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Available via license: CC BY-NC-ND 4.0
Content may be subject to copyright.
19 September 1996
Physics Letters B 384 (1996) 388-400
PHYSICS LETTERS B
Observation of events with an energetic forward neutron in deep
inelastic scattering at HERA
ZEUS Collaboration
M. Derrick, D. Krakauer, S. Magill, D. Mikunas, B. Musgrave, J.R. Okrasinski, J. Repond,
R. Stanek, R.L. Talaga, H. Zhang
Argonne National Laboratory, Argonne, IL, USA 51
M.C.K. Mattingly
Andrews University, Berrien Springs, Ml, USA
G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, P. Bruni,
G. Cara Romeo, G. Castellini ‘, L. Cifarelli2, F. Cindolo, A. Contin, M. Corradi, I. Gialas,
P. Giusti, G. Iacobucci, G. Laurenti, G. Levi, A. Margotti, T. Massam, R. Nania,
F. Palmonari, A. Polini, G. Sartorelli, Y. Zamora Garcia3, A. Zichichi
University and INFN Bologna, Bologna, ltaly41
C. Amelung, A. Bornheim, J. Crittenden, R. Deffner, T. Doeker4, M. Eckert, L. Feld,
A. Frey5, M. Geerts, M. Grothe, H. Hartmann, K. Heinloth, L. Heinz, E. Hilger,
H.-P. Jakob, U.F. Katz, S. Mengel 6, E. Paul, M. Pfeiffer, Ch. Rembser, D. Schramm7,
J. Stamm, R. Wedemeyer
Physikalisches Institut der Vniversitiif Bonn, Bonn, Germany38
S. Campbell-Robson, A. Cassidy, W.N. Cottingham, N. Dyce, B. Foster, S. George,
M.E. Hayes, G.P. Heath, H.F. Heath, D. Piccioni, D.G. Roff, R.J. Tapper, R. Yoshida
H.H. Wills Physics Laboratory University of Bristol, Bristol, VK50
M. Arneodo 8, R. Ayad, M. Capua, A. Garfagnini, L. Iannotti, M. Schioppa, G. Susinno
Calabria University, Physics Dept. and INFN, Cosenza, Italy41
A. Caldwellg, N. Cartiglia, Z. Jing, W. Liu, J.A. Parsons, S. Ritz lo, F. Sciulli, P.B. Straub,
L. Wai 11, S. Yang 12, Q. Zhu
Columbia University, Nevis Labs., Irvington on Hudson, N.E, VSA5’
P. Borzemski, J. Chwastowski, A. E&t-eys, Z. Jakubowski, M.B. Przybyciefi, M. Zachara,
L. Zawiej ski
Inst. qf Nuclear Physics, Cracow, Poland4i
0370-2693/96/$12.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved.
PII SO370-2693(96)00688-O
ZEUS Collaboration/Physics Letters B 384 (1996) 388-400 389
L. Adamczyk, B. Bednarek, K. Jeleri, D. Kisielewska, T. Kowalski, M. Przybycien,
E. Rulikowska-Zarebska, L. Suszycki, J. Zajac
Faculty of Physics and Nuclenr Techniques, Academy of Mining and Metallurgy, Cracow, Poland45
Z. Duliriski, A. Kotariski
Jagellonian Univ., Dept. of Physics, Cracow, Poland46
G. Abbiendi 13, L.A.T. Bauerdick, U. Behrens, H. Beier, J.K. Bienlein, G. Cases, 0. Deppe,
K. Desler, G. Drews, M. Flasihski ,
l4 D.J. Gilkinson, C. Glasman, P Giittlicher,
J. GroBe-Knetter, T. Haas, W. Hain, D. Hasell, H. Hefiling, Y. Iga, K.F. Johnson 15, P. Joos,
M. Kasemann, R. Klanner, W. Koch, U. K&z, H. Kowalski, J. Labs, A. Ladage, B. Lohr,
M. Lowe, D. Luke, J. Mainusch , .
l6 0 Mariczak, J. Milewski, T. Monteiro 17, J.S.T. Ng,
D. Notz, K. Ohrenberg, K. Piotrzkowski, M. Roco, M. Rohde, J. Roldan, U. Schneekloth,
W. Schulz, F. Selonke, B. Surrow, T. Vo13, D. Westphal, G. Wolf, U. Wollmer,
C. Youngman, W. Zeuner
Deutsches Elektronen-Synchrotron DESK Hamburg, Germany
H.J. Grabosch, A. Kharchilava ,
I8 S.M. Mari I’, A. Meyer, S. Schlenstedt, N. Wulff
DESY-IfH Zeuthen, Zeuthen, Germany
G. Barbagli, E. Gallo, P. Pelfer
University and INFN, Florence, Italy4’
G. Maccarrone, S. De Pasquale, L. Votano
INFN, Laboratori Nazionah di Frascaii, Frascati, Italy 41
A. Bamberger, S. Eisenhardt, T. Trefzger 20, S. Wolfle
Fakultiit fur Physik der Universitiit Freiburg i.Br.. Freiburg i.Br., Germany38
J.T. Bromley, N.H. Brook, P.J. Bussey, A.T. Doyle, D.H. Saxon, L.E. Sinclair, M.L. Utley,
A.S. Wilson
Dept. of Physics and Astronomy, University of Glasgow, Glasgow, UK 5o
A, Dannemann, U. Holm, D. Horstmann, R. Sinkus, K. Wick
Hamburg Universiiy, I. Institute of Exp. Physics, Hamburg, Germany38
B.D. Burow 21, L. Hagge l6 E. Lohrmann, G. Poelz, W. Schott, F. Zetsche
,
Hamburg University, II. Institute of Exp. Physics, Hamburg, Germany38
T.C. Bacon, N. Briimmer, I. Butterworth, V.L. Harris, G, Howell, B.H.Y. Hung,
L. Lamberti22, K.R. Long, D.B. Miller, N. Pavel, A. Prinias23, J.K. Sedgbeer, D. Sideris,
A.F. Whitfield
fmperial College London, High Energy Nuclear Physics Group, London, UK5’
U. Mallik, M.Z. Wang, S .M. Wang, J.T. Wu
University of Iowa Physics and Astronomy Dept., Iowa City, USA 5’
390 ZEUS Collaboration/Physics Letters B 384 (1996) 388-400
P. Cloth, D. Filges
Forschungszentrum Jiilich, Institut fiir Kernphysik, Jiilich, Germany
S.H. An, G.H. Cho, B.J. Ko, S.B. Lee, S.W. Nam, H.S. Park, SK. Park
Korea University, Seoul, South Korea43
S. Kartik, H.-J. Kim, R.R. McNeil, W. Metcalf, V.K. Nadendla
Louisiana State University, Dept. of Physics and Astronomy, Baton Rouge, LA, USAs
F. Barreiro, J.P. Fernandez, R. Graciani, J.M. Hernandez, L. Hervas, L. Labarga,
M. Martinez, J. de1 Peso, J. Puga, J. Terron, J.F. de Troconiz
Vniver. Autdnoma Madrid, Depto de Fisica Tecin’ca, Madrid, Spain4’
F. Corriveau, D.S. Hanna, 5. Hartmann, L.W. Hung, J.N. Lim, C.G. Matthews24, P.M. Patel,
M. Riveline, D.G. Stairs, M. St-Laurent, R. Ullmann, G. Zacek24
McGill University, Dept. of Physics, Montreal, Quebec, Canada36*37
T. Tsurugai
Meiji Gakuin University, Faculty of General Education, Yokohama, Japan
V. Bashkirov, B.A. Dolgoshein, A. Stifutkin
Moscow Engineering Physics Institute, Moscow, Russia 47
G.L. Bashindzhagyan , . . 25 PF Ermolov, L.K. Gladilin, Yu.A. Golubkov, V.D. Kobrin,
I.A. Korzhavina, V.A. Kuzmin, O.Yu. Lukina, A.S. Proskuryakov, A.A. Savin,
L.M. Shcheglova, A.N. Solomin, N.P. Zotov
Moscow State University, Institute of Nuclear Physics, Moscow, Russia48
M. Botje, F. Chlebana, J. Engelen, M. de Kamps, P. Kooijman, A. Kruse, A. van Sighem,
H. Tiecke, W. Verkerke, J. Vossebeld, M. Vreeswijk, L. Wiggers, E. de Wolf,
R. van Woudenberg26
NIKHEF and University of Amsterdam, Netherlands 44
D. Acosta, B. Bylsma, L.S. Durkin, J. Gilmore, C. Li, T.Y. Ling, P. Nylander, I.H. Park,
T.A. Romanowski 27
Ohio State University, Physics Department, Columbus, Ohio, USA 5L
D.S. Bailey, R.J. Cashmore , .
28 AM. Cooper-Sarkar, R.C.E. Devenish, N. Harnew,
M. Lancaster 2g, L. Lindemann, J.D. McFall, C. Nath, V.A. Noyes23, A. Quadt,
J-R. Tickner, H. Uijterwaal, R. Walczak, D.S. Waters, F.F. Wilson, T. Yip
Department of Physics, University of O.@rd, Oxford, VK50
A. Bertolin, R. Brugnera, R. Carlin, F. Dal Corso, M. De Giorgi, U. Dosselli, S. Limentani,
M. Morandin, M. Posocco, L. Stance, R. Stroili, C. Voci, F. Zuin
Dipartimento di Fisica dell’ Vniversita and INFN, Padova, Italy4’
J. Bulmahn, R.G. Feild 30, B.Y. Oh, J.J. Whitmore
Pennsylvanin State University, Dept. of Physics, University Park, PA, USA 52
ZHJS Collaboration/Physics Letters B 384 (I9961 388-400
G. D’Agostini, G. Marini, A. Nigro, E. Tassi
Dipartinzento di Fisica, Univ. ‘LA Sapienza’ and INFN, Rome, Italy41
J.C. Hart, N.A. McCubbin, T.P. Shah
Rutlzerford Appleton Laboratory, Chilton, Didcot, Oxon, UK50
39
E. Barberis, T. Dubbs, C. Heusch, M. Van Hook, W. Lockman, J.T. Rahn,
H.F.-W. Sadrozinski, A. Seiden, D.C. Williams
Ufliversity of California, Santa Cruz, CA, USAs’
J. Biltzinger, R.J. Seifert, 0. Schwarzer, A.H. Walenta, G. Zech
Faclzbereich Physik der Universitiit-GesamthocI-&zule Siegen, Germatzy38
H. Abramowicz, G. Briskin, S. Dagan 31, A. Levy 25
Sclzool of Physics, Tel-Aviv University, Tel Aviv, Israel@
J I
. . Fleck32 , M. Inuzuka, T. Ishii, M. Kuze, S. Mine, M. Nakao, I. Suzuki, K. Tokushuku,
K. Umemori, S. Yamada, Y. Yamazaki
Institute for Nuclear Study, University of Tokyo, Tokyo, Japand2
M. Chiba, R. Hamatsu, T. Hirose, K. Homma, S. Kitamura 33, T. Matsushita, K. Yamauchi
Tokyo Metropolitan University, Dept. uf Physics, Tokyo, Japan42
R. Cirio, M. Costa, M.I. Ferrero, S. Maselli, C. Peroni, R. Sacchi, A. Solano, A. Staiano
Universita di Torino, Dipartimento di Fisica Sperimentde and INFN, Torino, ItaZy41
M. Dardo
II Faculty of Sciences, Toriao Utziversiry and INFN - Alessandria, Italy41
D.C. Bailey, F. Benard, M. Brkic, C.-P. Fagerstroem, G.F. Hartner, K.K. Joo,
G.M. Levman, J.F. Martin, R.S. Orr, S. Polenz, CR. Sampson, D. Simmons, R.J. Teuscher
University of Toronto, Dept. of Physics, Toronto, Ont., Carzada36
J.M. Butterworth, C.D. Catterall, T.W. Jones, PB. Kaziewicz, J.B. Lane, R.L. Saunders,
J. Shulman, M.R. Sutton
University College London, Physics and Astronomy Dept., London, UK50
B. Lu, L.W. MO
Virginia Polytechnic Inst. and State University, Physics Dept., Blacksburg, VA. USA52
W. Bogusz, J. Ciborowski, J. Gajewski, G. Grzelak34, M. Kasprzak, M. Krzyianowski,
K . Muchorowski35, R.J. Nowak, J.M. Pawlak, T. Tymieniecka, A.K. Wroblewski,
J.A. Zakrzewski, A.F. Zarnecki
Warsaw University, Institute of Experimental Physics, Warsaw, Poland4’
M. Adamus
Institute for Nuclear Studies, Warsaw, Poiand45
392 ZEUS Collaboration/Physics Letters B 384 (1996) 388-400
C. Coldewey, Y. Eisenberg 31, D. Hochman, U. Karshon 31, D. Revel 31, D. Zer-Zion
Weimnn Institute, Nuclear Physics Dept., Rehovot, Israel 3g
W.F. Badgett, J. Breitweg, D. Chapin, R. Cross, S. Dasu, C. Foudas, R.J. Loveless,
S. Mattingly, D.D. Reeder, S. Silverstein, W.H. Smith, A. Vaiciulis, M. Wodarczyk
University of Wisconsin, Dept. oj’ Physics, Madison, WI, USA 5L
S. Bhadra, M.L. Cardy, C.-P. Fagerstroem, W.R. Frisken, K.M. Furntani, M. Khakzad,
W.N. Murray, W.B. Schmidke
York University, Dep. ctf Physics, North York, Oat., Canada36
Received 24 May 1996
Editor: L. Montanet
Abstract
In deep inelastic neutral current scattering of positrons and protons at the center of mass energy of 300 GeV, we observe,
with the ZEUS detector, events with a high energy neutron produced at very small scattering angles with respect to the
proton direction. The events constitute a fixed fraction of the deep inelastic, neutral current event sample independent of
Bjorken x and Q2 in the range 3. 10v4 < XBJ < 6. 10-s and 10 < Q2 < 100 GeV*.
1 Also at IROE Florence, Italy.
2 Now at Univ. of Salerno and INFN Napoli, Italy.
3 Supported by Worldlab, Lausanne, Switzerland.
4 Now as MINERVA-Fellow at Tel-Aviv University.
5 Now at Univ. of California, Santa Cruz.
6 Now at VDI-Technologiezentrum Dusseldorf.
7 Now at Commasoft, Bonn.
8Also at University of Torino and Alexander von Humboldt
Fellow.
9 Alexander von Humboldt Fellow.
lo Alfred P. Sloan Foundation Fellow.
‘t Now at University of Washington, Seattle.
l2 Now at California Institute of Technology, Los Angeles.
l3 Supported by an EC fellowship number ERBFMBICT 950172.
l4 Now at Inst. of Computer Science, Jagellonian Univ., Cracow.
l5 %sitor from Florida State University.
l6 Now at DESY Computer Center.
I7 Supported by European Community Program PRAXIS XXI.
l8 Now at Univ. de Strasbourg.
lg Present address: Dipartimento di Fisica, Univ. “La Sapienza”,
Rome.
*O Now at ATLAS Collaboration, Univ. of Munich.
*l Also supported by NSERC, Canada.
22 Supported by an EC fellowship.
23 PPARC Post-doctoral Fellow.
24 Now at Park Medical Systems Inc., Lachine, Canada.
25 Partially supported by DESY.
26 Now at Philips Natlab, Eindhoven, NL.
27 Now at Department of Energy, Washington.
28 Also at University of Hamburg, Alexander von Humboldt Re-
search Award.
29 Now at Lawrence Berkeley Laboratory, Berkeley.
3o Now at Yale University, New Haven, CT.
31 Supported by a MINERVA Fellowship.
32 Supported by the Japan Society for the Promotion of Science
(JSPS).
33 Present address: Tokyo Metropolitan College of Allied Medical
Sciences, Tokyo 116, Japan.
34 Supported by the Polish State Committee for Scientific Re-
search, grant No. 2P03B09308.
35 Supported by the Polish State Committee for Scientific Re-
search, grant No. 2PO3B09208.
36 Supported by the Natural Sciences and Engineering Research
Council of Canada (NSERC).
37 Supported by the FCAR of Quebec, Canada.
38 Supported by the German Federal Ministry for Education and
Science, Research and Technology (BMBF), under contract num-
bers 056BN191, 056FR19P, 056HH191, 056HH291,056SI791.
3g Supported by the MINERVA Gesellschaft ftlr Forschung GmbH,
the Israel Academy of Science and the U.S.-Israel Binational
Science Foundation.
4o Supported by the German Israeli Foundation, and by the Israel
Academy of Science.
41 Supported by the Italian National Institute for Nuclear Physics
(INFN).
42 Supported by the Japanese Ministry of Education, Science and
Culture (the Monbusho) and its grants for Scientific Research.
43 Supported by the Korean Ministry of Education and Korea
Science and Engineering Foundation.
44 Supported by the Netherlands Foundation for Research on Mat-
ZEUS Collaboration/Physics Leners B 384 (19961388-400 393
1. Introduction
The general features of the hadronic final state in
deep inelastic leptonnucleon scattering (DIS) are well
described by models inspired by Quantum Chromody-
namics (QCD) . In these models the struck quark and
the colored proton remnant evolve into a system of
partons which fragments into hadrons. Many of these
models neglect peripheral processes, which are char-
acterized by leading baryons.
A recent example of peripheral processes is the ob-
servation by ZEUS [ 11 and Hl [ 21 of DIS events with
large rapidity gaps. These events are distinguished by
the absence of color flow between the final state bary-
onic system and the fragments of the virtual photon,
and they have been interpreted as arising from diffrac-
tion. In the language of Regge trajectories, a pomeron
IP, with the quantum numbers of the vacuum, is ex-
changed between the proton and the virtual photon.
Another example is provided by meson ex-
change [ 3-81, which plays a major role in peripheral
hadronic scattering. In this process, the incoming pro-
ton fluctuates into a baryon and a meson. At HERA
energies, the lifetime of this state can be sufficiently
long that the lepton may interact with the meson. In
p + p transitions the exchange of neutral mesons
occurs together with diffractive scattering. These
contributions may be separable by measuring the
proton momentum distribution. On the other hand,
p 4 n transitions signal events where charged meson
ter (FOM).
45 Supported by the Polish State Committee for Scientific Re-
search, grants 115/E-343/SPUB/P03/109/95,2P03B 244 08~02,
~03, pO4 and ~05, and the Foundation for Polish-German Collab-
oration (proj. No. 506/92).
46 Supported by the Polish State Committee for Scientific Research
(grant No. 2 P03B 083 08) and Foundation for Polish-German
Collaboration.
47 Partially supported by the German Federal Ministry for Educa-
tion and Science, Research and Technology (BMBF).
48 Supported by the German Federal Ministry for Education and
Science, Research and Technology (BMBF), and the Fund of Fun-
damental Research of Russian Ministry of Science and Education
and by INTAS-Grant No. 93-63.
4g Supported by the Spanish Ministry of Education and Science
through funds provided by CICYT.
so Supported by the Particle Physics and Astronomy Research
Council.
s1 Supported by the US Department of Energy.
52 Supported by the US National Science Foundation.
exchange could dominate [9,10], regardless of the
neutron momentum. The pion, being the lightest me-
son, may provide the largest contribution to the cross
section. Isolation of the one pion exchange contribu-
tion would provide the opportunity to study virtual
gamma pion interactions and thereby determine the
structure function of the pion.
In order to study these issues we have installed a
hadronic calorimeter to detect high energy forward go-
ing neutrons produced in DIS (ep -+ en+anything)
at HERA. This paper reports the first observation of
such events, showing clear evidence of sizeable lead-
ing neutron production.
2. Experimenta setup
The data were collected with the ZEUS detector
during 1994 while HERA collided 153 ep bunches of
27.5 GeV positrons and 820 GeV protons. In addi-
tion, 15 unpaired bunches of positrons and 17 unpaired
bunches of protons circulated, permitting a measure-
ment of beam associated backgrounds. The data sam-
ple used in this analysis corresponds to an integrated
luminosity of 2.1 pb-‘.
The present analysis makes use of a test Forward
Neutron Calorimeter (FNC II) [ 1 l] installed at the
beginning of 1994 in the HERA tunnel at 6 = 0 de-
grees, Z = 102 m, downstream of the interaction
53 point . The layout of the beam line and calorimeter
is shown schematicaIly in Fig. 1. FNC II, located after
the final station of the ZEUS Leading Proton Spec-
trometer (LPS) , was an enlarged and improved ver-
sion of the original test Forward Neutron Calorimeter
(FNC I) which operated in 1993. The design, con-
struction and calibration of FNC II was similar to FNC
I [ 12,131. Both devices were iron-scintillator sand-
wich calorimeters read out with wavelength shifter
light guides coupled to photomultiplier tubes (PMT) .
The unit cell consisted of 10 cm of iron followed by
0.5 cm of SCSN-38 scintillator. FNC II contained 17
unit cells comprising a total depth of 10 interaction
lengths. It was 40 cm wide and 30 cm high, divided
vertically into three 10 cm towers read out on both
53 The ZEUS coordinate system is defined as right handed with
the Z axis pointing in the proton beam direction and the X axis
horizontal, pointing towards the center of HERA.
ZEUS Collaboration/Physics Letters B 384 (1996) 388-400
ty the inactive material in front of FNC II. The calorime-
O(mrod)
(cl (4
Fig. 1. (a) Schematic layout of the proton beam line viewed
from the side with FNC II (at Z = 101 m) below the beam pipe
and downstream of LPS stations SlS6. (b) Schematic drawing
of FNC II viewed from the top. (c) Front view of FNC II
showing the segmentation into three towers, and the projected
region of geometric aperture allowed by the HERA magnets. The
cross indicates the position of the zero degree line. (d) The
geometric acceptance as a function of polar angle (scattering
angle), integrated over azimuth.
sides. There was no longitudinal subdivision in the
readout.
The neutron calorimeter was situated downstream
of the HERA dipoles which bend the 820 GeV proton
beam upwards. Charged particles originating at the in-
teraction point were swept away from FNC II. The
aperture of the HERA magnets in front of FNC II lim-
ited the geometric acceptance as shown in Figs. 1 (c)
and (d) . Between these magnets and FNC II the neu-
trons encountered inactive material, the thickness of
which varied between one and two interaction lengths.
Two scintillation veto counters preceded the calorime-
ter:one30x25x5cm3,andone40x30x1cm’.These
counters were used offline to identify charged parti-
cles and thereby reject neutrons which interacted in
ter was followed by two scintillation counters, which
were used in coincidence with the front counters to
identify beam halo muons. The response of the coun-
ters to minimum ionizing particles was determined
with these muons.
Energy deposits in FNC II were read out using a
system identical to that of the ZEUS uranium scintil-
lator calorimeter (CAL). In addition the rate of sig-
nals exceeding a threshold of 250 GeV was recorded.
The accumulated counts provide the average counting
rate of FNC II for each run.
The other components of ZEUS have been de-
scribed elsewhere [ 141. The CAL, the central tracking
detectors (CTD,VXD) , the small angle rear tracking
detector (SRTD) which is a scintillator hodoscope in
front of the rear calorimeter close to the beam pipe,
and the luminosity monitor (LUMI) are the main
components used for the analysis of DIS events [ 151.
3. Kinematics of deep inelastic events
In the present analysis the two particle inclusive
reaction ep 4 ept+anything is compared with the
single particle inclusive reaction ep -+ e+anything.
In both cases the scattered positron and part of the
hadronic system, denoted by X, were detected in CAL.
Energetic forward neutrons were detected in FNC 11.
The two particle inclusive events are specified by four
independent kinematical variables: any two of XnJ, Q2,
y, and W for the scattered lepton; and any two of XL,
pr, and t for the leading baryon (see below).
Diagrams for one and two particle inclusive ep scat-
tering are shown in Figs. 2(a) and (b). The con-
ventional DIS kinematical variables describe the scat-
tered positron: Q2, the negative of the squared four-
momentum transfer carried by the virtual photon y*,
Q2 E -q2 = -(j$ - k’)2,
where k and k’ are the four-momentum vectors of
the initial and final state positron respectively; y, the
energy transfer to the hadronic final state
where P is the four-momentum vector of the incoming
proton; xnj, the Bjorken variable
ZEUS Collaboration/ Physics Leners B 384 (19%) 388-400 395
ZEUS 1994
(b)
3
1o3(d) lo X.,
100 200 300
(e) W (GeV) m W (GeV)
Fig. 2. (a) Diagram for the inclusive reaction ep + e-/-anything,
(b) for the two pxticle inclusive reaction ep -+ enfanything, a
special case of (a) where the hadronic system of mass W contains
a forward neutron. The parl of the hadronic system detected by
CAL is denoted by X and has a mass Mx. (c) A scatter plot
of Q2 versus .xn~ for DIS events, and (d) neutron tagged DIS
events with E,, > 400 GeV corresponding to (c). (e) A scatter
plot of Mx versus W for DIS events. The events in the band at
low Mx (larger dots) are the large rapidity gap events. (f) A
scatter plot of MX versus W for neutron tagged DIS events with
E,, > 400 GeV. The LRG events are plotted as squares.
where s is the center-of-mass (c.m.) energy squared
of the ep system; and W, the c.m. energy of the y*p
system,
w2 E (q+fy= Q=(l - XBJ)
XBJ + Mp2,
where M, is the mass of the proton.
The “double angle method” was used to determine
XBJ and Q2 [ 161. In this method, event variables are
derived from the scattering angle of the positron and
the scattering angle YH of the struck (massless) quark.
The latter angle is determined from the hadronic en-
ergy flow measured in the main ZEUS detector,
cGPx)2 + (CiPd2 - (C,(E -pz))2
cosyH = (CiPX>2 + (~;Pr)2 + (&(E - pz))2 ’
where the sums run over all CAL cells i, exclud-
ing those assigned to the scattered positron, and p =
(px, py , pz > is the momentum vector assigned to each
cell of energy E. The cell angles are calculated from
the geometric center of the cell and the vertex posi-
tion of the event. Final state particles produced close
to the direction of the proton beam give a negligi-
ble contribution to cosy~, since these particles have
(E-pz) N 0.
In the double angle method, in order that the
hadronic system be well measured, it is necessary
to require a minimum hadronic energy in the CAL
away from the beam pipe. A suitable quantity for this
purpose is the hadronic estimator of the variable y,
defined by
YJB _ CicE -PZ)
2E, ’
where E, is the electron beam energy.
The two independent kinematical variables describ-
ing the neutron tagged by FNC II are taken to be its
energy E,, and transverse momentum PT. These quanti-
ties are related to the four-momentum transfer squared
between the proton and the neutron, t, by
t N -p$ - (l iLXL) (M,2 _ xLM;) )
where A4,t is the mass of the neutron and XL 3 E,/Ep,
where Ep is the proton beam energy. The geometry
of FNC II and the HERA beam line limited the an-
gular acceptance of the scattered neutron to 6 5 0.75
mrad, and the threshold on energy deposits in FNC II
restricted XL to XL > 0.3.
The invariant mass of the hadronic system detected
in the calorimeter, Mx, can be determined from the
cell information in CAL, an approach similar to the
double angle method is applied to calculate Mx. Given
the energy, EH, the momentum, pi, and the polar an-
gle, tl~, of the hadronic system observed in the detec-
tor, the following formulae determine Mx: cos I~H =
Cipz/] CipJ, where the sum runs over all calorime-
ter cells i, excluding those assigned to the positron,
&j = Q2( 1 - y) / sin2 I)H, EH = 2E,y + PH cos OH,
&lx = l,/igqg.
The identification of neutral current deep inelastic
events uses the quantity 6 defined by
396 ZElJS Collaboration/Physics Letters B 384 (1996) 388-400
s= C(E-Pz),
i
where the sum runs over all CAL cells i. For fully
contained neutral current DIS events, and neglecting
CAL resolution effects and initial state radiation, 6 =
2E,.
We also use the variable vrnax which is defined as
the pseudorapidity,
77 z -1ntan (f3/2),
of the calorimeter cluster with energy greater than 400
MeV closest to the proton beam direction.
4. Monte Carlo simulation and studies
The response of FNC II was modeled by a Monte
Carlo (MC) simulation using the GEANT pro-
gram [ 171. The model was inserted into the full
simulation of the ZEUS detector and beam line. For
neutrons incident on the face of the calorimeter the
predicted energy resolution is approximately a( E,, ) =
2.0&,,, with E, in GeV. The predicted energy re-
sponse of the calorimeter is linear to better than 5%.
To aid the study of energetic neutron production
both in beam gas collisions and in DIS, a Monte
Carlo generator was written for one pion exchange,
which gives a cross section proportional to It\ . ( 1 -
XiJ1-2nqc&t) ( see, for example, [9] >, where
cl*(t) = ,&(t - mi) is the pion trajectory and LYE =
1 GeV2. The code uses, as a framework, the HER-
WIG program [ 181. Absorptive corrections to one
pion exchange have been widely discussed (see, for
example, [ 19-2 1 ] ) . To estimate such effects a simple
prescription which replaces ItI by ItI + rni in the nu-
merator of the above expression was used. In addition
to the one pion exchange model, the standard QCD
inspired DIS models ARIADNE [ 221, HERWIG, and
MEPS [ 231 were used to predict the forward neutron
production.
To compare data with the expectations of all these
models, the MC events produced by the generators
were fed through the simulation of the ZEUS detector.
5. Calibration and acceptance of FNC II
The relative gains of the PMTs were determined by
scanning each tower with a 6oCo gamma source us-
ing the procedure developed for the ZEUS CAL [ 241.
This was done at the end of the data taking period.
Beam gas data taken in HERA were used for calibra-
tion. These data were obtained after the proton beam
was accelerated to 820 GeV, but before positrons were
injected. To reject events where the neutrons had show-
ered in material upstream of FNC II, events were con-
sidered only when the energy deposited in the veto
counters was below that of a minimum ionizing par-
ticle.
The HERA beam gas interactions occur at c.m. en-
ergies similar to those of p -+ n data measured at Fer-
milab and the ISR [25] where neutron spectra were
found to be in good agreement with the predictions of
one pion exchange [9,25]. The energy scale of FNC
II was determined by fitting the observed beam gas
spectrum above 600 GeV to that expected from one
pion exchange, folded with the response of FNC II
as simulated by MC. The error in the energy scale is
estimated as 5%.
Proton beam gas data taken during a special run at
proton energies of 150, 300, 448, 560, 677, and 820
GeV showed that the energy response of FNC II was
linear to within 4%.
To correct for the drift in gains of the PMTs, pro-
ton beam gas data were taken with an FNC trigger
approximately every two weeks. The mean response
of each tower showed variations between calibration
runs at the level of 3%.
The overall acceptance for neutrons, An\lc, is in-
dependent of the acceptance of the main detector. To
determine Amc, the inactive material obscuring the
aperture had to be modeled. About half of the inactive
material was of simple geometric shape and included
in the ZEUS detector simulation. The remainder, con-
sisting mostly of iron between the beam line elements
and FNC II, was modeled by an iron plate. The thick-
ness of this plate was adjusted so that the resulting
MC energy spectrum of neutrons from beam gas in-
teractions matched the observed spectrum. Since the
interaction of neutrons in the material leads, in gen-
eral, to the loss of energy either by absorption and/or
by particle emission outside the acceptance of FNC
II, the observed energy spectrum is very sensitive to
ZEUS Collaboration/Physics Letters B 384 (1996) 388-400 397
the amount of inactive material upstream. Therefore,
in this study of inactive material, events in which the
neutrons began showering upstream of FNC 11 were
included in the spectrum; that is, no cut was made on
charged particles in the scintillator counters in front
of FNC II. The resulting thickness of the plate was
16f7 cm. Because of interactions in the inactive ma-
terial, only about 15% of neutrons with energy En >
250 GeV which pass through the geometric aperture
reach FNC II and survive the scintillator cuts. The ac-
ceptance is constant within 15% for neutrons with en-
ergy 400 < E, < 820 GeV scattered at a fixed angle
in the range 0 to 0.7 mrad.
The overall acceptance assuming one pion exchange
with the form described in Section 4 is 4.9f_:$% for
neutrons with En > 400 GeV and Jt( < 0.5 GeV2.
The error quoted is dominated by the systematic error
in estimating the amount of inactive material in front
of FNC II.
To study the effect of uncertainties in the theoreti-
cal form of the cross section for one pion exchange,
the part of the acceptance due to the geometric aper-
ture, as shown in Fig. 1 (d) , was calculated for several
proposed forms [ 7,9,10] _ It was found to vary from
approximately 32% to 35%. This part of the accep-
tance for p exchange varies between 10% and 30%,
depending on the model [ 7,261.
6. Triggering and data selection
The selection was almost identical to that used for
the measurement of the structure function Fz [ 151.
Events were filtered online by a three level trigger
system [ 141. At the first level DIS events were se-
lected by requiring a minimum energy deposition in
the electromagnetic section of the CAL. The thresh-
old depended on the position in the CAL and varied
between 3.4 and 4.8 GeV. For events selected with the
analysis cuts listed below, this trigger was more than
99% efficient for positrons with energy greater than
7 GeV, as determined by Monte Carlo studies.
At the second level trigger (SLT) , background was
further reduced using the measured times of energy
deposits and the summed energies from the calorime-
ter. The events were accepted if
8s~~ 3 CEi(l - COS Oi) > 24 GeV - 2E,,
i
where Ei and Bi are the energies and polar angles (with
respect to the primary vertex position) of calorimeter
cells, and E, is the energy deposit measured in the
LUMI photon calorimeter. For perfect detector reso-
lution and acceptance, Ss, is twice the positron beam
energy (55 GeV) for DIS events, while for photopro-
duction events, where the scattered positron escapes
down the beam pipe, Sst~ peaks at much lower values.
The full event information was available at the third
level trigger (TLT). Tighter timing cuts as well as
algorithms to remove beam halo muons and cosmic
muons were applied. The quantity 8nT was deter-
mined in the same manner as for 8s~~. The events
were required to have ST, > 25 GeV - 2E,. Finally,
events were accepted as DIS candidates if a scattered
positron candidate of energy greater than 4 GeV was
found.
In the analysis of the resulting data set, further se-
lection criteria were applied both to ensure accurate
reconstruction of the kinematical variables, and to in-
crease the purity of the sample by eliminating back-
ground from photoproduction. These cuts were:
EL > 8 GeV,
YJB > 0.04, ye < 0.95,
1x1 > 14 cm or [YI > 13 cm,
-40 < Z,,,,,, < 40 cm,
35 < 6 < 65 GeV,
where yc is y evaluated from the scattered positron
energy, EL, and angle; X and Y are the impact position
of the positron on the CAL as determined using the
SRTD. The cut on 1x1, IY[ is a fiducial volume cut
to avoid the region directly adjacent to the rear beam
pipe.
Beam conditions sometimes resulted in a large FNC
II counting rate from energy deposits above the thresh-
old of 250 GeV. Runs were rejected if the counting
rate, averaged over the run, was greater than 5 kHz in
order to reduce the probability of a beam gas interac-
tion randomly overlapping a true DIS event. Neutron
tagged events were selected by requiring that FNC II
show an energy deposit above threshold, and that the
398 ZEUS Collaboration/ Physics Letters B 384 (1996) 388-400
scintillation veto counters show an energy deposit be-
low that of a minimum ionizing particle.
This study is restricted to events with Q2 > 10
GeV2 [ 11. After these selections, 112k events re-
main containing 669 neutron tagged events constitut-
ing 0.6% of the sample.
7. Backgrounds
The counting rate of FNC II is predominantly due
to protons interacting with residual gas in the beam
pipe. As a result, the main background is due to the
random overlap of energetic neutrons from beam gas
interactions with genuine DIS events.
The fraction of beam gas triggers which survive the
scintillation counter charged particle veto was mea-
sured to be 54&4%. The average raw counting rate of
FNC II during the taking of ep data was 1.5 kHz leav-
ing an effective counting rate of 833 Hz after the cuts.
With 170 proton bunches in 220 HERA RF buckets
and a crossing time of 96 ns, the overlap probability of
a neutron with a random bunch was 1 .O . 10w4. Since
neutrons are tagged in 0.6% of the events,
signal 0.6. 1O-2
background = 1.0 . 1O-4 = 60.
Thus only 1.7% of the neutron tagged events result
from random overlaps. The same result is obtained if
the background is calculated on a run by run basis.
The small random coincidence rate was confirmed
by the rate of neutrons in non ep background events
(cosmic rays and beam halo muons), and in a sample
of random triggers.
For the DIS selection, the background from photo-
production was estimated to be less than 1% overall.
A sample of photoproduction events was studied to
rule out the possibility that the observed rate of neu-
trons in DIS was due to an anomalously large produc-
tion rate of neutrons in photoproduction. A fractional
rate in photoproduction comparable to that in DIS was
found, verifying that the photoproduction background
after the neutron tag was also less than 1%. The same
conclusion holds for the background from beam gas
interactions.
ZEUS1994
OOW 200 300
O’~“‘,‘~“,‘~,‘,‘~“~‘~’
-2.5 0 2.5 5 7.5
(a) W (GeV) Cc) %a.
Fig. 3. (a) The observed ratio of neutron tagged DIS events with
En > 400 GeV to all DIS events as a function of X~J, Q2 and W.
(b) The data points show the -qrnax distribution for tagged DIS
events with E,, > 400 GeV. The distribution for all DIS events
muhiplied by 0.45 IO-’ is superimposed as a histogram. (c)
The observed ratio of tagged DIS events with E,, > 400 GeV to
all DIS events as a function of T,,,~~.
8. Characteristics of events with a leading neutron
The production of neutron tagged events with neu-
tron energy E, > 400 GeV was studied as a function
of the lepton kinematical variables. Fig. 2(c) shows a
scatter plot of Q2 versus XBJ for a sample of 10k DIS
events which were not required to have a neutron tag.
All events in the full sample with a neutron tag are
shown in Fig. 2(d) . The neutron tagged events follow
the distribution of DIS events. This is demonstrated
quantitatively in Fig. 3 (a) which shows the ratio runt
of tagged events to all events, uncorrected for accep-
tance, as a function of XBJ, Q2 and W. Within the
statistical accuracy, Y,,, is consistent with being con-
stant. This is also true if we take the ratio as a func-
tion of Q2 in bins of XnJ (not shown). Averaged over
the XnJ and Q2 region the value of the ratio is TUnc =
0.45 i 0.02 i 0.02 % for En > 400 GeV. The first er-
ror is statistical and the second systematic. The latter
is dominated by the neutron energy scale uncertainty.
Further insight is gained by examining the scatter
plot of Mx versus W shown in Fig. 2(e) for the sam-
ple of 10k events. In this plot, there is a concentration
ZEUS Collaboration/Physics Letfers B 384 (1996) 388-400
of events at low Mx. These events are found to have
a large rapidity gap (LRG) , vmax < 2.0. The neutron
tagged events are distributed similarly to the full sam-
ple, as seen in Fig. Z(f). There is a concentration of
a few events with a rapidity gap at low Mx, but most
neutron tagged events are above the low Mx band.
The vmax distributions for all DIS events and for
neutron tagged DIS events are similar in shape for
qmax 2 2 (Fig. 3(b)), showing an exponential rise
for 2 5 vmax ,< 3.5. Note that for vmax 2 4 the dis-
tributions are strongly affected by limited acceptance
towards the forward beam hole 54.
For vmax 5 2.0 there are relatively fewer neutron
tags in the LRG events by a factor of about 2: the small
?;lmax events represent 7% of all DIS events, but only
3% of the neutron tagged DIS events. This is shown
in the plot of Y,,,~ as a function of vrnax in Fig. 3 (c) .
LRG events with a leading neutron are expected, for
instance, from diffractive production of a baryonic
system decaying to an energetic forward neutron and
from double peripheral processes, where a pomeron
is exchanged between the virtual photon and the vir-
tual pion emitted from the proton. This effect warrants
further study.
The measured fraction of DIS events with a leading
neutron with En > 400 GeV, FUnc = 0.45 f 0.02 f
0.02 %, can be compared with the predictions of
models for DIS at HERA. ARIADNE [22], which
is a colour dipole model including the boson gluon
fusion process, in general gives a good description of
the hadronic final state in DIS at HERA. The value of
FUnc predicted by ARIADNE is 0.13 f 0.05%, where
the error is due to the uncertainty in the acceptance.
This is a factor of about 3 less than that observed.
Fig. 4(a) shows the observed energy spectrum of
neutrons tagged above 250 GeV by FNC II. The
shape of the neutron energy distribution predicted by
ARIADNE fails to describe the data, as seen from
the dashed histogram in Fig. 4(a). The DIS models
MEPS 1231 and HERWIG [ 181 predict a higher rate
of neutrons by about a factor of 2 but still fail to
reproduce the observed energy spectrum.
The result of the one pion exchange Monte Carlo
calculation of the expected spectrum is superimposed
54 Values of qmax > 4.3 are an artifact of the clustering algorithm,
and may occur when particles are distributed in contiguous cells
around the beam pipe.
399
ZEUS 1994
(a) E. (GeV)
(c) 0’ (GeV*)
Fig. 4. (a) The energy distribution of neutrons tagged by FNC
II, uncorrected for acceptance. The solid points are data and the
histogram is the result of a one pion exchange DIS Monte Carlo
calculation normalized to the number of events greater than 400
GeV. The dashed histogram gives the prediction of ARIADNE
normalized to the same luminosity as the data. (b) and (c) The
variation of the mean EAV and width fls of the neutron energy
spectrum above 250 GeV as a function of XnJ and Q*.
on the energy spectrum in Fig. 4(a), normalized to
the total number of events above 400 GeV. There is
reasonably good agreement between the Monte Carlo
simulation and the data at energies above 400 GeV.
At lower energies, other exchanges, such as the p,
may become important. The neutron energy distribu-
tion shows no indication of varying with XnJ or Q2.
This is demonstrated in Fig. 4(b) and (c) where the
mean EAT and width UE of the neutron energy distri-
bution are shown as functions of XBJ and Q2.
If AFNC as determined for one pion exchange is
taken together with FUnc as measured in the data,
9.1+$% of DIS events have a neutron with energy
E,l > 400 GeV and ItI < 0.5 GeV2. The prescrip-
tion for absorptive corrections discussed in Section 4
decreases this fraction by about 8%.
9. Conclusions
We have observed energetic forward neutron pro-
duction in DIS at HERA. The neutrons are detected
400 ZEUS Collaboration/ Physics Letters B 384 (I 996) 388-400
at very small scattering angles, 6 5 0.75 mrad, and at
high xL = E,,/E,, XL > 0.3. Within present statistics
leading neutron production is a constant fraction of
DIS independent of xnJ and Q2 in the range 3. 10v4 <
xnJ < 6 . low3 and 10 < Q* < 100 GeV2. Further-
more, the neutron energy spectrum shows no variation
of its mean or width with XBJ and Q2. Neutrons with
energy E, > 400 GeV and ItI < 0.5 GeV2 account
for a substantial fraction (at the level of 10%) of DIS
events.
Acknowledgments
We acknowledge helpful discussions with E. Gots-
man, G. Ingelman, N. Nikolaev, F. Schrempp, A.
Szczurek and P. Zerwas. We thank F. Czempik, A.
Kiang, H. Schult, V. Sturm, and K. Westphal for their
help with the design and construction of the calorime-
ter. We also thank the HERA machine staff for their
forbearance during the operation of the FNC. We
especially appreciate the strong support provided by
the DESY Directorate.
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