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LEADING BARYON PRODUCTION at HERA Lorenzo Rinaldi On behalf of H1 and ZEUS Collaborations

LEADING BARYON PRODUCTION at HERA Lorenzo Rinaldi On behalf of H1 and ZEUS Collaborations. Motivations. Large fraction of events with a Leading Baryon (LB) in final state carrying high fraction of the proton beam momentum

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LEADING BARYON PRODUCTION at HERA Lorenzo Rinaldi On behalf of H1 and ZEUS Collaborations

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  1. LEADING BARYON PRODUCTION at HERA Lorenzo Rinaldi On behalf of H1 and ZEUS Collaborations

  2. Motivations • Large fraction of events with a Leading Baryon (LB) in final state carrying high fraction of the proton beam momentum • LB produced at small angle in forward direction: difficult detection • Production mechanism still not clear • Interest in LB study for next experiments @ LHC (absorptive corrections for diffractive Higgs, pile-up background...) Experimental results discussed in this talk: • Leading Proton (LP) spectra in DIS • Leading Neutron (LN) spectra in DIS and photoproduction • Dijet production with a Leading Neutron • Latest developments in theory • Comparison with models

  3. Leading baryon production in ep collisions 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...) LB production affected by absorption and rescattering effects: evidences of vertex factorization violation L.Rinaldi - Leading Baryons @ HERA

  4. Leading baryon detectors ZEUS 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) ZEUS Forward Neutron Calorimeter (FNC) • 10l lead-scintillator sandwich • σ/E =0.65/√E, ∆Eabs=2% • Acceptance qn<0.8 mrad, azimuthal coverage 30% • ZEUS Forward Neutron Tracker (FNT) • Scint. hodoscope @ 1λint, σx,y=0.23cm, σθ=22μrad H1 Forward Neutron Calorimeter (FNC) • Lead-scintillator calorimeter @ 107m from I.P. + veto hodoscopes • s(E)/E≈20%, neutron detection eff. 93±5% L.Rinaldi - Leading Baryons @ HERA

  5. Leading Proton: cross section vs xL Herwig MEPS Ariadne • Montecarlo samples (standard fragmentation): • Herwig (cluster model) • MEPS (parton shower,SCI) • Ariadne (CDM) Bad description of xL spectrum • Flat below diff. peak (1-xL)a, a~0 • Good description by reggeon-exchange model L.Rinaldi - Leading Baryons @ HERA

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

  7. LP: b-slopes vs xL • Different slopes in LEPTO • Better HERWIG • b-slope not well simulated by fragmentation models • No strong dependence observed on xL • observed fluctuations due to fit range • Good agreement with prediction from Reggeon-exchange model L.Rinaldi - Leading Baryons @ HERA

  8. LP summary • leading proton quantities 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 • Good description by reggeon-exchange model • Fragmentation models fail to describe LP production • Accurate measurements available for MC tunings And what about the leading neutrons?    L.Rinaldi - Leading Baryons @ HERA

  9. Leading Neutron: One-Pion-Exchange model O.P.E. partially explains the LN production a(t) and F2(xL,t) model dependent Longitudinal momentum spectrum and pT2 slopes discriminate between different parametrizations of fluxes L.Rinaldi - Leading Baryons @ HERA

  10. Rescattering model and absorption 1 Model 1: One pion exchange in the framework of triple-Regge formalism Nikolaev,Speth & Zakharov Re-scattering processes via additional pomeron exchanges (Optical Theorem) (hep-ph/9708290) (Kaidalov,) Khoze, Martin, Ryskin (KKMR) Enhanced absorptive corrections ( exclusive Higgs @ LHC), calculation of migrations, include also rand a2 exchange (different xL & pT dependences) (hep-ph/0602215, hep-ph/0606213) L.Rinaldi - Leading Baryons @ HERA

  11. Rescattering model and absorption 2 Model 2: calculations from D’Alesio and Pirner in the framework of target fragmentation (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 L.Rinaldi - Leading Baryons @ HERA

  12. LN: longitudinal momentum spectrum PHP DIS DIS D’Alesio & Pirner: • LN yield increases with xL due to increase in phase space: pT2 < 0.476 xL2 • LN yield decreases for xL1 due to kinematic limit s~Wa, a(sgp) a(sg*p) Wp2=(1-xL)Wp2 (1-xL) -0.1 • LN yield in PHP < yield in DIS  factorization violation • Models in agreement with data ! L.Rinaldi - Leading Baryons @ HERA

  13. LN: longitudinal momentum spectrum 2 PHP • KKMR predictions: Including migrations and other iso-vector exchanges, like r and a2 • Pure p exchange too high • Absorption and migrations effects reduces the LN yield and fit the data better • Additional r and a2 exchanges enhance the LN yield L.Rinaldi - Leading Baryons @ HERA

  14. LN: cross section vs pT2 in xL bins DIS • Exponential behavior with slope b • Intercept and exponential slope fully characterize the pT2 spectra L.Rinaldi - Leading Baryons @ HERA

  15. LN: intercepts and b-slopes vs xL • LN slope increases up to xL~0.8 • LP slope almost flat • Similar values in the range 0.7<xL<0.85 when p exchange dominates • intercept ~ cross section integrated over all pT2 rise towards xL~0 L.Rinaldi - Leading Baryons @ HERA

  16. LN b-slopes vs xL: Models 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 the data well KKMR model: good description of the data considering absorption effects and r,a2 exchange contributions L.Rinaldi - Leading Baryons @ HERA

  17. LN b-slopes: DIS & photoproduction p p,r,a2 slopes different in PHP and DIS in general agreement with expectation from absorption  more absorption @ small n-p size  depletion @ large pT  steeper slope in PHP DIS PHP Fit to exponential L.Rinaldi - Leading Baryons @ HERA

  18. LN b-slopes: DIS & photoproduction from KKMR model p p,r,a2 Other exchanges flatten the pT2 distributions in both, a bit more in PHP than in DIS L.Rinaldi - Leading Baryons @ HERA

  19. LN production compared to MC models Assuming the leading baryon production proceeds via the standard fragmentation process  do standard MC generators describe the data ? • Lepto+MEPSbest for xL spectra, but flat b-slope, • Lepto+Ariadne,RAPGAPin standard mode, CASCADEcannot describe any of the distributions: too few neutrons, too low xL, b-slopes too flat. L.Rinaldi - Leading Baryons @ HERA

  20. Dijet production with a LN DIS direct PHP resolved PHP xobs Re-scattering processes expected for resolved PHP: photon acts hadron-like  additional interactions between remnants and scattered partons Relevant variable: momentum fraction of the photon entering the hard sub-process. L.Rinaldi - Leading Baryons @ HERA

  21. Dijet production with a LN • DIS • gpjjXn • DIS • gpjjXn Lower values @ higher xL Comparable slopes L.Rinaldi - Leading Baryons @ HERA

  22. Dijet production with a LN: sjj+n/sjj ZEUS: RAPGAP/HERWIG-MI ( ) (Nucl.Phys.B596,3(2001)) ok Not ok H1: RAPGAP/PYTHIA-MI ( ) (Eur. Phys. J. C41 (2005) 273-286 ) Not ok ok No possibility to decide on factorization breaking due to re-scattering processes in resolved PHP (xg<1) with hadron-like photon. L.Rinaldi - Leading Baryons @ HERA

  23. Summary • High precision measurements of leading baryon production available from HERA • LP spectra measured and well described by Reggeon-exchange model • LN production characterized by rescattering effects (observed also in LP, not shown here): available models give a general good description of the data • MC generators in general fail to reproduce the measured quantities  need to understand the process of leading baryon production and the implementation of its mechanism in the generators. • theory provides now a lot of predictions, experimental measurements fundamental for model tuning • Leading baryon study remains an important topic in HEP with a direct impact on next experiments @ LHC L.Rinaldi - Leading Baryons @ HERA

  24. L.Rinaldi - Leading Baryons @ HERA

  25. LP absorption effects L.Rinaldi - Leading Baryons @ HERA

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