Observation of Events with an Energetic Forward Neutron in Deep Inelastic Scattering at HERA ZEUS Collab oration Abstract In deep inelastic neutral current scattering of p ositrons and protons at the center of mass energy of GeV we observe with the ZEUS detector events with a high energy neutron pro duced at very small scattering angles with resp ect to the proton direction The events constitute a xed fraction of the deep inelastic neutral current event sample indep endent of Bjorken x and Q in the range x BJ and Q GeV Intro duction The general features of the hadronic nal state in deep inelastic lepton nucleon scattering DIS are well describ ed by mo dels inspired by Quantum Chromo dynamics QCD In these mo dels the struck quark and the colored proton remnant evolve into a system of partons which fragments into hadrons Many of these mo dels neglect p eripheral pro cesses which are characterized by leading baryons A recent example of p eripheral pro cesses is the observation by ZEUS and H of DIS events with large rapidity gaps These events are distinguished by the absence of color ow b etween the nal state baryonic system and the fragments of the virtual photon and they have b een interpreted as arising from diraction In the language of Regge tra jectories a p omeron IP with the quantum numb ers of the vacuum is exchanged b etween the proton and the virtual photon Another example is provided by meson exchange which plays a ma jor role in p eripheral hadronic scattering In this pro cess the incoming proton uctuates into a baryon and a meson At HERA energies the lifetime of this state can b e suciently long that the lepton may interact with the meson In p p transitions the exchange of neutral mesons o ccurs together with diractive scattering These contributions may b e separable by measuring the proton momentum distribution On the other hand p n transitions signal events where charged meson exchange could dominate regardless of the neutron momentum The pion b eing the lightest meson may provide the largest contribution to the cross section Isolation of the one pion exchange contribution would provide the opp ortunity 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 going neutrons pro duced in DIS ep enanything at HERA This pa p er rep orts the rst observation of such events showing clear evidence of sizeable leading neutron pro duction Exp erimental setup The data were collected with the ZEUS detector during while HERA collided ep bunches of GeV p ositrons and GeV protons In addition unpaired bunches of p ositrons and unpaired bunches of protons circulated p ermitting a measurement of b eam asso ciated backgrounds The data sample used in this analysis corresp onds to an integrated luminosity of pb The present analysis makes use of a test Forward Neutron Calorimeter FNC I I installed at the b eginning of in the HERA tunnel at degrees Z m downstream of the interaction p oint The layout of the b eam line and calorimeter is shown schematically in Fig FNC I I lo cated after the nal station of the ZEUS Leading Proton Sp ectrometer LPS was an enlarged and improved version of the original test Forward The ZEUS co ordinate system is dened as right handed with the Z axis p ointing in the proton b eam direction and the X axis horizontal p ointing towards the center of HERA Neutron Calorimeter FNC I which op erated in The design construction and cali bration of FNC II was similar to FNC I Both devices were ironscintillator sandwich calorimeters read out with wavelength shifter light guides coupled to photomultiplier tub es PMT The unit cell consisted of cm of iron followed by cm of SCSN scintillator FNC II contained unit cells comprising a total depth of interaction lengths It was cm wide and cm high divided vertically into three cm towers read out on b oth sides There was no longitudinal sub division in the readout The neutron calorimeter was situated downstream of the HERA dip oles which b end the GeV proton b eam upwards Charged particles originating at the interaction p oint were swept away from FNC I I The ap erture of the HERA magnets in front of FNC II limited the geometric acceptance as shown in Figs c and d Between these magnets and FNC II the neutrons encountered inactive material the thickness of which varied b etween one and two interaction lengths Two scintillation veto counters preceded the calorimeter one x x cm and one x x cm These counters were used oine to identify charged particles and thereby reject neutrons which interacted in the inactive material in front of FNC I I The calorimeter was followed by two scintillation counters which were used in coincidence with the front counters to identify b eam halo muons The resp onse of the counters to minimum ionizing particles was determined with these muons Energy dep osits in FNC II were read out using a system identical to that of the ZEUS uranium scintillator calorimeter CAL In addition the rate of signals exceeding a threshold of GeV was recorded The accumulated counts provide the average counting rate of FNC II for each run The other comp onents of ZEUS have b een describ ed elsewhere The CAL the central tracking detectors CTDVXD the small angle rear tracking detector SRTD which is a scintillator ho doscop e in front of the rear calorimeter close to the b eam pip e and the luminosity monitor LUMI are the main comp onents used for the analysis of DIS events Kinematics of deep inelastic events In the present analysis the two particle inclusive reaction ep enanything is compared with the single particle inclusive reaction ep eanything In b oth cases the scattered p ositron and part of the hadronic system denoted by X were detected in CAL Energetic forward neutrons were detected in FNC I I The two particle inclusive events are sp ecied by four indep endent kinematical variables any two of x Q y and W for the scattered BJ lepton and any two of x p and t for the leading baryon see b elow L T Diagrams for one and two particle inclusive ep scattering are shown in Fig a and b The conventional DIS kinematical variables describ e the scattered p ositron Q the negative of the squared fourmomentum transfer carried by the virtual photon 0 Q q k k 0 where k and k are the fourmomentum vectors of the initial and nal state p ositron re sp ectively y the energy transfer to the hadronic nal state q P y k P where P is the fourmomentum vector of the incoming proton x the Bjorken variable BJ Q Q x BJ q P y s where s is the centerofmass cm energy squared of the ep system and W the cm energy of the p system Q x BJ M W q P p x BJ where M is the mass of the proton p The double angle metho d was used to determine x and Q In this metho d BJ event variables are derived from the scattering angle of the p ositron and the scattering angle of the struck massless quark The latter angle is determined from the hadronic energy H ow measured in the main ZEUS detector P P P E p p p Z Y X i i i cos P P P H E p p p Z Y X i i i where the sums run over all CAL cells i excluding those assigned to the scattered p ositron and p p p p is the momentum vector assigned to each cell of energy E The X Y Z cell angles are calculated from the geometric center of the cell and the vertex p osition of the event Final state particles pro duced close to the direction of the proton b eam give a negligible contribution to cos since these particles have E p H Z In the double angle metho d in order that the hadronic system b e well measured it is necessary to require a minimum hadronic energy in the CAL away from the b eam pip e A suitable quantity for this purp ose is the hadronic estimator of the variable y dened by P E p Z i y JB E e where E is the electron b eam energy e The two indep endent kinematical variables describing the neutron tagged by FNC II are taken to b e its energy E and transverse momentum p These quantities are related n T to the fourmomentum transfer squared b etween the proton and the neutron t by x p L T M x M t L n p x x L L where M is the mass of the neutron and x E E where E is the proton b eam energy n L n p p The geometry of FNC II and the HERA b eam line limited the angular acceptance of the mrad and the threshold on energy dep osits in FNC II restricted scattered neutron to x to x L L The invariant mass of the hadronic system detected in the calorimeter M can b e X determined from the cell information in CAL an approach similar to the double angle metho d is applied to calculate M Given the energy E the momentum p and the X H H p olar angle of the hadronic system observed in the detector the following formulae H P P pj where the sum runs over all calorimeter cells i p j determine M cos Z X H i i excluding those assigned to the p ositron p Q y sin E E y p cos H H e H H H q M E p X H H The identication of neutral current deep inelastic events uses the quantity dened by X E p Z i where the sum runs over all CAL cells i For fully contained neutral current DIS events and neglecting CAL resolution eects and initial state radiation E e We also use the variable which is dened as the pseudorapidity max ln tan of the calorimeter cluster with energy greater than MeV closest to the proton b eam direction Monte Carlo simulation