R. S inkus q, K. Wick q, E. B adura r, B.D. B urow r, A. Ftirtjes r.ll, E. Lohrmann r,

J. Milewski r, M. Nakahata r,12, N. Pavel r, G. Poelz r, W. Schott r, J. Terron r,7, E Zetsche r, T.C. Bacon s, R. Beuselinck s, I. Butterworth s, E. Gallo s, V.L. Harris s, K.R. Long s,

D.B. Miller s, E Morawitz s, A. Prinias s, J.K. Sedgbeer s, A. Vorvolakos s, A. Whitfield s, T. Bienz t,13, H. Kreutzmann t,14, U. Mallik t, E. McCliment t, M. Roco t, M.Z. Wang t, E Cloth u,

D. Filges u, S.H. An v, S.M. Hong v, C.O. Kim v, T.Y. Kim v, S.W. Nam v, S.K. Park v, M.H. Suh v, S.H. Yon v, R. Imlay w, S. Kartik w, H.-J. Kim w, R.R. McNeil w, W. Metcalf w,

V.K. Nadendla w, F. Barreiro x,15, G. Cases x, R. Graciani x, J.M. Hem~ndez x, L. Herv~is x,16, L. Labarga x,16, j. del Peso x, j. Puga x, J.E de Troc6niz x.17, F. IkraiamY, J.K. Mayer y,18,

G.R. Smith Y, F. Corriveau z, D.S. Hanna z, j. Hartmann z, L.W. Hung z, J.N. Lim z, C.G. Matthews z, J.W. Mitchell z.19, EM. Patel z, L.E. Sinclair z, D.G. Stairs z, M. St.Laurent z,

R. Ullmann z, V. Bashkirov aa, B.A. Dolgoshein aa, A. Stifutkin aa, G.L. Bashindzhagyan ab, EE Ermolov ab, L.K. Gladilin ab, Y.A. Golubkov ab, V.D. Kobrin ab, V.A. Kuzmin ab, E.N. Kuznetsov ab, A.A. Savin ab, A.N. Solomin ab, A.G. Voronin ab, N.E Zotov ab

S. Bentvelsen ac, M. Botje ac, F. Chlebana ac, A. Dake ac, j. Engelen ac, p. de Jong ac,20, M. de Kamps ac, p. Kooijman ac, A. Kruse ac, V. O'Dell ac,21, A. Tenner ac, H. Tiecke ac, W. Verkerke ac, M. Vreeswijk ac, L. Wiggers ac, E. de Wolf a~, R. van Woudenberg ac,

D. Acosta ad, B. Bylsma ad, L.S. Durkin ad, K. Honscheid ad, C. Li ad, T.Y. Ling ad, K.W. McLean ad, W.N. Murray ad, I.H. Park ad, T.A. Romanowski ad.22, R. Seidlein ad, D.S. Bailey ae, G.A. Blair ac'23, A. Byrne ae, R.J. Cashmore ae, A.M. Cooper-Sarkar ae, D. Daniels ae.24, R.C.E. Devenish ae N. Harnew ae M. Lancaster ae, P.E. Luffman ae,25,

ae ae ae ae ae • ae ae J. McFall , C. Nath , A. Quadt , H. Uijterwaal , R. Walczak , E E Wilson , T. Yip , G. Abbiendi af A. Bertolin af, R. Brugnera af, R. Carlin af, F. Dal Corso af, M. De Giorgi af, U. Dosselli af, F. Gasparini af S. Limentani af, M. Morandin af, M. POSOCCO af, L. S tanco af,

R. Stroili af, C. Voci af, j. Bulmahn ag, J.M. Butterworth ag, R.G. Feild ag, B.Y. Oh ag, J.J. Whitmore ag,26, G. D'Agostini ah, M. Guida ah,27, M. Iori ah, S.M. Mari an, G. Marini ah,

M. Mattioli ah, A. Nigro ah, J.C. Hart a~, N.A. McCubbin a,, K. Prytz ai, T.P. Shah m, T.L. Short a~, E. Barberis aj, N. Cartiglia aj, C. Heusch aj, M. Van Hook aj, B. Hubbard aj, W. Lockman aj,

H.E-W. Sadrozinski aj, A. Selden aj, D. Zer-Zion aj, j. Biltzinger ak, R.J. Seifert ak, A.H. Walenta ak, G. Zech ak, H. Abramowicz ae, S. Dagan ae.28, A. Levy ae,28, T. Hasegawa am,

M. HazulTli am, T. Ishii am, M. Kuze am, S. Mine am, yo Nagasawa am, T. Nagira am, M. Nakao am, I. Suzuki am, K. Tokushuku am, S. Yamada am, Y. Yamazaki am, M. Chiba an, R. Hamatsu an,

T. Hirose an, K. Homma an, S. Kitamura an, S. Nagayama an, Y. Nakamitsu an, R. Cirio ao, M. Costa ao, M.I. Ferrero ao, L. Lamberti ao, S. Maselli ao, C. Peroni ao, R. Sacchi ao, A. Solano ao,

A. Staiano ao, M. Dardo ap, D.C. Bailey aq, D. Bandyopadhyay aq, E Benard aq, S. Bhadra aq,29, M. Brkic aq, M.B. Crombie aq, D.M. Gingrich aq,30, G.F. Hartner aq, G.M. Levman aq,

J.E Martin aq, R.S. Orr "q, C.R. Sampson aq, R.J. Teuscher aq, EW. Bullock ~r, C.D. Catterall ar, J.C. Giddings ar, T.W. Jones ar, A.M. Khan ar, J.B. Lane a~, P.L. Makkar at, D. Shaw at,

2 3 0 ZEUS Collaboranon / Physics Letters B 332 (1994) 228-243

J. Shulman ar, K. Blankenship as, j. Kochocki as, B. Lu as, L.W. Mo as, W. Bogusz at, K. Charchuta at, j. Ciborowski at, j. Gajewski at, G. Grzelak at, M. Kasprzak at,

M. Krzy~anowski at, K. Muchorowski at, R.J. Nowak at, J.M. Pawlak at, T. Tymieniecka at, A.K. Wr6blewski at, J.A. Zakrzewski at, A.E Zarnecki at, M. Adamus an, y. Eisenberg av.28,

C. Glasman av, U. Karshon av,28, D. Revel av,28, A. Shapira av, I. Ali aw, B. Behrens aw, S. Dasu aw, C. Fordham aw, C. Foudas aw, A. Goussiou aw, R.J. Loveless aw, D.D. Reeder aw, S. Silverstein aw,

W.H. Smith aw, W.R. Frisken a y , K.M. Furutani ay, y. igaay a Argonne Natwnal Laboratory Argonne, IL, USA 46

b Umverstty and INFN Bologna, Bologna, Italy 36

e Physikahsches Instltut der Untversttiit Bonn, Bonn, Germany 33 d H H Wills Phystcs Laboratory Umversity of Brtstol, Bristol, UK 45

e Brookhaven Natwnal Laboratory Upton, L.L, USA46 f Calabria Umversity, Physics Dept and INFN, Cosenza, Italy 36

g Columbia University, Nevts Labs. Irvmgton on Hudson, N Y, USA 47

h Inst. of Nuclear Physzcs, Cracow, Poland 4° Faculty of Physics and Nuclear Techmques, Academy of Mmmg and Metallurgy, Cracow, Poland 40

J Jagelloman Umv, Dept. of Phystcs, Cracow, Poland 41 k Deutsches Elektronen-Synchrotron DESY Hamburg, Germany DESY-Zeuthen, Inst fur Hochenergzephys~k, Zeuthen, Germany

m University and INFN, Florence, Italy 36 n INFN, Laboratori Naztonali dt Frascatl, Frascatt, Italy 36

o Fakultdtfiir Phystk der Untversit?lt Fretburg t Br., Freiburg t.Br, Germany 33 P Dept of Physics and Astronomy, Universtty of Glasgow, Glasgow, UK 45 q Hamburg Universtty, L Instttute ofExp Phystcs, Hamburg, Germany 33 r Hamburg University, H lnstttute of Exp. Phystcs, Hamburg, Germany 33

s Imperial College London, High Energy Nuclear Physics Group, London, UK 45 t Umverstty of Iowa Phystcs and Astronomy Dept., lowa Czty, USA 46 u Forschungszentrum Jithch, lnstitutfflr Kernphystk, Jtihch, Germany

v Korea Umverstty, Seoul, Korea 38 w Loulslana State University, Dept of Physws and Astronomy Baton Rouge, LA, USA 46

x Umver. Autdnoma MadrM, Depto de Ffstca Tedrfca, Madrsd, Spatn 44 Y Untvers~ty of Manttoba, Dept of Phystcs, Winntpeg, Manttoba, Canada 31

z McGtll Umverszty, Dept. of Physics, Montreal, Quebec, Canada 31,32 aa Moscow Engmeermg Physics lnsntute, Moscow, Russta 42

ab Moscow State Umverstty, Institute of Nuclear Pyszcs, Moscow, Russta 43 ae NIKHEF and Untversity of Amsterdam, Netherlands 39

ad Ohto State Umverstty, Phystcs Department Columbus, Ohto, USA 46 ae Department ofPhysws, Universtty of Oxford, Oxford, UK 45

af Dtpartlmento di Fistca dell' Umversita and INFN, Padova, Italy 36

ag Pennsylvanta State Unwerslty, Dept. of Phystcs Unwerstty Park, PA, USA 47 ah Dtparttmento dz Ftstca, Univ. "La Saplenza' and INFN, Rome, Italy 36

a~ Rutherford Appleton Laboratory, Chtlton, Dtdcot, Oxon, UK 45 aj Umverslty of Cahforma Santa Cruz, CA, USA 46

ak Fachberetch Phystk der Umversttilt-Gesamthochschule Stegen, Germany 33 at School of Phystcs, Tel-Awv Untverstty, Tel Avtv, Israel 35

am Instttute for Nuclear Study, University of Tokyo, Tokyo, Japan 37 an Tokyo Metropohtan Umverstty, Dept of Physics, Tokyo, Japan 37

ao Universtta dt Torino, Dtparttmento d~ Fistca Spertmentale and INFN, Tormo, Italy 36 ap H Faculty of Sciences, Tormo Umverstty and INFN - Alessandrta, Italy 36

aq Umverstty of Toronto, Dept of Phystcs, Toronto, Ont, Canada 3t ar Unwerstty College London, Physics and Astronomy Dept, London, UK 45

as Virgmta Polytechmc lnst and State Umverstty, Phystcs Dept. Blacksburg, VA, USA 47 at Warsaw Umverstty, Institute of Expertmental Phystcs, Warsaw, Poland 4°

ZEUS Collaboratmn / Physics Letters B 332 (1994) 228-243

au Institute for Nuclear Studtes, Warsaw, Poland 4° av Weizmann lnstttute, Nuclear Physics Dept, Rehovot, lsrae134

aw University of Wisconsin, Dept. of Physics Madzson, WI, USA 46 ay York Universtty, Dept of Physics, North York, Ont., Canada 31

Received 21 April 1994 Editor. K Winter

231

Abstract

Events with a large rapidity gap in deep inelastic scattering with Q2 > 10 GeV 2 have been studied in the ZEUS detector. The properties of these events with W > 140 GeV are consistent with a leading twist diffractive production mechanism. In the

~jot > 4 GeV, 15% of the events are of the 1-jet type with negligible 2-jet production. The single jet laboratory frame, with ~ r - is back-to-back in azimuth with the scattered electron. No energy flow is observed between the jet and the proton direction. With a lower jet transverse energy cut 2-jet production is observed both in the laboratory and the y*p centre-of-mass systems, demonstrating the presence of hard scattering in the virtual photon proton interactions that give rise to large rapidity gap events.

I Supported by Woddlab, Lausanne, Switzerland. 2 Also at IROE Florence, Italy 3 Now at Univ of Pisa, Italy 4 Now with BONN DATA, Bonn. 5 Now a self-employed consultant 6 On leave from Jageilonian University, Cracow. 7 Supported by the European Community. 8 Now at Blohm & Voss, Hamburg. 9 Now at DESY lO Now with Herfurth GmbH, Hamburg 11 Now at CERN 12 Now at Institute for Cosmic Ray Research, University of Tokyo 13 Now with Messrs. Adobe, Santa Clara, CA 14 Now with Messrs TLC GmbH, Wiesbaden 15 On leave of absence at DESY, supported by DGICYT 16 Partially supported by Comumdad Aut6noma de Madrid, Spain. 17 Supported by Fundaci6n Banco Exterior 18 Now at Unw of Toronto 19 Now at Unlv of California, Davis, CA 20 Now at MIT, Cambridge, MA 21 Now at Fermflab, Batavia, IL. 22 Now at Department of Energy, Washragton 23 Now at RHBNC, Unw of London, England. 24 Fulbright Scholar 1993-1994 25 Now at Cambndge Consultants, Cambridge, UK 26 On leave and supported by DESY 1993-94 27 Now at Dip di Flslca, Umv. di Salerno, Italy 28 Supported by a MINERVA Fellowship. 29 Now at York Univ. and DESY 30 Now at Centre for Subatomic Research, Univ of Alberta, Canada and TRIUMF, Vancouver, Canada. 31 Supported by the Natural Sciences and Engmeenng Research Council of Canada 32 Supported by the FCAR of Quebec, Canada 33 Supported by the German Federal Ministry for Research and

1. Introduction

In a recent publ ica t ion [ 1 ] , we repor ted on the ob-

servat ion o f events wi th a large rapidi ty gap in e lec t ron

pro ton deep inelast ic scat ter ing ( D I S ) . The i r general

Technology (BMFT). 34 Supported by the MINERVA Gesellschaft fur Forschung GmbH, and by the Israel Academy of Science 35 Supported by the Israel Ministry of Energy, and by the German Israeli Foundation 36 Supported by the Itahan National InsUtute for Nuclear Physics (INFN) 37 Supported by the Japanese Ministry of Education, Soence and Culture (the Monbusho) and its grants for Soentific Research. 38 Supported by the Korean Ministry of Education and Korea Sci- ence and Engineenng Foundation. 39 Supported by the Netherlands Foundation for Research on Matter (FOM) 4o Supported by the Pohsh State Committee for Scientafic Research (grant No. 204209101) 41 Supported by the Polish State Commtttee for Scientific Research (grant No PB 861/2/91 and No. 2 2372 9102, grant No. PB 2 2376 9102 and No. PB 2 0092 9101). 42 Partially supported by the German Federal Mlmstry for Research and Technology (BMFT). 43 Supported by the German Federal Ministry for Research and Technology (BMFr), the Volkswagen Foundataon, and the Deutsche Forschungsgemem schaft ,u Supported by the Spanish Mmmtry of Education and Sctence through funds provided by CICYT. 45 Supported by the Particle Physics and Astronomy Research Council 46 Supported by the US Department of Energy 47 Supported by the US Natmnal Science Foundatmn

232 ZEUS Collaborauon /Physws Letters B 332 (1994) 228-243

properties were found to be inconsistent with the dom- inant mechanism of DIS, where colour is transferred between the scattered quark and the proton remnant, and suggested that the underlying mechanism was of a diffractive nature and also leading twist. Preliminary results of a similar nature have been shown by the H1 Collaboration [2].

Diffractive processes are generally understood to proceed through the exchange of a colourless object with the quantum numbers of the vacuum called the pomeron. Because of the absence of colour flow diffrac- tive processes should exhibit a high proportion of ra- pidity gap events at sufficiently high energies. How- ever, the true nature of the pomeron is far from clear. Ingelman and Schlein [ 3 ], following earlier work of Low and Nussinov [4] and Donnachie and Landshoff [ 5 ], proposed that the pomeron behaves like a hadron and suggested that it may have a partonic structure which could be probed by a hard scattering process. The UA8 experiment later observed events with two high p~ jets in diffractive p p interactions [6]. These results could be explained in terms of a partonic struc- ture with a hard parton distribution in the pomeron.

In this paper we report the observation of jets in large rapidity gap events in DIS. The data presented here were collected in the 1993 HERA running period and constitute an increase in statistics by a factor 20 over our initial study [ 1 ].

2. Experimental setup

2.1. HERA machine conditions

The experiment was performed at the electron- proton collider HERA using the ZEUS detector. Dur- ing 1993 HERA operated with bunches of electrons of energy Ee = 26.7 GeV colliding with bunches of protons of energy Ep = 820 GeV, with a time between bunch crossings of 96 ns. HERA is designed to run with 210 bunches in each of the electron and proton rings. For the 1993 data taking 84 paired bunches were filled for each beam and in addition 10 electron and 6 proton bunches were left unpaired for background studies. The electron and proton beam currents were typically 10 mA.

2.2. The ZEUS detector

ZEUS is a multipurpose magnetic detector whose configuration for the 1992 running period has been described elsewhere [7,8]. Here we give a brief de- scription concentrating on those parts of the detector relevant for the present analysis and those which were different for the 1993 running.

Charged particles are tracked by the inner tracking detectors which operate in a magnetic field of 1.43 T provided by a thin superconducting coil. Immedi- ately surrounding the beampipe is the vertex detec- tor (VXD) which consists of 120 radial cells, each with 12 sense wires. It uses a slow drift velocity gas (dimethylether) [9] and the presently achieved res- olution is 50 /zm in the central region of a cell and 150/zm near the edges. Surrounding the VXD is the central tracking detector (CTD) which consists of 72 cylindrical drift chamber layers, organised into 9 'su- perlayers' [ 10]. With the present understanding of the chamber, a spatial resolution of ,-~ 280/zm has been achieved. In events with charged particle tracks, using the combined data from both chambers, resolutions of 0.6 cm in Z and 0.1 cm in radius in the XY plane 48 are obtained for the primary vertex reconstruction. From gaussian fits to the Z vertex distribution, the rms spread is found to be 10.5 cm in agreement with the expecta- tion from the proton bunch length.

The solenoid is surrounded by a high resolution uranium-scintillator calorimeter divided into three parts, forward (FCAL) covering the pseudorapidity 49 region 4.3 _> r/ > 1.1, barrel (BCAL) covering the central region 1.1 > ~/ > -0 .75 and rear (RCAL) covering the backward region -0 .75 > r/ > -3 .8 . Holes of 20 x 20 cm 2 in the center of FCAL and RCAL are required to accommodate the HERA beam pipe. The resulting solid angle coverage is 99.7% of 47r. The calorimeter parts are subdivided into towers which in turn are subdivided longitudinally into elec- tromagnetic (EMC) and hadronic (HAC) sections. The sections are subdivided into cells, each of which is

48 The ZEUS coordinate system is defined as right handed with the Z axis pointing m the proton beam direction, hereafter referred to as forward, and the X axis honzontal, pointing towards the centre of HERA. 49 Pseudorapidlty r/ is defined as -In(tan ~), where the polar angle 0 is taken with respect to the proton beam direction from the nominal interaction point

ZEUS Collaboration/Physics Letters B 332 (1994) 228-243 233

viewed by two photomultiplier tubes• The calorimeter is described in detail in Refs. [ 11-13]•

The C5 beam monitor, a small lead-scintillator counter, located at Z = - 3 . 2 m was used to detect upstream proton beam interactions and to measure the timing and longitudinal structure of the proton and electron bunches. The vetowall detector, consisting of two layers of scintillator on either side of an 87 cm thick iron wall centered at Z = -7 .3 m was also used to tag off-beam background particles.

For measuring the luminosity as well as for tagging very small Q2 processes, we use two lead-scintillator calorimeters [ 14]. Bremsstrahlung photons emerging from the electron-proton interaction point (IP) at an- gles 0~ < 0.5 mrad with respect to the electron beam axis hit the photon calorimeter at 107 m from the IP. Electrons emitted from the IP at scattering angles less than or equal to 6 mrad and with energies 0.2Ee < E~e < 0.9Ee are deflected by beam magnets and hit the electron calorimeter placed 35 m from the IP.

lected for output by the TLT if t~ _= E,E, ( 1 - cos 0,) > 20GeV - 2E~,, where E,, 0, are the energy and polar angle (with respect to the nominal IP) of calorime- ter cells and E r is the energy measured in the photon calorimeter of the luminosity monitor. For fully con- tained events 6 ~ 2Ee = 53.4 GeV. Events from photo- production processes peak at low values of 8, because most of the scattered electrons remain within the rear beam pipe.

3. Kinematics of deep inelastic scattering

The kinematic variables used to describe deep in- elastic scattering events

e (k) + p (P) ~ e (k ~) +anything (1)

are the following: the negative of the squared four mo- mentum transfer carried by the virtual photon 50 :

Q2 = _q2 = - ( k - k')2; (2)

2.3. Trigger condi t ions

Data were collected with a three level trigger [7]. The First Level Trigger (FLT) is built as a deadtime free pipeline• The FLT for DIS events required a logical OR of three conditions on sums of energy in the EMC calorimeter cells: either the BCAL EMC energy ex- ceeded 3.4 GeV; or the RCAL EMC energy, excluding the towers immediately adjacent to the beam-pipe, ex- ceeded 2.0 GeV; or the RCAL EMC energy, including the beam-pipe towers, exceeded 3.75 GeV. For events with the scattered electron detected in the calorime- ter, the FLT was essentially independent of the DIS hadronic final state. The FLT acceptance was greater than 97% for Q2 > 10 GeV 2.

The Second Level Trigger (SLT) used information from a subset of detector components to differentiate physics events from backgrounds. The SLT rejected proton beam-gas events according to the event times measured in the rear calorimeter thereby reducing the FLT DIS triggers by an order of magnitude, but without loss of DIS events•

The Third Level Trigger (TLT) had available the full event information on which to apply physics-based filters• The TLT applied stricter cuts on the event times and also rejected beam-halo muons and cosmic muons. Events remaining after the above veto cuts were se-

the Bjorken variable:

Q2 x = - - ; (3)

2P -q

the variable which describes the energy transfer to the hadronic final state:

q . P Y = k. P ' (4)

and W the center-of-mass energy of the y*p system, where:

W 2 = ( q + P ) 2 - Q 2 ( l _ x ) + M p 2 (5) x

with Mp the proton mass. These variables, only two of which are independent,

can be determined either from the scattered electron or from the hadronic system. The variable y, calculated from the electron variables, is given by the expression

Ye = 1 Ete 1 - cosOte (6) Ee 2

where Ere, 0re denote the energy and angle of the scat- tered electron. This relation is valid for colliding beams

50 In the Q2 range covered by this data sample, ep interactions are described to sufficient accuracy for this analysis by the exchange of a virtual photon

234

in the zero mass approximation. Alternatively, y can be determined from the hadronic system, using the Jacquet-Blondel technique [ 15]:

~,(E, -pz , ) (7) YJn = 2 . Ee

where Pz, = E, • cos O, and 0, is the polar angle deter- mined using the Z-coordinate of the reconstructed IP in an event. The sum runs over the calorimeter ceils associated with the hadronic system.

Studies of the HERA kinematics have shown that it is advantageous, for the present analysis, to use the so-called double angle (DA) method, in which the angles of the scattered electron and the hadron systems are used to determine x and Q2. Quantities determined in this way will be identified with the subscript DA. Formulae to calculate Q2DA, WDA, XDA and YDA are

given in [ 16]. The invariant mass of the hadronic system X detected

in the calorimeter, M x , can be determined from the calorimeter cell information as follows [ 1 ]. Denote the energy, momentum and polar angle of the final hadronic system to be En, p/4 and On respectively; and Pi as the vector constructed from the energy E,, angle O, and the corresponding azimuthal angle ~bt of cell i, then:

~tpz, (8~ c o s 0 H - I ~ , t ' i l

p2 H = a 2 a ( 1 -- YDA) (9) sin 2 0/4

E/4 = PH COS 0/4 -~- 2EeYDa (10)

from which M x is determined by the definition M x =

4. DIS data s d e e t i o n cr i ter ia

The off-line selection of DIS events was similar to that described in our earlier publications [ 1,17]. Scat- tered electron candidates were selected by using the pattern of energy deposition in the calorimeter. The electron energy was required to be more than 5 GeV. The electron finder algorithm used in this analysis was optimised to have high efficiency at low energies at a cost of somewhat lower purity compared to the one

ZEUS Collaboration/Physics Letters B 332 (1994) 228-243

used in our earlier study. The efficiency for finding iso- lated electrons in this energy range was greater than 97%. Furthermore we demanded: - Q2DA >_ 10 GeV2; - YJB >- 0 .04 , to give sufficient accuracy for DA re-

construction; - 6 >_ 35 GeV, to control radiative corrections and

photoproduction background; - Ye < 0.95, to reduce photoproduction background; - scattered electrons whose impact points (X, Y) in

the RCAL are inside a square of 32 x 32 cm 2 centered on the beam axis were rejected;

- a vertex, as reconstructed from VXD+CTD tracks, was required with I ZI ___ 30 cm and a radial distance from the beam line R < 10 cm.

A total of 38192 events was selected in this way corre- sponding to a total integrated luminosity of 0.55 pb - l . This sample was estimated to have a contamination from beam gas background of less than 0.5%, as de- termined from the number of events produced by un- paired electron and proton bunches. The background in the total DIS sample due to photoproduction was estimated to be about 16% from a fit to the shape of the 6 distribution before the above cut on 6 was applied [ 17]. An additional source of background is elastic QED Compton scattering. Seventy-eight such events were removed by a suitable algorithm, which finds two electromagnetic clusters in the calorimeter and one track in the CTD.

5. The M o n t e Car lo s i m u l a t i o n

The expected final states from DIS were modelled using two different sets of generators, the first one for the description of standard DIS processes and the second to model pomeron exchange reactions. We shall use the term pomeron exchange as a generic name to describe the process which is responsible for creating large rapidity gap events.

Events from standard DIS processes with first order electroweak corrections were generated with HERA- CLES [ 18]. This was interfaced using DJANGO [ 19] to ARIADNE 4.0 [20] for modelling the QCD cas- cade. The fragmentation into hadrons was performed with the Lund string hadronization model [21 ] as im- plemented in JETSET [22]. The proton parton den- sities were chosen to be the MRSD -t set [ 23 ] which

ZEUS Collaboratton / Physws Letters B 332 (1994) 228-243

represents closely our structure function results [ 17]. Note that these Monte Carlo codes do not contain ex- plicit contributions from diffractive y*p interactions.

In order to model the DIS hadronic final states with a large rapidity gap we have studied three Monte Carlo event samples, two of which were generated by POMPYT [24]. POMPYT is a Monte Carlo realisa- tion of factorisable models for high energy diffractive processes, where within the PYTHIA [ 25 ] framework, the beam proton emits a pomeron, whose constituents take part in a hard scattering process with the virtual photon or its constituents. The quark momentum den- sity in the pomeron for this analysis was taken to be either hard:

/3f(/3) = constant./3( 1 - / 3 ) (11)

or soft:

/3f(/3) = constant. (1 - /3 )5 , (12)

where/3 denotes the fraction of the pomeron momen- tum carried by the quark. Note that the hard quark density in POMPYT is the same as that proposed by Donnachie and Landshoff [26], up to a normalisation constant. The third sample was generated following the Nikolaev-Zakharov (NZ) model [27] which was interfaced to the Lund fragmentation scheme [28]. The NZ model, which is not factorisable, assumes that the exchanged virtual photon fluctuates into a qq pair which interacts with a colourless two-gluon system emitted by the incident proton. The resulting effective /3 distribution lies between the two parameterisations chosen above for POMPYT.

NZ events were produced within the following ranges of generator parameters: 0.0001 < x < 0.05; - 1 . < t < -0.0001 GeV 2 where t is the momentum transfer squared between the virtual photon and the proton and 3.0 < (M*) 2 < 10000. GeV 2 where M* is the invariant mass of the hadronic system produced by the diffractive excitation of the virtual photon. POMPYT events were produced within the following ranges of generator parameters: 0.9 < XF < 0.9997 where XF is the ratio of the longitudinal momentum of the scattered proton to that of the beam proton; -10 . < t < 0. GeV 2 (mos tof theeventshave- t < 2. GeV 2) and the invariant mass of the M* plus scattered electron system was required to be greater than 5 GeV. For both POMPYT and NZ events Q2 > 4 GeV 2 was

235

required at the generator level. With these parameter settings the cross section predictions of these models differ by an order of magnitude. In this paper we con- sider only the predicted shapes of distributions. Monte Carlo event statistics are comparable to those of the data. QED radiative processes were not simulated for diffractive events, but with the selection cuts of Sec- tion 4, radiative corrections to the rates of DIS events are below 10% [ 17]. All Monte Carlo events were passed through the standard ZEUS detector and trigger simulations and the event reconstruction package [ 7 ].

6. Results

6.1. Events with a large rapidity gap

Following our previous study [ 1 ] we define r/max as the maximum pseudorapidity of all calorimeter clusters in an event, where a cluster is defined as an isolated set of adjacent cells with summed energy above 400 MeV. The distribution of '?/max is shown in Fig. la, uncorrected for detector effects. Values of r/max _> 4.3, which are outside the calorimeter acceptance, occur when energy is deposited in many contiguous cells around the beam pipe in the proton direction. The dip in the r/max distribution at r/max " ~ 1.1 is a detector effect. The bulk of the events is centered around r/max ~ 4. In addition to this region of large r/max, a second class of events is observed which has r/max ( 1.5, called large rapidity gap events. A total of 1973 such events is found. The photoproduction background for these events is about 3% as inferred from the shape of the 8 distribution restricted to events with r/max < 1.5. Other backgrounds for these events are negligible.

The standard DIS model, shown as a dotted his- togram in Fig. 1 a, gives a reasonable account of the shape of the "//max distribution for values above 1.5 but cannot account for the excess of events at lower values. To investigate how well a combination of standard DIS processes and diffractive interactions could describe this distribution, the fractions of standard DIS Monte Carlo events and pomeron events are adjusted to give the best fit to the r/max distribution, normalising the to- tal number of Monte Carlo events to the data. In Fig. la the fit with the hard POMPYT model is shown as a solid histogram, that with the soft POMPYT model as dash-dotted histogram and that with the NZ model

236 ZEUS Collaboration ~Physics Letters B 332 (1994) 228-243

=<

~10 ~

"0

"0 10 ~

10 2

lO

I

16 ~ (_9

ff "o

"o z6 ~ -w-

Z

z6 ~

!

(D V -1 ~;~10 o "o

7 - o ..K-

, ~16 ; "

• ZEUS DATA - HARD POMPY'r ~e- ~ = -- NZ r:J ~- -- SOFT PO.PYT p

L F"

- 2 0 2 4, 6

~ m~x

(a) >

~ 1 2 0

100

80

60

40

20

ZEUS data

• ~ /~> 1.5 • ~ < 1.5

,o .oo , , o . o

W~(Ge V)

• ZEUS DATA

S 10 15 20 Mx~5(~eV I o

(c) I

(.9

"D

Z "0

~ . z6 ~

1 •5 ~m- • ZEUS DATA

- HARD POMPYT

° " - ' - ---, - NZ

. . . . i . . . . i . . . . [ . . . . i , ,

so ~oo zso 200 W~2saG( eV"

1(~ "~

10

• ZEUS DATA ~,~ q m < 1.5 - HARD POMPYF

. . . . . NZ

.... ~ . . . . . . . i

,o' Q~,(Gev, lO °

Ce) X "o

Z -o

Z

15:

1():'

10"

~__< 1.5 .e. • ZEUS DATA

10 -4 10 -~

(b)

(d)

(0

10 - I

XoA

ZEUS Collaboration / Physics Letters B 332 (1994) 228-243 237

Fig 1 (a) The distribution of the calorimeter cluster with maximum rapidity, ~/max, for the DIS sample. The dotted histogram shows the standard DIS Monte Carlo sample alone, the other histograms show fits to the data using a combination of standard DIS Monte Carlo plus hard POMPYT events (full), or plus soft POMPYT events (dash-dotted) or plus NZ events (dashed) The combined total of Monte Carlo events has been normalised to the data (b) The correlation between the mass of the hadronic system Mx and WoA (c) The distributton of Mx an large rapidity gap events with comparisons from the hard POMPYT and NZ models, see text for detmls. (d) The distribution of WoA for events with a large raptthty gap with the same model comparisons (e) Q2oA and (f) Bjorken XDA distrlbutmns for events with 7/max < 1.5 with model comparisons as labelled.

as a dashed histogram. We find that a qualitative de- scription of the data can be obtained with about 10% of all events coming from either the hard POMPYT or the NZ model. The soft POMPYT model does not give a good representation of the data and will not be considered further. From these fits we also find that the background of DIS events in the large rapidity gap sample is about 7%.

The correlation between Mx and WDA is displayed in Fig. lb where events with 'r/max < 1.5 are plotted with larger dots. Note, in this and all following figures, the data shown are uncorrected for detector effects and the 'r/max cut. The large rapidity gap events are distinct from the standard events; they are characterized by small Mx values, typically Mx <_ 20 GeV. In Fig. lc the Mx distribution is shown for events with ?/max < 1.5. From the NZ and hard POMPYT models we learn that the reconstructed Mx tends to overestimate the true value by about 7% at 10 GeV, the discrepancy increasing approximately linearly with Mx. The resolution of Mx is roughly constant at a value of about 15%, for values of Mx above 4 GeV. For both models the acceptance due to the "r/max cut is about 40% and high values of Mx are preferentially suppressed. The hard POMPYT model, and to a lesser extent also the NZ model, are seen to predict fewer events for Mx > 10 GeV. We note that the 7% standard DIS Monte Carlo events populate mainly the region Mx < 10 GeV and do not account for the difference. In Figs. ld-f the distributions of WDA, Q2 a and XDA are shown for T/max < 1.5. From Fig. 1 we see that neither the hard POMPYT nor the NZ models, with the parameter settings given in Section 5, can describe all details of our data; however the gross features of the data are well enough described for the purposes of this analysis.

The features of large rapidity gap events displayed in Fig. 1 do not change if the threshold energy used in the cluster definition is varied by 4-100 MeV.

We define the variable ~: by the relation

M 2 ± 1"12 x T ~OA . (13)

= W2a + a ~ a

If large rapidity gap events are interpreted as due to pomeron exchange, then ~: is the fraction of the proton's momentum carried by the pomeron. For the data with a large rapidity gap, due to acceptance cuts, ~: lies in the range 5.6 - 10 -4 to 2 .0 .10 -2 with a mean value < ~: > of 3.2- 10 -3.

6.2. The x, 0 2 and W dependence

In Regge phenomenology the amplitudes for two- body scattering by pomeron exchange are approxi- mately independent of the centre-of-mass energy while Reggeon exchange amplitudes have inverse power law dependences. The contribution to y*p scattering of a subprocess due to pomeron exchange should depend weakly on W, whereas ~r or p exchange would give a contribution falling approximately as W -4 or W -2. About the same W dependences would be expected in ep scattering for the relative contributions of these sub- processes to the total DIS sample. In order to examine the W dependence we study the ratio r of the num- ber of events with r/max < 1.5 to the total number of DIS events. In this ratio several factors drop out, such as the flux of virtual photons, which are common for events with and without a large rapidity gap. The de- pendence of r on WDA is shown in Fig. 2a. The figure also shows (as a histogram) the relative acceptance of the "0'max cut as a functionof WOA. For Woa > 140 GeV it can be seen that the acceptance corrections are in- dependent of WDA. At smaller Woa values, the accep- tance decreases since the final state hadronic system is boosted in the forward direction. The contribution of the large rapidity gap events to the DIS cross section is constant, within errors, for WOA > 140 GeV, suggest- ing a diffractive type of interaction. For WOA >_ 140 GeV the fraction of events with 97max < 1.5 is 7 - 8% of the total DIS sample.

238 ZEUS Collaborauon /Physics Letters B 332 (1994) 228-243

L.-- 0 . 1

0 . 0 8

0 . 0 6

0 • 0 4

0 . 0 2

0

++++4--+- + , ' ~ _ _ + + ,

, ÷ , - ÷

. ,

, i I , r I i r t i , , I , , , I , , , I , , , I , , , I , , , I ,

6 0 0 0 1 0 0 1 2 0 1 4 0 1 6 0 1 0 0 2 0 0 2 2 0 2 4 0

WDA(GeV)

(o)

L . o.1~ 0 . 0 8

0 . 0 6

0 . 0 4 ,

0 . 0 2

W . . > 1 4 0 G e V 0 . 0 0 0 3 < X = . < 0 . 0 0 0 6

10 12 14 16 18 20 22

(b)

2& 26 28 30

Qg,,(c v

0 . 1 --

0 . 0 8

0 . 0 6 --

0 . 0 4

O . 0 2

0 i i 1 0

W.,,,> 14-0 G e V 0 . 0 0 0 6 < X ~ < 0 . 0 0 1 2

I . . . . I . , , , I , , , , I , , , , I , , * , I , , , , I , , ,

1 5 2 0 2 5 3 0 3 5

(c)

40 45 50

Qg,,(cev

L- .

0 . 0 ' 7

0 . 0 6

0 . 0 5

0 . 0 4 ,

0 . 0 3

0 . 0 2

0 . 0 3 .

0 l O

L W..> 14-0 GeV

L

20 30 40 50 60 70

0.0012<X..<0.0024 (d)

80 90 I00

Qg,,,(G eV")

ZEUS Collaboranon / Phystcs Letters B 332 (1994) 228-243 239

Frg 2. (a) The ratio r of the number of events wtth r/max < 1.5 to the total number of DIS events as a function of WDA. The histogram shows the acceptance of the r/max cut m arbitrary umts (b), (c) and (d) The ratio r for WDa > 140 GeV as a function of Q2DA for three intervals m Bjorken XOA 3 -- 6 10 -4, 6 -- 12 10 -4 and 12 -- 24. 10 -4 respectively.

In Figs. 2b-d we show the ratio r as a function of Q2 A for three different intervals in Bjorken XDA. The data were restricted to values WOA >_ 140 GeV where Monte Carlo calculations show that the acceptance is flat in Q2DA. Since the total DIS sample shows a leading twist behaviour, the near constancy of r w i t h Q2DA sug- gests that the production mechanism responsible for the large rapidi ty gap events should also be a leading twist effect.

6.3. Jet structure

Table 1 (a) Percentage of large rapidity gap events with jets ra the ep frame for two values of the minimum transverse jet energies. (b) Percentage of large rapidity gap events for jets m the ),*p frame with a minimum transverse jet energy of 2 GeV

(a) ep Data Hard POMPYT NZ

E•et > 4 GeV 1 jet 15± 1% 21 5% 23% 2jets 0.34-0.1% 05% 06%

E jet > 2 GeV 1 jet 664- 2% 76% 69% 2jets 54- 0.5% 22% 3 8%

(b) "y*p Data Hard POMPYT NZ

In order to see whether the process leading to large rapidity gap events contains a hard component we stud- ied the je t structure of the final state.

Two types of je t studies were performed. In the first study ( e p ) , je t production was analysed in the labora- tory system with respect to the beam axis to see how the transverse momentum of the electron was balanced by the hadronic system. The second study (9,* P) was done in the virtual photon proton centre-of-mass system. We searched for je t structures using a cone based je t finding algori thm in pseudorapidi ty (~7), azimuth (~b) space [29,30] . The cone radius R = (A~b 2 + A~72)½ in the algori thm was set to 1 unit and calorimeter cells with EMC ( H A C ) energy below 60 MeV (110 MeV) were excluded. Also those cells associated with the scat- tered electron were removed when performing the je t search. Clusters were formed around cells with trans- verse energy greater than 300 MeV.

6.3.1. Jet studies in the ep system A cluster was called a je t if its transverse energy

E~ t in the laboratory with respect to the beam axis was larger than 4 GeV. An example of a 1-jet event is shown in Fig. 3a. Of the 1973 DIS events with ~Tm~x < 1.5 we found 294 events of the 1-jet type and 6 events of the 2-jet category. Jet rates for data and Monte Carlo sim- ulations are given in Table la. Both the hard POMPYT and NZ models predict je t rates that are somewhat higher than the data. The total hadronic transverse en- ergy distribution is shown in Fig. 4a for all events with 1"/max < 1.5 and for those with > 1 je t (hashed) and 2 jets (cross-hashed) in the final s ta te . For Er > 10

E.jet r >2GeV 1jet 594-05% 20% 4.6% 2jets 354-04% 05% 22% 3jets 04-4-01% -

GeV practically all events are of the 1-jet type. In Fig. 4b, the distribution of the azimuthal angular difference At~e_je t between the scattered electron and the je t is displayed. It shows that the scattered electron and the je t are preferentially back-to-back in azimuth. Fig. 4c shows tile transverse je t energy for events with 7/max <

1.5 containing at least one jet. Both the hard POMPYT and NZ models give reasonable accounts of the data.

Even if the cross section for je t production in y*pomeron interactions were large, we would not expect to see sizeable 2-jet production in the HERA system because of the requirement that E~ t > 4 GeV and the rather small centre-of-mass energy of the y*pomeron system. The average momentum of the pomeron for our event sample with 77max < 1.5 is small, < ~: > .Ep = 2.63 GeV. If the m i n i m u m E~ t requirement is lowered from 4 to 2 GeV the 1- and 2-jet rates increase as shown in Table la . A minimum jet energy of 2 GeV can be used because the 7]max cut removes the proton remnant in the FCAL. Also as we describe below in Section 6.4, the je t profiles in rapidity and azimuth show that there is li t t le hadronic activity outside the jets.

6.3.2. Jet studies in the 3/*p system The question whether the observed jets are a mere

kinematic artifact of the necessity to balance the trans- verse momentum of the scattered electron is investi-

240 ZEUS Collaboranon / Physics Letters B 332 (1994) 228-243

'!I C~ % - . . i!ii!! o I.: ~-"

E O.

F - -

360

[deg] 180 -2 0 2 11

0

. . . . . . . . . . . . . . . . . . - . . .

360

180 0 2 [deg] -2 TI

0

[ • DATA ~l~< l.G

I" I ~ - HARD POMPTT ~_< 1.5 I- - .z

' ~ ' ,

~ 0 . 6

O ° l

o . ~

(c) > 0

~0o$

Z O.G '

0.4

0.2

0

• DATA ~'( 1 .,5 0 DATA ~,,.> 1.5 - HARD POUWT 'z'/_< 1.5 -NZ ~.<1.5

"i

~ ~ . . . . + .~,.~

i -1 -0.5 0

i(d)

0.5 ! 1.3

A* (rodions) Fig. 3 (a) Transverse energy deposition m 7/ -- ~b space for a large rapidity gap event with one hadronlc .let balancing the electron's transverse momentum. (b) A slmdar display for a large rapidity gap two-jet event. (c), (d) The transverse energy weighted profiles for jets with rljet < 0 m events with r/max < 1.5 (filled circles) and with r/max > 1 5 (open circles), (c) shows the rapid]ty profile with a hermsphere cut and (d) the azimuthal profile excluding energy deposits with r /> 1.5, see text The profiles for Jets m events with a large rapidity gap are compared with expectations of the hard POMPYT and NZ models (full and dashed histograms respectavely).

gated in the T*P system. To boos t to the T*p system

the 4 - m o m e n t u m o f the virtual photon is first recon- structed us ing D A variables. F r o m M o n t e Car lo studies

the uncertainty in the total hadronic t ransverse energy,

E~, generated by the Lorentz t ransformat ion is o f the

order o f 650 M e V rms. In Fig. 4d we show the E~

ZEUS Collaboration /Phystcs Letters B 332 (1994) 228-243 241

T > (1)

0

I . ~ 102 " 0

z " 0

10

1

0

eD s y s t e m ZEUS DATA

5 10 15 2 0

ET (GeV)

I (o) I > (.9

z "O

10

7"P s y s t e m ZEUS DATA

I

o 2 4 @ 8 10 12 14 E;(GeV)

(d)

<::] 1 e n ~ v ~ t e m • ZEUS DATA "O - r - ~ . . . . . . - HARD POMF'YT

_.z r

z°=

0 50 1 0 0 1 5 0

A*o_~t (degrees)

(b) :~ 0 . 5

z

• 0 . 4 z

0 . 3

0 . 2

0 . 1

0 0

7"P s y s t e m • ZEUS DATA - HARD POUPYT

- NZ

i 5O 1 0 0 1 5 0

A4)[,t_~ (degrees)

(e)

7 >

ep system • ZEUS DATA - HARD POMPYT

~ -- NZ

z

"- 1 5 2 ~

& 6 8 10 12 14 E;~(GeV)

(c) I 1 > ~)

0

i

Z

Z

• y'p system • ZEUS DATA - HARD POMPYT -- NZ

---~---: !

~ ' r- ....

( f )

3 4 5 6 o'/je~ 8 9 E, (GeV)

242 ZEUS Collaboration / Physics Letters B 332 (1994) 228-243

Fig 4 (a) The distnbutaon of the total hadromc transverse energy seen in the calorimeter, ET, for DIS events with a large rapidity gap and those with, m addition, > 1 (hashed) and >_ 2 jets (cross-hashed) A jet Is required to have at least 4 GeV transverse energy with respect to the beam directaon (b) The difference in azimuthal angle between the scattered electron and the jet (c) The jet transverse energy m the laboratory for events m the DIS sample with a large rapidity gap. (d) The total hadronic energy transverse to the virtual photon dlrecuon, E~., for DIS events with a large rapidity gap and those with, m additaon, >_ 1 (hashed), > 2 (cross-hashed) or 3 jets (solid) in the final state Here a jet is required to have at least 2 GeV with respect to the virtual photon direction (e) The thfference in azimuthal angle between the two jets in the T*P centre-of-mass system (2-jet sample) (f) The distribution of the jet energy transverse to the virtual photon dlrecuon for the 1- and 2-jet samples. In figures (b), (c), (e) and (f) the data are shown as black dots with errors and the results from the hard POMPYT and NZ models as full and dashed histograms respectively

distribution for large rapidity gap events, and observe transverse energies as large as 15 GeV.

In this study the minimum transverse jet energy re- quired, with respect to the T* axis, was 2 GeV. With this definition 117 events were found of the 1-jet type, 69 of the 2-jet type and 7 of the 3-jet type. Jet rates in the T*P system for data and Monte Carlo event sam- ples are given in Table lb. The events with > 1, _> 2 and 3 je ts are shown in Fig. 4d as hashed, cross-hashed and solid histograms respectively. Jet production is the dominant mechanism for E~- values larger than 7 GeV. The NZ model gives je t rates in better agreement with the data than the hard POMPYT model, though both models reproduce the overall features of the data in the T*P frame shown in Figs. 4e and 4f below.

In Fig. 4e the difference in azimuth for the 2-jet events (A~bjet_jet) is displayed, showing that the jets are preferentially back-to-back. In Fig. 3b we show an example of a 2-jet event, which satisfies the 2-jet criteria in both the ep and T*P frames. In Fig. 4f we show, for events with > 1 jet , the je t transverse energy distribution (with respect to the T* direction) where jets with transverse energies, E~. jet, as large as 7 GeV are observed. The same holds for the sub-sample of 2- je t events. A momentum transfer squared t between the virtual photon and the incident proton could contribute to the je t transverse energy in the T*P frame. However the analysis of the je t transverse energy in the X system shows good agreement with that in the y*p system demonstrating that a non-zero t value is not the cause of the h a r d E~, Jet spectrum. Taking these observations together we have evidence for hard scattering processes in both the T*P system and the X system, producing large rapidity gap events (T/max < 1.5) in ep scattering.

6.4. Energy f low around the j e t axis

The transverse energy flow around the jet axis in the ep system was studied for 1-jet events with and

without a rapidity gap as a function of A~b and AT/ with respect to the je t axis. A~b and AT/are defined as

the differences ~bcell - q~jet axis and T/eeu - 1"]jet axis re- spectively. The 1-jet DIS sample with T/max > 1.5 was restricted to the kinematic region 10 - 4 < X ~ 10 - 2

and 10 < Q2 _< 100 GeV 2, where most of the events

with T/max < 1.5 lie. To avoid any bias from the T/max cut the jets for both samples were restricted to the re- gion T/jet < 0. The transverse energy weighted flows around the jet axis with respect to At /and A~b are shown in Figs. 3c and d respectively. In the case o f the AT/ plot only transverse energy deposits in the hemisphere defined by the je t axis are included. Positive AT/ val- ues are closer to the forward (proton beam) direction. For the A~b plot transverse energy deposits with T/ > 1.5 were excluded. The following conclusions can be drawn. The jets are strongly coll imated and the energy flow in the central core of the je t is very similar for events with a large rapidity gap and for standard DIS events (T/max > 1.5). However, while standard DIS events have a significant amount of transverse energy flow between the je t and the proton direction, there is practically no such energy flow seen for large rapidity gap events in the AT/ distribution. These results con- firm our earlier conclusion [ 1 ] that, for large rapid- ity events, there is no colour flow between the je t and the proton direction. Both the hard POMPYT and NZ models give good descriptions of the je t profiles for events with T/max < 1.5.

7. Summary and conclusions

We have investigated the production of events with a large rapidity gap at HERA energies in the DIS regime for Q2 > 10 GeV 2 with a twentyfoldincrease in statis- tics compared to our previous analysis [ 1 ]. Focussing on the region W > 140 GeV we find that the events with a large rapidity gap, defined by T/max < 1.5, ac-

ZEUS Collaboration/Physics Letters B 332 (1994) 228-243 243

count for 7 - 8% of all DIS events, without acceptance corrections. The ratio r of events with r/max < 1.5 to all DIS events is constant with W suggesting a diffrac- tive production mechanism. Since the total DIS sample shows a leading twist behaviour and the ratio r is found to be approximately constant with Q2, the large rapid- ity gap events are also consistent with leading twist. The analysis of the hadronic system in the ep frame shows that for total transverse energies ET _> 10 GeV basically all events are of the 1-jet type, when the jet is required to have more than 4 GeV transverse energy with respect to the beam direction. The jet is found to be back-to-back with the scattered electron in the transverse plane. The analysis of the hadronic system in the T*P centre-of-mass frame shows that for trans- verse jet energies greater than 2 GeV with respect to the virtual photon direction, small but significant two- jet production is observed with transverse jet energies up to 7 GeV. The two jets are produced preferentially back-to-back in azimuth. This demonstrates the pres- ence of a hard scattering process in the virtual pho- ton proton interaction in large rapidity gap events. The hard POMPYT and NZ models give fair descriptions of the shapes of the distributions studied in the large rapidity gap event sample. The Q2 independence of r together with the observation of high Er jets in the T*P system and the noted absence of colour flow in- dicate that a natural interpretation is the interaction of the virtual photon with partons in a colourless object inside the proton.

Acknowledgements We thank Profs. J. Bartels and G. Kramer for valu-

able discussions. The strong support and encourage- ment of the DESY Directorate: Prof. B.H. Wiik, Drs. H. Krech, J. May, Profs. G.A. Voss, and A. Wagner have been invaluable, as well as the support given by Dr. G. S6hngen.

The experiment was made possible by the inven- tiveness and the diligent efforts of the HERA machine group who continued to run HERA most efficiently during 1993.

The design, construction, and installation of the ZEUS detector has been made possible by the inge- nuity and dedicated effort of many people from inside DESY and from the home institutes who are not listed as authors. Their contributions are acknowledged with

great appreciation.

[1] ZEUS Collab., M Demck et al., Phys. Lett. B 315 (1993) 481

[2] H1 Collab., A. De Roeck, invited talk at the Europhysics Conf on HEP, Marsetlle 1993 and DESY 94-005. J. B Dmnton, invited talk at the XVI International Symposium on Lepton-Photon Interactions, Comell 1993 and RAL-94-012.

[31 G Ingelman and E Schlein, Phys. Lett. B 152 (1985) 256 [4] E E Low, Phys. Rev. D 12 (1975) 163;

S Nussinov, Phys Rev Lett. 34 (1975) 1286, Phys. Rev D 14 (1976) 246.

[ 5 ] A Donnachie and E V. Landshoff, Nucl Phys B 244 ( 1984 ) 322

[6] UA8 Collab., A. Brandt et al., Phys Lett B 297 (1992) 417. [7] The ZEUS Detector, Status Report 1993, DESY 1993. [8] ZEUS Collab., M. Derrick et al., Phys Lett. B 293 (1992)

465 [9] C. Alvisi et al., Nuel. Instr Meth A 305 (1991) 30.

[ 101 C.B. Brooks et al., Nucl. Instr Meth. A 283 (1989) 477; B Foster et al, Nucl. Instr Meth. A 338 (1994) 254

[111 A. Andresen et al, Nucl Instr Meth. A 309 (1991) 101 [12] A Bernstem et al., Nucl. Instr. Meth. A 336 (1993) 23 [13] A Caldwell et al, Nucl Instr Meth A 321 (1992) 356. [ 141 J Andmszk6w et al, DESY 92-066 (1992) [ 15] F Jacquet and A Blondel in Proc. of the study for an ep

facility in Europe, DESY 79/48(1979), 391-394, ed. U Amaldi.

[ 16] S Bentvelsen, J Engelen and P Kooijman, Proc of the Workshop on Physics at HERA, Vol 1, DESY (1992) 23.

[17] ZEUS Collab., M. Demck et al, Phys Lett B 316 (1993) 412

[ 18 ] A. Kwiatkowskl, H Splesberger and H J. M6hring, Proc. of the Workshop on Physics at HERA, Vol. 3, DESY (1992) 1294.

[ 19] G. A. Schuler and H. Splesberger, Proc. of the Workshop on Physms at HERA, Vol. 3, DESY (1992) 1419

[20] ARIADNE 4 0 Program Manual, L LOnnblad, DESY-92-046 (1992).

[21] B. Andersson et al., Phys. Rep. 97 (1983) 31. [22] T Sjbstrand, Comp Phys Comm 39 (1986) 347;

T. Sj6strand and M Bengtsson, Comp Phys Comm. 43 (1987) 367.

[23] A D Martin, WJ Stifling and RG Roberts, Phys. Lett. B 306 (1993) 145.

[24] P Bmm and G. Ingelman, DESY 93-187, to be published m the proceedings of the Europhysms Conf on HEP, Marseille 1993.

[25] H-U Bengtsson and T. Sjbstrand, Comp. Phys Comm 46 (1987) 43, T SjOstrand, CERN-TH 6488/92.

[26] A. Donnachle and P.V Landshoff, Phys Lett B 191 (1987) 309, Nucl. Phys B 303 (1988) 634.

[27] N.N. Nikolaev and B G. Zakharov, Z. Phys. C 53 (1992) 331 [28] A. Solano, Ph D Thesis, Unwerstty of Torino 1993,

unpnbhshed. [ 29 ] UA 1 Collab, G Amlson et al, Phys Lett B 123 (1983) 115 [30] J. Huth et al, FERMILAB-Conf-90-249-E