ZEUS. dσ/dx L ZEUS. /dx L. dσ LP. 1/σ inc. x L ZEUS

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1 EPJ Web of Conferences 5, 8 () DOI:.5/ epjconf/ 58 C Owned by the authors, published by EDP Sciences, Leading Hadron Production at HERA Armen Buniatyan,a Deutsches Elektronen Synchrotron, DESY Notkestrasse 85, 67 Hamburg, Germany Abstract. Data from the recent measurements of very forward baryon and photon production with the H and detectors at electron-proton collider HERA are presented and compared to the theoretical calculations and Monte Carlo models. Results are presented of the production of ing protons, neutrons and photons in deep inelastic scattering (ep e px, ep e nx, ep e γx) as well as the ing neutron production in the photoproduction of dijets (ep e j jxn). he forward baryon and photon results from the H and Experiments are compared also with the models of the hadronic interactions of high energy Cosmic Rays. he sensitivity of the HERA data to the differences between the models is demonstrated. Introduction he QCD hardness scale for secondary particles production in semi-inclusive deep inelastic scattering (DIS) gradually decreases from the photon virtuality, Q, which determines the hard scale in the virtual photon fragmentation (current) region, to a soft hadronic scale in the proton fragmentation region. In the processes with low Q (photoproduction) the hard scale of the interaction can be guaranteed by the presence in the final state of the hadronic jets or heavy quarks. Particle production at very small angles with respect to the proton beam direction (forward direction) in a process with a hard scale provides a testing ground for the theory of strong interactions in the soft regime and is important for the theoretical understanding of proton fragmentation. Measurements of forward particles also provide important constraints for the modelling of the high energy air showers and thereby are very valuable for the understanding of high energy Cosmic Ray data []. In this presentation, the recent results of the H and Collaborations of the forward proton, neutron and photon production in electron-proton and positron-proton interactions at HERA collider are reported. Leading Baryons at HERA In ep scattering at HERA a significant fraction of events contains in the final state a proton or a neutron, which carry a large fraction of the incoming proton energy. Although a fraction of these ing baryons (LB) may result from the hadronisation of the proton remnant, the t-channel exchange of the colour singlet virtual particles a Armen.Buniatyan@desy.de (corresponding to the diagram shown in Figure ) is expected to contribute significantly [ 5]. he production of the ing neutron (LN) in the virtual exchange model occurs through the exchange of isovector states, andπ + exchange is expected to dominate. For the ing proton (LP) production, isoscalar exchanges also contribute, including diffractive Pomeron mediated interactions. e p t γ* π, IP, IR e X p,n Figure. Leading baryon production ep exn via the colour singlet exchange processes. In the simple exchange picture, the cross section is factorised into two parts (vertex factorisation): one factor describes the probability of the emission of a colour neutral object from the beam proton, the other describes the scattering of virtual photon with the exchanged particle. For instance, the cross section of LN production viaπ + -meson exchange can be written as dσ γ p nx= f π/p (, t) dσ γ π X. () Here f π/p is the flux of virtual pions in the proton, which is constrained from the low energy hadronic data. Such a his is an Open Access article distributed under the terms of the Creative Commons Attribution License., which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at or

2 EPJ Web of Conferences reaction can thus be used to probe the partonic structure of the exchanged pion. In order to measure the ing protons and neutrons, the H and Experiments have been equipped with dedicated detectors. he protons were measured with position sensitive detectors (Roman Pots) placed along the proton beam downstream of the interaction point. he neutrons were measured with -scintillator forward calorimeters (FNC) at the zero-degree point; magnet apertures limited neutron detection to scattering angles less than.75 mrad.. Leading Proton and Leading Neutron cross sections as a function of dσ/d [nb] 8 6 H Data RAPGAP-π DJANGO LEPO-SCI-GAL.65 RAPGAP-π +. DJANGO p <. GeV H..6.8 he cross section of LP production in DIS normalised to the inclusive DIS cross section (/σ tot dσ LP /d ), measured as a function of the longitudinal momentum fraction of the scattered proton, is shown in Figure [7]. Here = E p /E beam p, where E beam p is the energy of a beam proton. he LP rate is approximately flat up to the diffractive peak at, where it increases by a factor of about six. In the upper part of Figure the distribution is compared to the predictions of Monte Carlo (MC) models DJANGOH [8] and RAPGAP [9], which are based on standard fragmentation. hese models don t reproduce the flat dependence of the cross section below the diffractive peak. he same data are compared to a Regge-based model [6] which incorporates the isovector and isoscalar exchanges and includes the Pomeron exchange for diffraction. A good de- (/σ ep ejjx )dσ ep ejjxn /d pb - ep ejjxn Q < GeV p <.76 GeV Systematic uncertainty RAPGAP RAPGAP - π exch. PYHIA-SCI /σ inc dσ LP /d -.8 pb - p <.5 GeV Q > GeV 5<W<5 GeV Szczurek et al. Pomeron πn Reggeon π Figure. Differential cross section of the ing neutron production as a function of in the angular rangeθ n <.75 mrad in DIS, in the phase space defined by the photon virtuality 6 < Q < GeV and the inelasticity.<y<.6 (up) and in the photoproduction of dijets in the kinematic range defined by Q < GeV and the jet transverse energies and pseudorapidities > 7.5 (6.5)GeV,.5<η jet, <.5 (bottom). Predictions of the MC simulations are compared with the measurements. E jet() /σ inc dσ LP /d pb - p <.5 GeV Q > GeV 5<W<5 GeV Djangoh+SCI+MEPS Djangoh+SCI+ Rapgap+MEPS Rapgap Figure. Normalised ing proton production cross section in DIS as a function of, compared to the Monte Carlo models (up) and Regge-based model [6] (bottom). scription of the distribution is obtained by adding a substantial contribution of isoscalar Reggeon exchanges, which turn out to be the dominant processes below the diffractive peak. Figure shows the cross sections of LN production as a function of in DIS [] and in the photoproduction of dijets []. he DIS measurement is compared with the prediction of RAPGAP MC [9], which here generates exclusively theπ + -exchange process, and the standard fragmentation model is simulated with DJANGOH [8]. For the comparison with the photoproduction data, the RAPGAP simulation incorporates the standard fragmentation and theπ + -exchange processes. he photoproduction measurement is also compared with the PYHIA MC [], which includes the simulation of soft colour interactions (SCI) [], in which the production of diffraction-like configurations is enhanced via non-perturbative colour rearrangements between the outgoing partons. Both distribu- 8-p.

3 ISVHECRI tions are well described by the combination of the standard fragmentation andπ + -exchange models. At large values the contribution fromπ + -exchange dominates. Figure compares the distributions for the ing protons and neutrons restricted to the same p range (p <. GeV ). If the LP production proceeded only through isovector exchange, as LN production must, there would be half as many LP as LN. he data instead has approximately twice as many LP as LN. hus, exchanges of particles with different isospins such as isoscalars must be invoked for LP production d σ/(d dp ) [nb/gev ]. < <.. p [GeV ].58 < < < <.5.. p [GeV ].66 < <.7 H Preliminary < < p [GeV ].7 < <.8 /σ inc dσ LB /d pb - e + p e + Xp p <. GeV Q > GeV 5<W<5 GeV pb - e + p e + Xn p <. GeV Q > GeV 5<W<5 GeV p [GeV ].8 < <.9. p [GeV ].5. p [GeV ].9 < <.. p [GeV ]. p [GeV ] H Data (Prel.).65 RAPGAP-π. DJANGO.65 RAPGAP-π +. DJANGO Figure 5. Double differential cross section of the ing neutron production in DIS as a function of p and of the neutron. Data are compared with predictions of the RAPGAP pion exchange and DJANGOH MC simulations as well as combination of those two simulations. Figure. he rate /σ inc dσ LB /d for the ing proton (dots) and the ing neutron (open circles) production in DIS restricted to p <. GeV range.. Leading Neutron and p cross sections in DIS he measurement of the double differential cross section of LN production in DIS as a function of and the squared transverse momentum p of the neutron is shown in Figure 5 []. he best description of the data is achieved by the combination of the standard fragmentation andπ + -exchange models, similar to the single differential distribution shown in Figure. Assuming the dominance of theπ + -exchange mechanism for LN production the shape of p (or the squared four momentum transfer at proton vertex t) distribution is directly related to the pion flux f π/p (see Eq.). Since the p distributions from Figure 5 have almost exponential shapes in all bins, they can be fitted by an exponential function a( ) exp ( b( )p ). he obtained values of the p slope parameters in bins, b( ), are shown in Figure 6 together with the slopes obtained using the different parameterisations of the pion flux [ 5, 5, 6]. Most of the shown predictions describe the data within uncertainties.. Leading Baryon production rate in DIS It is instructive to investigate the relative yield of the ing baryon production as a function of the DIS kinematic parameters, with respect to the inclusive DIS events. b [GeV - ] 8 6 H Preliminary H Data (Prel.) KPP Bishari Holtmann GKS FMS-monopole FMS-dipole xl Figure 6. b-slopes of the p distributions of the ing neutrons in DIS compared with the different parameterisations of pion flux. he hypothesis of limiting fragmentation suggests that the production of ing baryons in the proton fragmentation region is independent of the photon virtuality Q and Bjorken x variable. Figure 7 displays the LP production rate r LP as a function of x in bins Q and Figure 8 shows the ratio of semi-inclusive structure function of the ing neutron production F LN to the proton structure function F (i.e. the LN production rate) as a function of Q in bins of 8-p.

4 EPJ Web of Conferences x and. he ratios are almost independent of x and Q, as expected from the hypothesis of limiting fragmentation. r LP().8 pb -.< <.9 p <.5 GeV Q = GeV Q =7. GeV Q =. GeV Q = GeV Q = GeV Q =7 GeV Q =88 GeV - - x Figure 7. he ing proton production rate in DIS r LP as a function of Bjorken x in bins Q. x H Data Average F LN() (Q,x, )/F (Q,x) H =.7 =.6 =.55 =.6 =.7 =.8 = Q [GeV ] Figure 8. he ratio of the semi-inclusive structure function F LN (Q, x, ) to the proton structure function F (Q, x).. Photoproduction of dijets with LN he photoproduction of dijets in events with LN is measured in the kinematic range defined by Q < GeV and the jet transverse energies and pseudorapidities E jet() > 7.5 (6.5) GeV,.5<η jet, <.5. he differential cross sections as a function of the event variables xγ OBS, x OBS p and W are presented in Figure 9. Here, xγ OBS and x OBS p are respectively the fractions of the four-momenta of the photon and the proton, which participate in the hard interaction, and W is the centre-of-mass energy of theγp system. Also shown are the cross sections of inclusive photoproduction of dijets and the ratios of the dijet cross sections calculated with the requirement of LN in the final state to the inclusive dijet cross sections. As one can see from the ratio distributions, the LN sample has a significantly smaller contribution at low xγ OBS. he cross sections are roughly flat as a function of W; the yield exhibits a decrease with increasing W and x OBS p. For the LN sample, RAPGAP overestimates the cross section at low xγ OBS while PYHIA- SCI underestimates the cross section at high xγ OBS. Neither model can reproduce the dependence of the neutron yield on xγ OBS and W. he RAPGAP model predicts a small decrease of the neutron yield with x OBS p, which however is less pronounced than in the data. his dependence of the neutron yield indicates a violation of vertex factorisation. dσ/dx OBS γ (nb) dσ/dx OBS γ (nb) Ratio.5 x OBS γ.75.5 p <.76 GeV.5 x OBS..5 ep ejjx ep ejjxn >. γ.5 x OBS γ dσ/dw (nb/gev) dσ/dw (nb/gev) Ratio..5. pb -.5 Q < GeV Energy scale ep ejjx W (GeV) ep ejjxn RAPGAP PYHIA-SCI W (GeV) 5 5 W (GeV) dσ/dlog p ) (nb) dσ/dlog p ) (nb) Ratio ep ejjx - - log p ep ejjxn - - log p - - log p ) ) ) Figure 9. Differential cross sections of dijet photoproduction with ing neutrons and inclusive dijet photoproduction as a function of xγ OBS, W and log p ). he ratios between cross sections are also given..5 Comparison of Forward Baryon results from HERA with Cosmic Ray interaction models he measurements of forward particles at HERA may provide valuable information for the physics of ultra-high energy Cosmic Rays (CR). he models which are used for CR analyses can be used to make predictions for HERA kinematics, which then can be compared to the experimental measurements. Here the comparisons are presented of HERA measurements with the several models of hadronic interaction developed for the simulation of air shower cascades: EPOS [7], QGSJE [8, 9], QGSJE II [, ] and SIBYLL [, ]. 8-p.

5 ISVHECRI Comparison of the ing proton and the ing neutron spectra measured at HERA with the predictions of the CR models [] are shown in Figure. While all tested models give reasonable description of the ing proton data, they differ significantly in the predictions for the ing neutrons. hus, HERA forward particle measurements are sensitive to the differences between the models and can be used for further tuning of model parameters. dσ/d - - e+p e+n+x H Data (EPJ C68 () 8) SIBYLL. QGSJE II QGSJE p < *.69 GeV ] - [GeV dσ/dp dσ/d H Data Lepto H.... p [GeV] H Data Lepto H MC/Data MC/Data H Data Lepto H.... p [GeV] 6 H Data H 5 Lepto Figure. he normalised cross sections for the production of forward photons in the pseudorapidity rangeη>7.9 in DIS in the kinematic region 6<Q < GeV and.5<y<.6 as a function of the transverse momentum p and longitudinal momentum fractions x L of ing photon. he ratios of the measured cross sections to the LEPO and MC model predictions are shown in the right sides of the plots. Figure. Comparison of the ing neutron (up) and the ing proton (bottom) distributions measured in DIS at HERA with the predictions of the Cosmic Ray interaction models. (he LN data from [] are normalised to the total inclusive DIS cross section in the quoted kinematic range.) Forward photon spectra in DIS he production of photons at very small angles with respect to the proton beam direction is studied in DIS at HERA [5] by the H Experiment. he forward photons are measured in the electromagnetic section of the FNC calorimeter. For the most energetic forward photon in the pseudorapidity rangeη>7.9 (ing photon) the cross sections are presented as a function of its transverse momentum p and longitudinal momentum fraction of the incoming proton x L. In Figure the data are compared with DJAN- GOH [8] MC model predictions, in which higher order QCD effects are simulated using ing log parton showers as implemented in LEPO [6], or using the Colour Dipole Model () as implemented in ARIADNE [7]. In Figure the measurements are also compared with the predictions of Cosmic Ray hadronic interaction models. All models overestimate the total rate of forward photons. he shapes of measured distributions are well described by LEPO. predicts harder and p spectra. he QGSJE models predict slightly softer spectra, while the EPOS and SIBYLL models predict harder spectra, but reasonably describe the shape of p distribution. he normalised forward photon production cross section as a function of virtual photon-proton centre-of-mass energy, W, is shown in Figure. It is determined as the ratio of the cross section of forward photon production to the inclusive DIS cross section in each W bin. Within uncertainties, the measured ratio is consistent with a constant value, suggesting that the shapes of the W distributions are similar for inclusive DIS events and events which contain forward photons. he measurements are compared with the MC predictions of DIS models and the models of hadronic interactions (EPOS, SIBYLL and two versions of QGSJE). he models indicate slight W dependence of the ratios.. Analysis of Feynman scaling with the forward photons he forward photon production has been studied also as a function of Feynman-x variable. his variable is defined as = p /W, where p is the longitudinal momentum of forward photon in the CMS frame of the virtual photon - proton system. his measurement allows the Feyn- 8-p.5

6 EPJ Web of Conferences ] - [GeV dσ/dp dσ/d H Data SIBYLL. QGSJE II QGSJE H.... p [GeV] H Data SIBYLL. QGSJE II QGSJE H MC/Data MC/Data H Data SIBYLL. QGSJE II QGSJE H.... p [GeV] 6 H Data H 5 SIBYLL. QGSJE II QGSJE / σ DIS γ σ DIS / σ DIS γ σ DIS H Data (prel.) correlated uncertainty Lepto H Preliminary 5 5 W [GeV] H Preliminary Figure. he normalised cross sections for the production of forward photons in the pseudorapidity rangeη>7.9 in DIS in the kinematic region 6<Q < GeV and.5<y<.6 as a function of the transverse momentum p and longitudinal momentum fractions x L of ing photon. he ratios of the measured cross sections to Cosmic Ray hadronic interaction model predictions are shown in the right sides of the plots.... H Data (prel.) SIBYLL. QGSJE II QGSJE 5 5 W [GeV] man scaling, which predicts the independence of distributions from the total centre-of-mass energy W, to be tested. Figure shows the measured cross sections for forward photons in three W intervals, normalised to the inclusive DIS cross section in corresponding W intervals. he measurements are compared to the predictions of Cosmic Ray interaction models. he differences between the measurements and predictions have similar behaviour as for x L distributions shown above in Figure. In order to study the energy dependence of distributions in details, the ratios are made of the cross sections in different W intervals. he ratios of the cross sections in the ranges <W< 9 GeV and 9<W< 5 GeV to that in the range 7<W< GeV, are shown in the upper and lower plots of Figure 5, respectively. he ratio distributions for the data are consistent with the unity within uncertainties, confirming the validity of Feynman scaling for forward photon production in the measured kinematic range. A similar behaviour is predicted by and LEPO MC models. he CR models indicate the deviations from the scaling, i.e. the rate of forward photons becomes lower with increasing energy W. Summary he production of the forward protons, neutrons and photons has been studied in DIS, and the production of ing neutrons in the photoproduction of dijets. he contributions from the fragmentation processes and from the exchange of colour-neutral particles are required to describe the ing baryon data. he p spec- Figure. he fraction of DIS events with forward photons in the kinematic region 6<Q < GeV and.5<y<.6 and the pseudorapidity of the photonη>7.9. as a function of W. he predictions of MC models are compared to the measurements. tra of the forward neutrons show sensitivity to the pion flux parameterisations. he hadronic interaction models of Cosmic Rays describe well the ing proton data, but differ in the predictions for ing neutron energy spectra. he production of the forward photons has been studied in DIS as a function of the longitudinal momentum fraction, the transverse momentum p and Feynmanx variable. Predictions of all tested Monte Carlo models overestimate the rate of forward photon production. he models predict different spectra in and p ; none of them can describe the forward photon data in rate and in shape. Within the measured kinematic range the spectra are insensitive to the virtual photon-proton centre-of-mass energy W, confirming the validity of Feynman scaling. he present measurements may to further understanding of proton fragmentation in collider and Cosmic Ray experiments. Acknowledgements Finally, I wish to thank the organisers for the excellent organisation and atmosphere of the conference which I enjoyed very much. 8-p.6

7 ISVHECRI dσ/d dσ/d dσ/d < W < GeV H Preliminary H Data (prel.) SIBYLL. QGSJE II QGSJE < W < 9 GeV H Preliminary H Data (prel.) SIBYLL. QGSJE II QGSJE < W < 5 GeV H Preliminary H Data (prel.) SIBYLL. QGSJE II QGSJE Figure. Normalised cross sections of forward photon production in DIS as a function of in the regionη > 7.9, 6<Q < GeV a and.5<y<.6 in three W intervals. he data are compared to predictions of Cosmic Ray hadronic interaction models. References [] A. Bunyatyan, A. Cooper-Sarkar, C. Diaconu, R. Engel, C. Kiesling et al., pp (9) [] J.D. Sullivan, Phys. Rev. D5, 7 (97) [] M. Bishari, Phys. Lett. B8, 5 (97) [] H. Holtmann et al., Phys. Lett. B8, 6 (99) [5] B. Kopeliovich, B. Povh, I. Potashnikova, Z. Phys. C7, 5 (996),hep-ph/969 [6] A. Szczurek, N.N. Nikolaev, J. Speth, Phys. Lett. B8, 8 (998),hep-ph/976 [7] S. Chekanov et al. ( Collaboration), JHEP 6, 7 (9),8.6 [8] K. Charchula, G.A. Schuler, H. Spiesberger, Comput. Phys. Commun. 8, 8 (99), DJANGOH. [9] H. Jung, Comp. Phys. Commun. 86, 7 (995), RAPGAP. [] F.D. Aaron et al. (H Collaboration), Eur. Phys. J. C68, 8 (),.5 [] S. Chekanov et al. ( Collaboration), Nucl. Phys. B87, (),99. dσ/d (<W<9 GeV) dσ/d ( 7<W< GeV) dσ/d (9<W<5 GeV) dσ/d ( 7<W< GeV)..9.8 H Preliminary H Data (prel.).7 SIBYLL..6 QGJSE II.5 QGJSE H Preliminary H Data (prel.).7 SIBYLL..6 QGJSE II.5 QGJSE Figure 5. Ratios of normalised cross sections of forward photons production in DIS from the different W intervals shown in Figure as a function of. Upper plot shows the ratio of the cross section in <W< 9 GeV interval to the cross section in 7<W < GeV interval. he lower figure shows the ratio of the cross section in 9<W < 5 GeV interval to the cross section in 7<W< GeV interval. Predictions of Cosmic Ray hadronic interaction models are compared to the measurement. [] M.S. Sjostrand,., P.Z. Skands, JHEP 5, 6 (6),hep-ph/675 [] A. Edin, G. Ingelman, J. Rathsman, Phys. Lett. B66, 7 (996),hep-ph/95886 [] H Collaboration, Leading Neutron p distribution, Hprelim- (), h.desy.de/h/www/publications/htmlsplit/hprelim-.long.html [5] K.J. Golec-Biernat, J. Kwiecinski, A. Szczurek, Phys. Rev. D56, 955 (997),hep-ph/975 [6] L.L. Frankfurt, L. Mankiewicz, M.I. Strikman, Z. Phys. A, (989) [7] K. Werner, F.M. Liu,. Pierog, Phys. Rev. C7, 9 (6),hep-ph/56 [8] N.N. Kalmykov, S.S. Ostapchenko, Phys. Atom. Nucl. 56, 6 (99) [9] N.N. Kalmykov, S.S. Ostapchenko, A.I. Pavlov, Nucl. Phys. Proc. Suppl. 5B, 7 (997) 8-p.7

8 EPJ Web of Conferences [] S.S. Ostapchenko, Phys. Rev. D7, 6 (6), hep-ph/5559 [] S.S. Ostapchenko, AIP Conf. Proc. 98, 8 (7), [] J. Engel,.K. Gaisser,. Stanev, P. Lipari, Phys. Rev. D6, 5 (99) [] E.J. Ahn, R. Engel,.K. Gaisser, P. Lipari,. Stanev, Phys. Rev. D8, 9 (9),96. [] Monte Carlo simulation of Cosmic Ray hadronic interaction models provided by anguy Pierog [5] F. Aaron et al. (H Collaboration), Eur.Phys.J. C7, 77 (),6.59 [6] G. Ingelman, A. Edin, J. Rathsman, Comput. Phys. Commun., 8 (997),hep-ph/96586 [7] L. Lönnblad, Comput. Phys. Commun. 7, 5 (99), ARIADNE. 8-p.8