Photoproduction of mesons associated with a leading neutron

b

ron

i,,ci,

ang,

g

Physics Letters B 590 (2004) 143–160

www.elsevier.com/locate/physlet

Photoproduction ofD∗± mesons associated with a leading neut

ZEUS Collaboration

S. Chekanov, M. Derrick, D. Krakauer, J.H. Loizides1, S. Magill, S. Miglioranzi1,B. Musgrave, J. Repond, R. Yoshida

Argonne National Laboratory, Argonne, IL 60439-4815, USA 44

M.C.K. Mattingly

Andrews University, Berrien Springs, MI 49104-0380, USA

P. Antonioli, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. BrunG. Cara Romeo, L. Cifarelli, F. Cindolo, A. Contin, M. Corradi, S. De Pasquale

P. Giusti, G. Iacobucci, A. Margotti, A. Montanari, R. Nania, F. Palmonari, A. PesG. Sartorelli, A. Zichichi

University and INFN Bologna, Bologna, Italy 35

G. Aghuzumtsyan, D. Bartsch, I. Brock, S. Goers, H. Hartmann, E. Hilger, P. IrrgH.-P. Jakob, O. Kind, U. Meyer, E. Paul2, J. Rautenberg, R. Renner, A. Stifutkin,

J. Tandler, K.C. Voss, M. Wang, A. Weber3

Physikalisches Institut der Universität Bonn, Bonn, Germany 32

D.S. Bailey4, N.H. Brook, J.E. Cole, G.P. Heath, T. Namsoo, S. Robins, M. Win

H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 43

M. Capua, A. Mastroberardino, M. Schioppa, G. Susinno

Physics Department, Calabria University and INFN, Cosenza, Italy 35

J.Y. Kim, Y.K. Kim, J.H. Lee, I.T. Lim, M.Y. Pac5

Chonnam National University, Kwangju, South Korea 9

A. Caldwell6, M. Helbich, X. Liu, B. Mellado, Y. Ning, S. Paganis, Z. Ren,W.B. Schmidke, F. Sciulli

Nevis Laboratories, Columbia University, Irvington on Hudson, NY 10027, USA 45

0370-2693/$ – see front matter 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.physletb.2004.03.076

http://www.elsevier.com/locate/physletb

144 ZEUS Collaboration / Physics Letters B 590 (2004) 143–160

jski

,

,

n,

J. Chwastowski, A. Eskreys, J. Figiel, A. Galas, K. Olkiewicz, P. Stopa, L. Zawie

Institute of Nuclear Physics, Cracow, Poland 39

L. Adamczyk, T. Bołd, I. Grabowska-Bołd7, D. Kisielewska, A.M. Kowal, M. Kowal,T. Kowalski, M. Przybycien, L. Suszycki, D. Szuba, J. Szuba8

Faculty of Physics and Nuclear Techniques, A GH-University of Science and Technology, Cracow, Poland 46

A. Kotanski9, W. Słominski

Department of Physics, Jagellonian University, Cracow, Poland

V. Adler, U. Behrens, I. Bloch, K. Borras, V. Chiochia, D. Dannheim, G. DrewsJ. Fourletova, U. Fricke, A. Geiser, P. Göttlicher10, O. Gutsche, T. Haas, W. Hain,

S. Hillert11, B. Kahle, U. Kötz, H. Kowalski12, G. Kramberger, H. Labes, D. Lelas,H. Lim, B. Löhr, R. Mankel, I.-A. Melzer-Pellmann, C.N. Nguyen, D. Notz,

A.E. Nuncio-Quiroz, A. Polini, A. Raval, L. Rurua, U. Schneekloth, U. StössleinG. Wolf, C. Youngman, W. Zeuner

Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany

S. Schlenstedt

DESY Zeuthen, Zeuthen, Germany

G. Barbagli, E. Gallo, C. Genta, P.G. Pelfer

University and INFN, Florence, Italy 35

A. Bamberger, A. Benen, F. Karstens, D. Dobur, N.N. Vlasov

Fakultät für Physik der Universität Freiburg i.Br., Freiburg i.Br., Germany 32

M. Bell, P.J. Bussey, A.T. Doyle, J. Ferrando, J. Hamilton, S. Hanlon, D.H. SaxoI.O. Skillicorn

Department of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom 43

I. Gialas

Department of Engineering in Management and Finance, University of Aegean, Greece

T. Carli, T. Gosau, U. Holm, N. Krumnack, E. Lohrmann, M. Milite, H. Salehi,P. Schleper, S. Stonjek11, K. Wichmann, K. Wick, A. Ziegler, Ar. Ziegler

Institute of Exp. Physics, Hamburg University, Hamburg, Germany 32

ZEUS Collaboration / Physics Letters B 590 (2004) 143–160 145

n,

v

C. Collins-Tooth, C. Foudas, R. Gonçalo13, K.R. Long, A.D. Tapper

Imperial College London, High Energy Nuclear Physics Group, London, United Kingdom 43

P. Cloth, D. Filges

Forschungszentrum Jülich, Institut für Kernphysik, Jülich, Germany

M. Kataoka14, K. Nagano, K. Tokushuku15, S. Yamada, Y. Yamazaki

Institute of Particle and Nuclear Studies, KEK, Tsukuba, Japan 36

A.N. Barakbaev, E.G. Boos, N.S. Pokrovskiy, B.O. Zhautykov

Institute of Physics and Technology of Ministry of Education and Science of Kazakhstan, Almaty, Kazakhstan

D. Son

Center for High Energy Physics, Kyungpook National University, Daegu, South Korea 37

K. Piotrzkowski

Institut de Physique Nucléaire, Université Catholique de Louvain, Louvain-la-Neuve, Belgium

F. Barreiro, C. Glasman16, O. González, L. Labarga, J. del Peso, E. Tassi, J. TerróM. Vázquez, M. Zambrana

Departamento de Física Teórica, Universidad Autónoma de Madrid, Madrid, Spain 42

M. Barbi, F. Corriveau, S. Gliga, J. Lainesse, S. Padhi, D.G. Stairs, R. Walsh

Department of Physics, McGill University, Montréal, Québec, Canada H3A 2T8 31

T. Tsurugai

Faculty of General Education, Meiji Gakuin University, Yokohama, Japan 36

A. Antonov, P. Danilov, B.A. Dolgoshein, D. Gladkov, V. Sosnovtsev, S. Suchko

Moscow Engineering Physics Institute, Moscow, Russia 40

R.K. Dementiev, P.F. Ermolov, Yu.A. Golubkov17, I.I. Katkov, L.A. Khein,I.A. Korzhavina, V.A. Kuzmin, B.B. Levchenko18, O.Yu. Lukina, A.S. Proskuryakov,

L.M. Shcheglova, S.A. Zotkin

Institute of Nuclear Physics, Moscow State University, Moscow, Russia 41


N. Coppola, S. Grijpink, E. Koffeman, P. Kooijman, E. Maddox, A. Pellegrino,S. Schagen, H. Tiecke, J.J. Velthuis, L. Wiggers, E. de Wolf

NIKHEF and University of Amsterdam, Amsterdam, Netherlands 38

N. Brümmer, B. Bylsma, L.S. Durkin, T.Y. Ling

Physics Department, Ohio State University, Columbus, OH 43210, USA 44

A.M. Cooper-Sarkar, A. Cottrell, R.C.E. Devenish, B. Foster, G. Grzelak,C. Gwenlan19, S. Patel, P.B. Straub, R. Walczak

Department of Physics, University of Oxford, Oxford, United Kingdom 43

A. Bertolin, R. Brugnera, R. Carlin, F. Dal Corso, S. Dusini, A. Garfagnini,S. Limentani, A. Longhin, A. Parenti, M. Posocco, L. Stanco, M. Turcato

Dipartimento di Fisica dell’Università and INFN, Padova, Italy 35

E.A. Heaphy, F. Metlica, B.Y. Oh, J.J. Whitmore20

Department of Physics, Pennsylvania State University, University Park, PA 16802, USA 45

Y. Iga

Polytechnic University, Sagamihara, Japan 36

G. D’Agostini, G. Marini, A. Nigro

Dipartimento di Fisica, Università ‘La Sapienza’ and INFN, Rome, Italy 35

C. Cormack21, J.C. Hart, N.A. McCubbin

Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, United Kingdom 43

C. Heusch

University of California, Santa Cruz, CA 95064, USA 44

I.H. Park

Department of Physics, Ewha Womans University, Seoul, South Korea

N. Pavel

Fachbereich Physik der Universität-Gesamthochschule Siegen, Germany

H. Abramowicz, A. Gabareen, S. Kananov, A. Kreisel, A. Levy

Raymond and Beverly Sackler Faculty of Exact Sciences, School of Physics, Tel-Aviv University, Tel-Aviv, Israel 34

M. Kuze

Department of Physics, Tokyo Institute of Technology, Tokyo, Japan 36


ndage

T. Fusayasu, S. Kagawa, T. Kohno, T. Tawara, T. Yamashita

Department of Physics, University of Tokyo, Tokyo, Japan 36

R. Hamatsu, T. Hirose2, M. Inuzuka, H. Kaji, S. Kitamura22, K. Matsuzawa

Department of Physics, Tokyo Metropolitan University, Tokyo, Japan 36

M.I. Ferrero, V. Monaco, R. Sacchi, A. Solano

Università di Torino and INFN, Torino, Italy 35

M. Arneodo, M. Ruspa

Università del Piemonte Orientale, Novara, and INFN, Torino, Italy 35

T. Koop, J.F. Martin, A. Mirea

Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7 31

J.M. Butterworth23, R. Hall-Wilton, T.W. Jones, M.S. Lightwood, M.R. Sutton4,C. Targett-Adams

Physics and Astronomy Department, University College London, London, United Kingdom 43

J. Ciborowski24, R. Ciesielski25, P. Łuzniak26, R.J. Nowak, J.M. Pawlak, J. Sztuk27,T. Tymieniecka28, A. Ukleja28, J. Ukleja29, A.F. Zarnecki

Institute of Experimental Physics, Warsaw University, Warsaw, Poland 47

M. Adamus, P. Plucinski

Institute for Nuclear Studies, Warsaw, Poland 47

Y. Eisenberg, L.K. Gladilin30, D. Hochman, U. Karshon, M. Riveline

Department of Particle Physics, Weizmann Institute, Rehovot, Israel 33

D. Kçira, S. Lammers, L. Li, D.D. Reeder, M. Rosin, A.A. Savin, W.H. Smith

Department of Physics, University of Wisconsin, Madison, WI 53706, USA 44

A. Deshpande, S. Dhawan

Department of Physics, Yale University, New Haven, CT 06520-8121, USA 44

S. Bhadra, C.D. Catterall, S. Fourletov, G. Hartner, S. Menary, M. Soares, J. Sta

Department of Physics, York University, Ontario, Canada M3J 1P3 31


0,

e

.

rgy

03B

03B

E-mail address: [email protected] (R. Yoshida).1 Also affiliated with University College London, London, UK.2 Retired.3 Self-employed.4 PPARC Advanced Fellow.5 Now at Dongshin University, Naju, South Korea.6 Now at Max-Planck-Institut für Physik, München, Germany.7 Partly supported by Polish Ministry of Scientific Researchand Information Technology, Grant No. 2 P03B 12225.8 Partly supported by the Israel Science Foundation, and Ministry of Science, and Polish Ministry of Scientific Research and Information

Technology, Grant No. 2 P03B 12625.9 Supported by the Polish State Committee for Scientific Research, Grant No. 2 P03B 09322.

10 Now at DESY group FEB.11 Now at University of Oxford, Oxford, UK.12 On leave of absence at Columbia University, Nevis Labs., NY, USA.13 Now at Royal Holoway University of London, London, UK.14 Also at Nara Women’s University, Nara, Japan.15 Also at University of Tokyo, Tokyo, Japan.16 Ramón y Cajal Fellow.17 Now at HERA-B.18 Partly supported by the Russian Foundation for Basic Research, Grant 02-02-81023.19 PPARC Postdoctoral Research Fellow.20 On leave of absence at The National Science Foundation, Arlington, VA, USA.21 Now at University of London, Queen Mary College, London, UK.22 Present address: Tokyo Metropolitan University of Health Sciences, Tokyo 116-8551, Japan.23 Also at University of Hamburg,Alexander von Humboldt Fellow.24 Also at Łódz University, Poland.25 Supported by the Polish State Committee for Scientific Research, Grant No. 2 P03B 07222.26 Łódz University, Poland.27 Łódz University, Poland, supported by the KBN Grant 2 P03B 12925.28 Supported by German Federal Ministry for Education and Research (BMBF), POL 01/043.29 Supported by the KBN Grant 2 P03B 12725.30 On leave from MSU, partly supported by University of Wisconsin via the US–Israel BSF.31 Supported by the Natural Sciences and Engineering Research Council of Canada (NSERC).32 Supported by the German Federal Ministry for Education andResearch (BMBF), under contract numbers HZ1GUA 2, HZ1GUB

HZ1PDA 5, HZ1VFA 5.33 Supported by the MINERVA Gesellschaft für Forschung GmbH, theIsrael Science Foundation, the US–Israel Binational Scienc

Foundation and the Benozyio Center for High Energy Physics.34 Supported by the German–Israel Foundation and the Israel Science Foundation.35 Supported by the Italian National Institute for Nuclear Physics (INFN).36 Supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and its grants for Scientific Research37 Supported by the Korean Ministry of Education and Korea Science and Engineering Foundation.38 Supported by the Netherlands Foundation for Research on Matter (FOM).39 Supported by the Polish State Committee for Scientific Research, Grant No. 620/E-77/SPB/DESY/P-03/DZ 117/2003-2005.40 Partially supported by the German Federal Ministry for Education and Research (BMBF).41 Partly supported by the Russian Ministry of Industry, Science andTechnology through its grant for Scientific Research on High Ene

Physics.42 Supported by the Spanish Ministry of Education and Science through funds provided by CICYT.43 Supported by the Particle Physics and Astronomy Research Council, UK.44 Supported by the US Department of Energy.45 Supported by the US National Science Foundation.46 Supported by the Polish State Committee for Scientific Research, Grant No. 112/E-356/SPUB/DESY/P-03/DZ 116/2003-2005, 2 P

13922.47 Supported by the Polish State Committee for Scientific Research, Grant No. 115/E-343/SPUB-M/DESY/P-03/DZ 121/2001-2002, 2 P

07022.


ector in

he

lusive

Received 19 January 2004; accepted 27 March 2004

Available online 6 May 2004

Editor: W.-D. Schlatter

Abstract

The photoproduction ofD∗±(2010) mesons associated with a leading neutron has been observed with the ZEUS detep collisions at HERA using an integrated luminosity of 80 pb−1. The neutron carries a large fraction,xL > 0.2, of the incomingproton beam energy and is detected at very small production angles,θn < 0.8 mrad, an indication of peripheral scattering. TD∗ meson is centrally produced with pseudorapidity|η| < 1.5, and has a transverse momentumpT > 1.9 GeV, which islarge compared to the average transverse momentum of the neutron of 0.22 GeV. The ratio of neutron-tagged to incD∗production is 8.85±0.93(stat.)+0.48

−0.61(syst.)% in the photon–proton center-of-mass energy range 130< W < 280 GeV. The datasuggest that the presence of a hard scale enhances the fraction of events with a leading neutron in the final state. 2004 Elsevier B.V. All rights reserved.

een

mall

ly aap-cle-

ro-sed-en

ro-mesrd

er-

iousnstic

ionis

oton

n.omas atly

ionnsis-

ns,

edonsantther

1. Introduction

Events containing a leading neutron have bstudied inep collisions at HERA[1–4]. The neutronscarry a large fraction of the incoming proton beaenergy,xL > 0.2, and are produced at very smscattering angles,θn < 0.8 mrad, indicative of aperipheral process.

The small transverse momenta(pT ) which char-acterize leading baryon production processes impsoft scale, which means that a non-perturbativeproach is required to model such events. Partiexchange models within Regge theory[5], in par-ticular the one-pion-exchange model (OPE)[6–8],are often applied to describe leading neutron pduction. Charm production, in contrast, can be uto investigate parton dynamics because the charmquark mass provides the hard scale necessary tosure the applicability of perturbative quantum chmodynamics (pQCD). Therefore the study of charproduction in events with a leading neutron givinformation on the interplay between soft and hascales.

This Letter presents measurements ofD∗± photo-production associated with a leading neutron. Diffential cross sections and ratios to inclusiveD∗± photo-production are reported. These results extend prevZEUS studies[1,3,4]of leading-neutron production idijet and inclusive photoproduction and deep inelascattering.

-

2. Charm and neutron production

An important process in charm photoproductis boson–gluon fusion (BGF). At leading order thcorresponds to the direct component where the phcouples directly to a high-transverse-momentumcc

pair which interacts with a gluon from the protoAnother contribution to the cross section comes frthe resolved component, where the photon actssource of partons, interacting with the proton mosvia charm excitation processes,cq → cq andcg → cg

[9].The mechanism for leading neutron product

is not well understood. In the following subsectiosome models for neutron production are briefly dcussed.

2.1. Fragmentation models

In fragmentation models of partons into hadrosuch as the cluster model[10] or the Lund stringmodel[11], a certain fraction of neutrons is expectin the final state. In this case the leading neutrare produced by fragmentation of the proton remnusing the same mechanism as is used for the ofinal state hadrons. Such models predict a softerxL

distribution than that measured[1–4].


ngeon-i-d

asgr

thef-

be

.fe-

onhe

Ftri-re-uc-are

RVreonith

tion

in

ec-st ofmat-canro-

fer-,ron

ex-

ell

ofncey ofe-ndsent

nt

2.2. One-pion-exchange model

Previous studies have shown that particle-exchamodels[6–8,12–15]describe data on leading neutrproduction both at HERA[1–4] and at hadroproduction experiments[16–25]. In such models the transtion amplitude forp → n is dominated by OPE anthe electroproduction cross section can be writtenthe convolution of a function describing the splittinof a proton into aπn system, i.e., the pion flux factofπ/p(xL, t), and theeπ cross section:

(1)dσep→e′nX

dxL dt= fπ/p(xL, t)σ eπ (s′),

wheret is the squared four-momentum transfer atproton vertex,s′ = s(1− xL) is the squared center-omass energy of theeπ system ands is that of theep.

The flux factors found in the literature canexpressed in general as[8,15]

fπ/p(xL, t) = 1

4π

2g2pπp

4π

−t

(t − m2π )2

(2)× (1− xL)1−2α(t)[F(xL, t)

]2,

whereg2pπp/(4π) ∼ 14.5 is thepπp coupling con-

stant,mπ is the pion mass andα(t) is defined belowThe form-factorF(xL, t) accounts for the finite size othe nucleon and pion. Examples of flux factor paramtrizations are:

• f1 [26]:

(3)

F(xL, t) = exp

[R2 (t − m2

π)

(1− xL)

], α(t) = 0,

whereR = 0.6 GeV−1 andF(xL, t) is the light-cone form factor.

• f2 [8]:

(4)F(xL, t) = 1, α(t) = απ (t),

whereαπ (t) � t (with t in GeV2) is the Reggetrajectory of the pion.

• f3 [27]:

(5)F(xL, t) = exp

[b(t − m2

π

)], α(t) = απ (t),

whereb = 0.3 GeV−2 andF(xL, t) is the expo-nential form factor.

• f4 [28]:

(6)F(xL, t) = (1− m2π/Λ2)

(1− t2/Λ2), α(t) = 0,

whereΛ = 0.25 GeV andF(xL, t) is the mono-pole form factor.

The termσeπ in Eq. (1) involve the parton distri-bution in the pion. Charm production in associatiwith a leading neutron is potentially sensitive to tgluon content of the pion in OPE models via the BGprocess. Parametrizations for the pionic parton disbution function (PDF) available in the literature weobtained by performing fits toπN scattering data, assuming some parametrization for the nucleon strture function. Examples of such parametrizationsthose by Owens[29] which come from fits onJ/Ψ

and dimuon production data. The more recent Gparametrizations[30] assume a valence-like structufor the pion at a certain low scale. This distributiis dynamically evolved and the results combined wthe constraints imposed by prompt photon producdata on the pionic gluon density.

2.3. Rescattering effects

From the factorization hypothesis expressedEq. (1), it is expected that the ratios,r, of neutron-tagged to inclusive cross sections for different eltroproduction processes are about the same. Mothe dependence of the cross sections on the kineics of the processes cancels; remaining differencesbe attributed to differences between the pion and pton energies and their PDFs. However, larger difences than these may arisefrom neutron absorptionwhich can occur through rescattering of the neuton the exchanged photon[14,31]. With increasing sizeof the virtual photon more rescattering may bepected.

Inclusive photoproduction cross sections are wdescribed by vector meson dominance models[32–34],where the dipole associated with the photon ishadronic size. In dijet photoproduction, the preseof the hard scale given by the transverse energthe jets implies smaller dipole size. In the infinitmomentum frame the smaller dipole size correspoto the enhancement of the direct photon componat high transverse energyET . Such an enhanceme


to. Inardre-

ringlved-

di-S),gra-:

–orf-

an

isen-

n-on-ed

lu-

e-hebdi-

siann,

inal

toear)heell.un-

–terc-sec-ec-eers

w-ured

rster-C.eu-ich

of-ter

re

gy

and

nalf

has been observed at HERA[35]. Absorptive effectsin dijet photoproduction, therefore, are expectedbe smaller than those in inclusive photoproductionthe case of charm photoproduction, an additional hscale is provided by the mass of the charm quark. Pvious ZEUS measurements have shown that requithe presence of charm further suppresses the resocomponent[36] compared to inclusive dijet photoproduction. Therefore rescattering inD∗ photoproduc-tion may be further suppressed in comparison tojet photoproduction. In deep inelastic scattering (DIat sufficiently high photon virtuality, little rescatterinshould occur. Within such a picture, therefore, thetios are expected to have the following relationship

rγp < rjj < rD∗ � rDIS.

3. Experimental conditions

The integrated luminosity of 80.2 ± 1.8 pb−1

used for this measurement was collected at theep

collider HERA with the ZEUS detector during 19982000, when HERA collided 27.5 GeV electronspositrons48 with 920 GeV protons, giving a center-omass energy of 318 GeV.

A detailed description of the ZEUS detector cbe found elsewhere[37]. A brief outline of the com-ponents that are most relevant for this analysisgiven below. Charged particles are tracked in the ctral tracking detector (CTD)[38], which operates in amagnetic field of 1.43 T provided by a thin supercoducting solenoid. The central tracking detector csists of 72 cylindrical drift chamber layers, organizin nine superlayers covering the polar-angle49 region15◦ < θ < 164◦. The transverse-momentum resotion for full-length tracks isσ(pT )/pT = 0.0058pT ⊕0.0065⊕ 0.0014/pT , with pT in GeV.

The high-resolution uranium-scintillator calorimter (CAL) [39] consists of three parts: the forward, tbarrel and the rear calorimeters. Each part is su

48 Hereafter, bothe+ ande− are referred to as electrons.49 The ZEUS coordinate system is a right-handed Carte

system, with theZ axis pointing in the proton beam directioreferred to as the “forward direction”, and theX axis pointing lefttowards the center of HERA. The coordinate origin is at the nominteraction point.

vided transversely into towers and longitudinally inone electromagnetic section and either one (in ror two (in barrel and forward) hadronic sections. Tsmallest subdivision of the calorimeter is called a cThe calorimeter energy resolutions, as measuredder test-beam conditions, areσ(E)/E = 0.18/

√E for

electrons andσ(E)/E = 0.35/√

E for hadrons, withE in GeV.

The forward neutron calorimeter (FNC)[40,41]was installed in the HERA tunnel atθ = 0 degrees andatZ = 106 m from the interaction point in the protonbeam direction. It is a lead-scintillator calorimewhich is segmented longitudinally into a front setion, seven interaction lengths deep, and a reartion, three interaction lengths deep. The front stion is divided vertically into 14 towers, allowing thseparation of electromagnetic and hadronic showfrom the energy-weighted vertical width of the shoers. The energy resolution for neutrons, as measin a beam test, isσ(En)/En = 0.65/

√En, with neu-

tron energy,En, in GeV. Three planes of veto counteare used to reject events in which particles had inacted with the inactive material in front of the FNMagnet apertures limit the FNC acceptance to ntrons with production angles less than 0.8 mrad, whcorresponds to transverse momentapT < Enθmax =0.74xL GeV. The mean value ofpT for the data is0.22 GeV.

The luminosity was determined from the ratethe bremsstrahlung processep → eγp, where the photon was measured with a lead-scintillator calorime[42,43]located atZ = −107 m.

4. Kinematics

The kinematics of photoproduction at HERA aspecified by the photon virtuality,Q2, and the photon–proton center-of-mass energy,W . The electron–protoncenter-of-mass energy,

√s, is related toW by W2 =

ys wherey is the fraction of the electron beam enercarried by the photon in the proton rest frame.

To describe the processep → e′D∗±nX, fouradditional variables are used: two for the neutrontwo for the charmed meson. They are:

• (xL, θn), the fractional energy and productioangle of the produced neutron; only about h


ethisof

ra-

ectn

NC

ingosi-cks

edas

rgyo

iredrue

en-

dst

Theine-ns-s to

hepid-

thede-

1.80fer-

asforksand

omrgys,theredrikeederse

than

orethelowele

thean

ingons

aUSnd

of the data have aθn measurement, thereforall results discussed here are integrated overvariable up to the maximum accepted angle0.8 mrad;

• (pT , η), the transverse momentum and pseudopidity of the producedD∗± meson.

The measurement was performed in the followingkinematic region:Q2 < 1 GeV2, 130< W < 280 GeV,|η(D∗)| < 1.5,pT (D∗) > 1.9 GeV,xL > 0.2 andθn <

0.8 mrad.

5. Event selection

5.1. Trigger

A three-level trigger system was used to selevents online[37,44]. The selection was based oenergy deposits, tracking and event timing. The Fwas not used in the trigger.

5.2. Photoproduction selection

Photoproduction events were selected offline uscuts based on the reconstructed primary vertex ption, CAL energy deposits and the reconstructed traof charged particles. Events with a well-identifielectron candidate in the CAL were removed. It wrequired that

∑i (Ei − pZ,i) > 7 GeV, where the sum

runs over all CAL cells andpZ,i is theZ component ofthe momentum vector assigned to each cell of eneEi . Tracking and CAL information was combined tform energy flow objects (EFOs)[45,46]. A cut wasmade on the Jacquet–Blondel[47] estimator ofW2,W2

JB = yJBs, whereyJB = ∑i (Ei − EZ,i)/2Ee, and

EZ,i = Ei cosθi ; Ei is the energy of EFOi with po-lar angleθi with respect to the measuredZ-vertex ofthe event. The sum runs over all EFOs. It was requthatWJB < 265 GeV. These cuts correspond to a tW range of 130< W < 280 GeV andQ2 < 1 GeV2

with the medianQ2 ≈ 10−3 GeV2.

5.3. D∗±(2010) reconstruction

The inclusive charm sample was selected by idtifying events containing a charmed meson. TheD∗±selection cuts are based on the decay channel:D∗+ →

(D0 → K−π+)π+s (+charge conjugate), whereπs

indicates the “slow” pion[48]. Only tracks assigneto the primary event vertex and with hits in at leathree superlayers of the CTD were considered.combinatorial background was reduced and the kmatic phase space defined by requiring: the traverse momentum of the kaon and pion candidatesatisfy pT (K) > 0.45 GeV,pT (π) > 0.45 GeV andpT (πs) > 0.12 GeV; the transverse momentum of tD∗± to be greater than 1.9 GeV and the pseudoraity of theD∗± to satisfy|η(D∗±)| < 1.5.

Since no particle identification was performed,K andπ masses were alternately attributed to thecay products of the candidateD0 meson. ThoseD0

candidates that had an invariant mass betweenand 1.92 GeV were required to have a mass difenceM = M(Kππs) − M(Kπ) between 0.1435and 0.1475 GeV. The combinatorial background westimated from the mass-difference distributionwrong-charge combinations, in which both tracforming theD0 candidates have the same chargethe third track has the opposite charge.

5.4. Neutron reconstruction

Events with a leading neutron were selected frthe inclusive charm sample by requiring a large enedeposit (En > 184 GeV) in the FNC. Protons, photonand neutrons are separated by their position indetector, as well as by the shower width. Scatteprotons are deflected by the HERA magnets and stthe top part of the FNC. Photons can be identifiand removed from the sample because the transvspread of electromagnetic showers is much lessthat of hadronic showers.

Events with particles that started to shower befreaching the FNC were removed by requiring thatscintillator veto counter had an energy deposit bethat of a minimum-ionizing particle. Events with widshowers, inconsistent with originating from a singhigh-energy hadron, were removed.

After the cuts on shower position and width andveto counter, the contamination of particles other thneutrons in the sample is negligible. The remainbackground is from the random overlap of the neutrcoming from proton beam-gas interactions withphotoproduction event observed in the central ZEdetector. The contamination from this backgrou


ge

wrge

Fig. 1. The data points show the neutron-taggedM distribution for right-charge track combinations. The solid line shows the wrong-charcombinations normalized to the right-charge combinations in the region 0.15 < M < 0.165 GeV outside theD∗± mass window0.1435< M < 0.1475 GeV which is shown shaded. The signal observed in theM(Kπ) distribution for events within the mass windo0.1435< M < 0.1475 GeV is shown as an inset. The solid line also shows the wrong-charge combinations normalized to the right-chacombinations as before.

ove

lee.

on,ck-ive

dif-

-

foron-

of

source was less than a few percent[4] as estimatedby comparing the rate of FNC energy deposits ab400 GeV with the bunch-crossing rate.

5.5. Final event sample

TheM distribution for the neutron-taggedsampis shown in Fig. 1, along with the wrong-chargcombinations. A prominentD∗± signal is observedThe signal observed in theM(Kπ) distribution forevents within the mass window 0.1435< M <

0.1475 GeV is shown as an inset.After the wrong-charge background subtracti

298± 31 D∗± mesons were found. The same baground subtraction procedure, applied to the inclusD∗± sample, gave 14743± 253 events.

6. Monte Carlo simulation and acceptancecorrections

A GEANT-based[49] Monte Carlo (MC) simula-tion was used to calculate selection efficiencies andcorrection factors for the charmed meson. Threeferent event generators were used: RAPGAP 2.08/06[50] for evaluating the nominal corrections, HER-WIG 6.301 [51] and PYTHIA 6.156 [52] as system-atic checks. RAPGAP and PYTHIA use the Lundstring model for hadronization. HERWIG uses a cluster model. The events generated with RAPGAP for ac-ceptance calculations were produced using OPEthe production of the leading neutron, with the piflux factor fromEqs. (2) and (3)and the GRV parametrization [30] for the pion PDF. Inclusive RAP-GAP, employing the Lund string model instead


m-eu-in-sed

sesionron

u-

ep-

sseas

tlyted,

ichularns

val-

NC,

de.

ys-n-s):

re

by

-om

C

its

ctshereinty

in

he

of

eith

thethe

lyineal-thewl-

ea-osecan-

OPE for neutron production, was produced for coparisons to the final measurements. The leading ntron is also produced via the Lund string modelPYTHIA . In HERWIG, it is produced via the cluster model. The proton PDFs parametrizations uwere CTEQ5L[53] for PYTHIA and HERWIG, andCTEQ4D[54] for inclusive RAPGAP. The photon PDFGRV-G LO [55] was used in PYTHIA and HERWIG,and GRS LO[56] in RAPGAP. The mass of the charmquark was set to 1.5 GeV. The fraction ofc quarkshadronizing to aD∗ meson was set tof (c → D∗) =0.235[57]. Both direct and resolved photon procesfor charm production were generated, in proportto their predicted cross sections. Effects of neutrescattering were not taken into account in the simlation.

For all the MC samples used to evaluate the acctances, events with at least oneD∗± decaying in theappropriate decay channel were selected and pathrough the ZEUS detector and trigger simulationswell as the event-reconstruction package.

Since theD∗ and neutron were independendetected and their kinematics largely uncorrelathe acceptances for the two particles factorize. Theselection efficiencies and correction factors for theneutron calculated for previous analyses[4] wereused. The overall acceptance of the FNC, whincludes the beam-line geometry and the angdistribution of the neutrons, is about 25% for neutrowith xL > 0.2 andθn < 0.8 mrad.

The differential cross section forD∗± photopro-duction associated with a leading neutron was euated in terms of a given variableY as dσ/dY =N/(AFNC · A(D∗) · B · Y) whereN is the number ofD∗ found in the final sample in a bin of sizeY , AFNCis the acceptance for the neutron detection in the FAD∗ is the acceptance for theD∗ reconstruction andB is the branching ratio for the selected decay moA value ofB = 2.57%[58] was used.

7. Systematic uncertainties

For theD∗± measurement the major sources of stematic uncertainty are listed below (the relative ucertainty onD∗± acceptance is shown in parenthese

d

• the selection of photoproduction events andD∗±candidates. Variations were made in theWJB(+4.5%) andZ vertex(+1.5

−0.3%) cuts;• the pT of the pion and kaon cuts. These we

varied according to their resolutions(+1.2−1.7%);

• variation of the mass windows. TheM windowused for the extraction of theD∗± was widenedsymmetrically by 0.5 MeV. TheM(D0) win-dow was widened and reduced symmetrically5 MeV (+3.4

−3.6%);• the M region for normalization of the wrong

charged combinations. This was changed fr0.15–0.165 GeV to 0.15–0.163 GeV(+2.4%);

• the MC model dependence. HERWIG (−4.8%)

and PYTHIA (+2%) were used instead of RAP-GAP;

• the fraction of resolved photon events in the Mwas lowered by 20% and raised by 10%(+0.8

−1.8%);• the CAL energy scale. This was varied within

uncertainty of±3% (±0.9%).

An extensive discussion of the systematic efferelated to the neutron measurement is given elsew[4]. Here, the major sources of systematic uncertaand their effect on the FNC acceptance (shownparentheses) is listed:

• the uncertainty in the angular distribution of tneutrons (±4% for xL < 0.82, ±7% for xL >

0.82);• the uncertainty in the overall FNC energy scale

±2% (less than 4% effect forxL < 0.82,+14−16% for

xL > 0.82);• the normalization uncertainty arising from th

proton–beam-gas interactions overlapping wphotoproduction events, the uncertainty inamount of dead material in the beam line, anduncertainties from the veto cuts(±5%).

All above errors were added in quadrature separatefor the positive and negative variations to determthe overall systematic uncertainty. The overall normization has additional uncertainties of 2.2% due toluminosity measurement and 2.5% due to the knoedge of branching ratios.

Sources of systematic uncertainty in the ratio msurement were studied in a similar manner to thfor the cross-section measurements. There is a


ain-o-

tri-the

-m

y

tic

s

nd

iveicnge

are

ies

cellation between the common systematic uncertties originating from the selection of inclusive photproduction events, the selection ofD∗± candidatesand the background estimation. The remaining conbutions are those from the model dependence ofacceptance corrections used inthe evaluation of the inclusiveD∗± photoproduction cross sections and frothe neutron measurement uncertainties.

8. Results

The integrated cross section for the reactionep →e′D∗±nX in the kinematic regionQ2 < 1 GeV2,130 < W < 280 GeV, |η(D∗)| < 1.5, pT (D∗) >

1.9 GeV,xL > 0.2 andθn < 0.8 mrad is

2.08± 0.22(stat.)+0.12−0.18(syst.) ± 0.05(BR) nb,

where the final uncertaintyarises from the uncertaintof the branching ratios for theD∗ and D0. The

luminosity uncertainty was included in the systemauncertainty.

Table 1 and Fig. 2 show the differential crossections for neutron-taggedD∗± production as afunction of W , pT (D∗) and η(D∗). The differentialcross section as a function ofxL is shown inTable 1andFig. 3.

The inclusiveD∗± cross section was measured afound to agree with previous measurements[36] ina similar kinematic range.Table 2and Fig. 4 showthe measured ratios of neutron-tagged to inclusD∗± production as a function of different kinematvariables. Over the whole measured kinematic rathe ratio is

rD∗ = 8.85± 0.93(stat.)+0.48−0.61(syst.)%,

which is shown superposed onFig. 4. The χ2 perdegree of freedom with respect to the overall ratio0.27, 1.65 and 0.09 for theW , pT (D∗) and η(D∗)distributions. Within the experimental uncertaint

Table 1Values of the differential cross sections for neutron-taggedD∗± photoproduction (Q2 < 1 GeV2 and θn < 0.8 mrad) with respect toW ,pT (D∗±), η(D∗±) andxL

W range (GeV) dσ/dW ± (stat.) ± (syst.) (nb/GeV)

130–160 0.0226± 0.0054+0.0067−0.0025

160–188 0.0214± 0.0036+0.0027−0.0004

188–226 0.0111± 0.0026+0.0008−0.0007

226–280 0.0080± 0.0017+0.0007−0.0017

pT (D∗±) range (GeV) dσ/dpT (D∗±) ± (stat.) ± (syst.) (nb/GeV)

1.9–2.3 1.991± 0.531+0.317−0.421

2.3–2.73 1.089± 0.261+0.144−0.168

2.73–3.8 0.364± 0.090+0.055−0.018

3.8–15 0.038± 0.004+0.003−0.004

η(D∗±) range dσ/dη(D∗±) ± (stat.) ± (syst.) (nb)

(−1.5)–(−0.72) 0.914± 0.126+0.087−0.067

(−0.72)–(−0.15) 0.858± 0.147+0.164−0.055

(−0.15)–(+0.42) 0.665± 0.161+0.222−0.064

(+0.42)–(+1.5) 0.436± 0.132+0.061−0.186

xL range dσ/dxL ± (stat.) ± (syst.) (nb)

0.2–0.46 2.18± 0.48+0.15−0.20

0.46–0.64 3.70± 0.64+0.25−0.34

0.64–0.82 4.29± 0.65+0.32−0.42

0.82–1.0 0.456± 0.381+0.073−0.086


thetions

Fig. 2. The points show the differential cross sections for neutron-taggedD∗± production as a function ofW , pT (D∗) andη(D∗) for xL > 0.2andθn < 0.8 mrad. The error bars displayed on the plots denote the statisticaluncertainty (inner) and the quadratic sum of the statistical andsystematic uncertainties (outer). The uncertainties due to the luminosity measurement and the branching ratios are not shown. The predicof Monte Carlo models normalized to the data are also shown.

ata,

d-

nsosscal-

y in

rgyb-ro-

entalsst

RVthe

on-

iceatheis

neutron-taggedD∗± production is compatible withbeing a constant fraction of inclusiveD∗± production,independent of theD∗± kinematics.

9. Discussion

The experimental results inFigs. 2 and 3(a)arecompared to the predictions, normalized to the dof the MC models RAPGAP with OPE for leading-neutron production, HERWIG, PYTHIA and inclusiveRAPGAP. The predicted cross sections from the moels are 3.0 nb for HERWIG, 4.6 nb for PYTHIA , 2.6 nbfor inclusive RAPGAP and 2.0 nb for RAPGAP withOPE. However, the normalization of these simulatioboth for the inclusive and the neutron-tagged crsections is unreliable since the hard process isculated at LO in QCD. InFig. 2, the agreement inshape between data and MC models is satisfactor

all cases.Fig. 3(a), however, shows that only RAPGAP

with OPE agrees with the measured neutron enedistribution seen in the data. A similar result was otained in the study of neutron-tagged dijet photopduction[3].

In principle the data allow the pion PDF to bprobed in the range of the parton fractional mome10−3 < xπ < 10−2. Fig. 3(b) shows the differentiacross section inxL compared to the predicted crosections from RAPGAP with OPE, based on differenparametrizations for the pion structure function: Gset 1 and Owens sets 1 and 2. The shape ofdata has little sensitivity, as in the case of neutrtagged dijet photoproduction[3], to the choice of thepion structure function. Even with an extreme choof a pion structure function, e.g., a completely flgluon distribution, or a parametrization identical to tproton structure function, little variation in shapeseen in the predictions.


e

d);aintyment

Fig. 3. The points show the differential cross sections for neutron-taggedD∗± production as a function ofxL. The histograms show thpredictions of Monte Carlo models (a) RAPGAP with OPE (solid histogram), HERWIG (dashed), PYTHIA (dotted), and inclusive RAPGAP

(dashed-dotted); (b) RAPGAP with OPE and pion PDF parametrizations from GRV set 1 (solid), Owens sets 1 (dashed) and 2 (dotte(c) RAPGAP with OPE and flux factorsf1–f4 from Eqs. (3), (4). The error bars displayed on the plots denote the statistical uncert(inner) and the quadratic sum of the statistical and the systematic uncertainties (outer). The uncertainties due to the luminosity measureand the branching ratios are not shown. All distributions are normalized to the data. The numbers in (c) are the normalization factors.

ic-

r-tors

ape

r-ies

m

for

for

theeatic

in-

Fig. 3(c) shows the data compared to the predtions of RAPGAP with OPE for the four flux factorsdiscussed inSection 2.2and GRV parametrization fothe pion PDF. All RAPGAP distributions are normalized to the data and the resulting normalization facare given in the figure. Fluxesf1 andf3 give similarresults, being compatible with the data both in shand normalization. The fluxesf2 andf4 are disfavoredby the shape of the data.

The independence ofrD∗on W , pT (D∗) and

η(D∗), shown inFig. 4, supports the hypothesis of vetex factorization, in agreement with previous studof leading-neutron production[1,3,4]. The predictedratios from the models are: 0.24 for HERWIG, 0.29for PYTHIA , and for RAPGAP with OPE: 0.18 usingthe pion PDF parametrizations from GRV, 0.076 froOwens set 1 and 0.085 from Owens set 2.

The ratio for charm production,rD∗, is in agree-

ment with the analogous ratio previously measuredneutron-tagged DIS,Q2 > 4 GeV2 [4],

rDIS = 8.0± 0.5%,

but lies above the ratio previously measuredneutron-tagged inclusive photoproduction atW =207 GeV[4],

rγp = 5.7± 0.4%,

as expected from the rescattering effects withinOPE model (seeSection 2.3). The errors quoted arthe quadratic sum of the statistical and systemuncertainties.

For photoproduced dijets the neutron-tagged toclusive ratio has only been measured forxL > 0.49.In this kinematic region, the measuredD∗± ratio and


the
Fig. 4. The ratio of neutron-taggedD∗± production to inclusiveD∗± production as a function ofW , pT (D∗) andη(D∗) for xL > 0.2 andθn < 0.8 mrad. The error bars displayed on the plots denote the statisticaluncertainty (inner) and the quadratic sum of the statistical andsystematic uncertainties (outer). The line superposedon the figures shows the overall ratio of neutron-taggedD∗± to inclusiveD∗± events.
for-

hy-

d

Table 2Values of the ratios of the differential cross sections for neutron-tagged to inclusiveD∗± photoproduction (Q2 < 1 GeV2, xL > 0.2andθn < 0.8 mrad) with respect toW , pT (D∗±) andη(D∗±)

W range (GeV) rD∗ ± (stat.) ± (syst.)

130–160 0.089± 0.022+0.004−0.055

160–188 0.101± 0.017+0.006−0.025

188–226 0.075± 0.018+0.012−0.004

226–280 0.090± 0.019+0.006−0.022

pT (D∗±) range (GeV) rD∗ ± (stat.) ± (syst.)

1.9–2.3 0.137± 0.038+0.030−0.007

2.3–2.73 0.105± 0.026+0.026−0.005

2.73–3.8 0.058± 0.014+0.011−0.003

3.8–15 0.087± 0.010+0.004−0.009

η(D∗±) range rD∗ ± (stat.) ± (syst.)

(−1.5)–(−0.72) 0.093± 0.013+0.005−0.025

(−0.72)–(−0.15) 0.097± 0.016+0.007−0.024

(−0.15)–(+0.42) 0.085± 0.021+0.007−0.028

(+0.42)–(+1.5) 0.080± 0.024+0.004−0.022

their corresponding ratios previously measuredDIS, inclusive photoproduction[4], and photoproduction of dijets with transverse energyEjet

T > 6 GeV[3]are:

rD∗(xL > 0.49) = 6.55± 0.76(stat.)+0.35

−0.45(syst.)%,

rDIS(xL > 0.49) = 5.8± 0.3%,

rjj (xL > 0.49) = 4.9± 0.4%,

rγp(xL > 0.49) = 4.3± 0.3%.

The results are compatible with the rescatteringpothesis described inSection 2.3.

10. Summary

The photoproduction ofD∗± mesons associatewith a leading neutron has been studied inep interac-tions at HERA in the kinematic regionQ2 < 1 GeV2,130 < W < 280 GeV, |η(D∗)| < 1.5, pT (D∗) >

1.9 GeV, θn < 0.8 mrad andxL > 0.2. The MonteCarlo models RAPGAP, HERWIG and PYTHIA give


lyThec-

a

issticthen,nce

nal

ghinerrkof

g toot

icse

4

9)

96

37

h

65

20

83)

84)

7

7

b-

6)

.27

1

11

23

2)

6

r.on

0;

a satisfactory description of theD∗± kinematics, butonly RAPGAP with one-pion exchange satisfactoridescribes the leading-neutron energy distribution.results show sensitivity to the choice of pion flux fator. The ratio of neutron-taggedD∗± photoproductionto inclusive D∗± photoproduction isrD∗ = 8.85 ±0.93(stat.)+0.48

−0.61(syst.)%.The ratio of neutron-taggedD∗± photoproduction

to inclusive D∗± photoproduction is constant asfunction ofW , pT (D∗) andη(D∗), in agreement withthe hypothesis of vertex factorization. This ratioconsistent with the analogous ratio in deep inelascattering, but both are about 30% higher thancorresponding ratio for inclusive photoproductiosuggesting that the presence of a hard scale enhathe fraction of events with a leading neutron in the fistate.

Acknowledgements

We thank the DESY Directorate for their stronsupport and encouragement, and the HERA macgroup for their diligent efforts. We are grateful fothe support of the DESY computing and netwoservices. The design, construction and installationthe ZEUS detector have been made possible owinthe ingenuity and effort of many people who are nlisted as authors.

This study was only made possible by the physinsight and hard work of G. Levman, to whom we owa great debt of gratitude.

References

[1] ZEUS Collaboration, M. Derrick, et al., Phys. Lett. B 38(1996) 388.

[2] H1 Collaboration, C. Adloff, et al., Eur. Phys. J. C 6 (199587.

[3] ZEUS Collaboration, J. Breitweg, et al., Nucl. Phys. B 5(2000) 3.

[4] ZEUS Collaboration, S. Chekanov, et al., Nucl. Phys. B 6(2002) 3.

[5] P.D.B. Collins, An Introduction to Regge Theory and HigEnergy Physics, CambridgeUniv. Press, Cambridge, 1977.

[6] H. Yukawa, Proc. Phys. Math. Soc. Jpn. 17 (1935) 48.[7] J.D. Sullivan, Phys. Rev. D 5 (1972) 1732.[8] M. Bishari, Phys. Rev. Lett. B 38 (1972) 510.

s

[9] ZEUS Collaboration, S. Chekanov, et al., Phys. Lett. B 5(2003) 87.

[10] B.R. Webber, Nucl. Phys. B 238 (1984) 492.[11] B. Andersson, G. Gustafson, B. Söderberg, Z. Phys. C

(1983) 317.[12] V.R. Zoller, Z. Phys. C 53 (1992) 443.[13] S.N. Ganguli, D.P. Roy, Phys. Rep. 67 (1980) 201.[14] B.G. Zakharov, V.N. Sergeev, Sov. J. Nucl. Phys. 38 (19

947;B.G. Zakharov, V.N. Sergeev, Sov. J. Nucl. Phys. 39 (19448.

[15] R.D. Field, G.C. Fox, Nucl. Phys. B 80 (1974) 367.[16] A.R. Erwin, et al., Phys. Rev. Lett. 6 (1961) 628.[17] E. Pickup, D.K. Robinson, E.G. Salant, Phys. Rev. Lett.

(1961) 192;E. Pickup, D.K. Robinson, E.G.Salant, Phys. Rev. Lett.(1961) 472, Erratum.

[18] B. Robinson, et al., Phys. Rev. Lett. 34 (1975) 1475.[19] J. Engler, et al., Nucl. Phys. B 84 (1975) 70.[20] W. Flauger, F. Mönnig, Nucl. Phys. B 109 (1976) 347.[21] J. Hanlon, et al., Phys. Rev. Lett. 37 (1976) 967;

J. Hanlon, et al., Phys. Rev. D 20 (1979) 2135.[22] G. Hartner, Ph.D. Thesis, McGill University, 1977 (unpu

lished).[23] Y. Eisenberg, et al., Nucl. Phys. B 135 (1978) 189.[24] V. Blobel, et al., Nucl. Phys. B 135 (1978) 379.[25] H. Abramowicz, et al., Nucl. Phys. B 166 (1980) 62.[26] H. Holtmann, et al., Phys. Rev. Lett. B 338 (1994) 363.[27] B. Kopeliovich, B. Povh, I. Potashnikova, Z. Phys. C 73 (199

125.[28] L.L. Frankfurt, L. Mankiewicz, M.I. Strikman, Z. Phys. A 334

(1989) 343.[29] J.F. Owens, Phys. Rev. D 30 (1984) 943.[30] M. Gluck, E. Reya, A. Vogt, Z. Phys. C 53 (1992) 651.[31] K.J.M. Moriarty, et al., Phys. Rev. D 16 (1976) 130;

A.G. Azcarate, Phys. Rev. D 17 (1978) 3022;N.N. Nikolaev, J. Speth, B.G. Zakharov, hep-ph/9708290;U. D’Alesio, H.J. Pirner, Eur. Phys. J. A 7 (2000) 109.

[32] T.H. Bauer, et al., Rev. Mod. Phys. 50 (1978) 261;T.H. Bauer, et al., Rev. Mod. Phys. 51 (1979) 407, Erratum

[33] ZEUS Collaboration, S. Chekanov, et al., Nucl. Phys. B 6(2002) 3.

[34] H1 Collaboration, S. Aid, et al., Z. Phys. C 69 (1995) 27.[35] ZEUS Collaboration, J. Breitweg, et al., Eur. Phys. J. C

(1998) 109;ZEUS Collaboration, J. Breitweg, et al., Eur. Phys. J. C(1999) 35;ZEUS Collaboration, S. Chekanov, et al., Eur. Phys. J. C(2002) 615;HI Collaboration, C. Adloff, et al., Eur. Phys. J. C 25 (20013.

[36] ZEUS Collaboration, J. Breitweg, et al., Eur. Phys. J. C(1999) 67.

[37] ZEUS Collaboration, U. Holm (Ed.), ZEUS DetectoStatus report (unpublished), DESY, 1993, availablehttp://www-zeus.desy.de/bluebook/bluebook.html.

[38] N. Harnew, et al., Nucl. Instrum. Methods A 279 (1989) 29

http://www-zeus.desy.de/bluebook/bluebook.html


81;

01;6;3...

4)

k,gy,

92-

b-

1

the9,

4-

65.

0)

5)

66

B. Foster, et al., Nucl. Phys. B (Proc. Suppl.) B 32 (1993) 1B. Foster, et al., Nucl. Instrum. Methods A 338 (1994) 254.

[39] M. Derrick, et al., Nucl. Instrum. Methods A 309 (1991) 77;A. Andresen, et al., Nucl. Instrum. Methods A 309 (1991) 1A. Caldwell, et al., Nucl. Instrum. Methods A 321 (1992) 35A. Bernstein, et al., Nucl. Instrum. Methods A 336 (1993) 2

[40] S. Bhadra, et al., Nucl. Instrum. Methods A 354 (1995) 479[41] S. Bhadra, et al., Nucl. Instrum. Methods A 394 (1997) 121[42] J. Andruszków, et al., Preprint DESY-92-066, DESY, 1992;

ZEUS Collaboration, M. Derrick, et al., Z. Phys. C 63 (199391.

[43] J. Andruszków, et al., Acta Phys. Pol. B 32 (2001) 2025.[44] W.H. Smith, K. Tokushuku, L.W. Wiggers, in: C. Verker

W. Wojcik (Eds.), Proceedings of Computing in High-EnerPhysics (CHEP), Annecy, France, September 1992, CERNGeneva, Switzerland, 1992, p. 222. Also in preprint DESY-150B.

[45] G.M. Briskin, Ph.D. Thesis, Tel Aviv University, 1998 (unpulished).

[46] ZEUS Collaboration, J. Breitweg, et al., Eur. Phys. J. C(1998) 81.

[47] F. Jacquet, A. Blondel, in: U. Amaldi (Ed.), Proceedings ofStudy for anep Facility for Europe, Hamburg, Germany, 197p. 391. Also in preprint DESY 79/48.

[48] S. Nussinov, Phys. Rev. Lett. 35 (1975) 1672.[49] R. Brun, et al., GEANT3, Technical report CERN-DD/EE/8

1, CERN, 1987.[50] H. Jung, Comput. Phys. Commun. 86 (1995) 147.[51] G. Marchesini, et al., Comput. Phys. Commun. 67 (1992) 4[52] T. Sjöstrand, Comput. Phys. Commun. 135 (2001) 238.[53] CTEQ Collaboration, H.L. Lai, et al., Eur. Phys. J. C 12 (200

375.[54] H.L. Lai, et al., Phys. Rev. D 55 (1997) 1280.[55] M. Gluck, E. Reya, A. Vogt, Phys. Rev. D 45 (1992) 3986.[56] M. Gluck, E. Reya, M. Stratmann, Phys. Rev. D 51 (199

3220.[57] L. Gladilin, hep-ex/9912064.[58] Particle Data Group, K. Hagiwara, et al., Phys. Rev. D

(2002) 010001.

Documents

Photoproduction of mesons associated with a leading neutron