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LEADING BARYON PRODUCTION IN ep COLLISION Lorenzo Rinaldi On behalf of ZEUS Collaboration

LEADING BARYON PRODUCTION IN ep COLLISION Lorenzo Rinaldi On behalf of ZEUS Collaboration. Introduction and experimental setup Leading Proton (LP) in DIS (EPS03) Leading Neutron (LN) p T 2 distribution in DIS and photoproduction, rescattering and absorption (NEW)

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LEADING BARYON PRODUCTION IN ep COLLISION Lorenzo Rinaldi On behalf of ZEUS Collaboration

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  1. LEADING BARYON PRODUCTION IN ep COLLISION Lorenzo Rinaldi On behalf of ZEUS Collaboration • Introduction and experimental setup • Leading Proton (LP) in DIS (EPS03) • Leading Neutron (LN) pT2 distribution in DIS and photoproduction, rescattering and absorption (NEW) • Leading Neutron tagged D* production (ZEUS Coll., Phys. Lett. B 590 (2004) 143) • Conclusions

  2. Introduction • Large fraction of events with a Leading Baryon (LB) in final state • Production mechanism still not clear • Model in use: Lepton variables Q2, W, x, y Vertex factorization p,IR,IP N,P LB variables: pT2, xL=ELB/Ep t=(p-p’)2 N,P p’ p’ Virtual particle exchange • LP: neutral iso-scalar iso-vector (p,IR,IP) • LN: charged iso-vector(p+,r+,...) • LB also from p fragmentation in double dissociative diffraction Standard fragmentation • LB from hadronization of p remnant • Implemented in MC models (Cluster, Lund strings...) L.Rinaldi - Leading Baryons Production

  3. ZEUS leading baryon detectors Leading Proton Spectrometer (LPS) • 6 stations each made by 6 Silicon-detector planes • Stations inserted at 10sbeam from the proton beam during data taking • sxL < 1% spT2~ few MeV2 (better than p-beam spread ~ 50 - 100 MeV) L.Rinaldi - Leading Baryons Production

  4. ZEUS leading baryon detectors 2 • Forward Neutron Calorimeter (FNC) • 10 lI Pb-sci. sandwich • s/E = 0.65%/√E • spT2 ~ MeV2 (dominated by p-beam spread) Forward Neutron Tracker (FNT) • Scintillator hodoscope at 1lI • sx,y= 0.23 cm  sq= 22 mrad Beampipe hole FNT Kinematic regions covered by the angular acceptance of FNC Acceptance window (qn<0.75 mrad) L.Rinaldi - Leading Baryons Production

  5. Leading proton: cross section vs xL Herwig(1-xL)1.0 MEPS: (1-xL)1.0 Ariadne: (1-xL)1.4 DIS{ ~ Flat distribution below the diffractive peak • Montecarlo samples (standard fragmentation): • Herwig (cluster model) • MEPS (parton shower,SCI) • Ariadne (CDM) MonteCarlo models fail to describe the measured leading baryon xL spectrum. L.Rinaldi - Leading Baryons Production

  6. Leading proton: cross section vs pT2 Data distribution ~ exponential Fit to exponential in each xL bin: L.Rinaldi - Leading Baryons Production

  7. Leading proton: b-slopes No strong b-slope dependence of xL observed Also the pT2 is not well simulated. L.Rinaldi - Leading Baryons Production

  8. Leading proton summary • leading proton features have been measured with high precision • ~ flat cross section vs xL below the diffractive peak • approximate exponential fall of pT2 cross section • No visible dependence of pT2 slopes vs xL • Differences found with models predictions • Accurate measurement available for MC tunings And what about the neutrons?    L.Rinaldi - Leading Baryons Production

  9. Leading Neutron: One-Pion-Exchange model • O.P.E. explains features of leading neutron data: • production rate • xL spectrum • factorization a(t) and F2(xL,t) model dependent pT2 slopes discriminate between production models (flux) L.Rinaldi - Leading Baryons Production

  10. Leading Neutron cross section vs pT2 in xL bins • Cross sections normalized to inclusive process • Fit to exponential in xL bins L.Rinaldi - Leading Baryons Production

  11. Leading Neutron b-slopes vs xL OPE models: • Dominant at 0.6<xL<0.9 • (non-) Reggeized flux, different form factors with different parameters • none of the models seem to decribe well the data Best models: • FMS-mono [Frankfurt, Mankiewicz, Strickman, Zeit. Phys.A334 (1989) 343] • FMS-dipole [Frankfurt, Mankiewicz, Strickman,Zeit. Phys.A334 (1989) 343] • GKS [Golec-Biernat, Kwiecinski, Szczurek,Phys. Rew.D56 (1997) 3955] • Bishari0 [Bishari, Phys. Lett.B38 (1972) 510] L.Rinaldi - Leading Baryons Production

  12. Rescattering model and absorption Model 1: (D’Alesio and Pirner, EPJ A7(2000) 109) • more absorption when photon size larger (small Q2)  less neutrons detected in photoproduction • more absorption when mean p-n system size (‹rnp›) smaller at low xL  less neutrons detected at low xL • more absorption  fewer neutrons detected with higher pT2  larger b-slope expected in photoproduction Model 2: (Nikolaev,Speth and Zakharov, hep-ph/9708290) a=e,p • absorption from additional pomeron exchange • implies large uncertainties to pion pdf description L.Rinaldi - Leading Baryons Production

  13. Rescattering model: measurements • Comparison DIS and photoproduction: • LN in photoproduction cross section smaller than DIS (factorization violation) • At lower xL less neutrons observed in photoproduction • Same trend in rescattering (absorption) of D’Alesio and Pirner L.Rinaldi - Leading Baryons Production

  14. Rescattering model: DIS & photoproduction DIS Db>0 : Measurement consistent with predictions PHP Fit to exponential L.Rinaldi - Leading Baryons Production

  15. Leading Neutron tagged D* photoproduction Hard scale pT>1.9 GeV p+ soft scale pT<0.3 GeV n s=2.08+0.22(stat.) +0.12 -0.18 (syst.)+0.05(B.R) nb Information on interplay between soft and hard scales • Kinematic region: • Q2<1 GeV2 • 130<W<280 GeV • |h(D*)|<1.5 • pT(D*)>1.9 GeV • xL>0.2 • qn<0.8 epe’D*+nX D*+(D0K-p+)ps+ and c.c. L.Rinaldi - Leading Baryons Production

  16. Leading Neutron tagged D* photoproduction Standard Fragmentation OPE fi = parametrization of flux factor Ratio to inclusive processes Ratio LN tagged to inclusive D* production rD*=8.85+0.93(stat.)+0.48-0.61(syst.) % Ratio LN tagged to inclusive DIS rDIS=8.0+0.5 % Ratio LN tagged to inclusive PHP rPHP=5.7+0.4 % rPHP<rDIS≈rD* rescattering suppressed by the precence of a hard scale LN production models • Fragmentation models also fail to describe leading neutron • OPE gives a good description of data • OPE model sensitive to pion flux parametrisation L.Rinaldi - Leading Baryons Production

  17. Conclusion and outlook • High precision measurements on leading baryon available • Leading proton features well measured • High precision measurement of leading neutron pT2 slopes both in DIS and photoproduction • Absorption effects visible in data • Leading particles production mechanism is still not completely understood • Theoretical predictions differ from measurements • Need to improve leading baryon production models • New results input to tune the models L.Rinaldi - Leading Baryons Production

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