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LEADING PROTON PRODUCTION in DIS at HERA

LEADING PROTON PRODUCTION in DIS at HERA. Lorenzo Rinaldi. Introduction. Leading protons (LP) carry large fraction of proton beam-energy (x L ) and have small transverse momentum (p T ) LP production mainly soft process, non perturbative-QCD

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LEADING PROTON PRODUCTION in DIS at HERA

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  1. LEADING PROTON PRODUCTION in DIS at HERA Lorenzo Rinaldi

  2. Introduction • Leading protons (LP) carry large fraction of proton beam-energy (xL) and have small transverse momentum (pT) • LP production mainly soft process, non perturbative-QCD • Alternative approach needed (example: Reggeon-exchange) • Possible implication on LHC (simulation of forward protons) and extended cosmic ray showers Results discussed in this talk: Measurement of LP production in DIS (Q2>3 GeV2, 45<W<225 GeV), for leading proton with 0.5<xL<1 (diffractive and not-diffractive regimes) and pT2<0.5 GeV2: • LP longitudinal momentum spectrum • LP transverse momentum spectrum and slopes • LP rate vs DIS variables x,Q2 • LP-tagged structure function • Comparison to models and other data

  3. Leading proton production in ep collisions p,IR,IP p LP cross sections vs structure functions: (QCD-based approach) Standard fragmentation • LP from hadronization of p remnant • Implemented in MC models (Cluster, Lund strings...) Virtual particle exchange p, IR, IP LP also from p fragmentation in double dissociative diffraction Kinematics Lepton vertex variables: x, Q2, (W, y) LP variables: pT2,xL=ELP/Ep LP and DIS variables assumed to be independent (vertex factorization)

  4. The ZEUS Leading Proton Spectrometer (LPS) • 6 stations (s1s6) each made by 6 Silicon-detector planes • Stations positioned at 20  90m from I.P. • Stations inserted at 10sbeam from the proton beam during data taking • sxL < 1% spT2~ few MeV2 (better than p-beam spread ~ 50-100 MeV) • Full LPS set used in this analysis (xL>0.5, possibility to extend xL>0.32)

  5. Cross section vs xL pT2<0.04 GeV2 pT2<0.5 GeV2 Non diffractive region Cross section normalised to inclusive DIS cross section Flat below diffractive peak DIS and photoproduction data are compatible

  6. Cross section vs pT2 in bins of xL and slopes Exponential behaviour Fit to A*exp(-b*pT2) shown with stat. error No strong dependence of b on xL

  7. Rates to inclusive DIS Structure function ratio Q2 Information on LP production as a function of DIS variables Measurement for 0.5<xL<0.92 (NO diffraction) rLP(3) almost independent of x and Q2 with average value ~0.4 x

  8. Rates to inclusive DIS - 2 ~17% of DIS events have a LP with 0.5<xL<0.92, almost independently of x and Q2 No clear evidence of vertex factorization violation

  9. LP-tagged structure functions Q2 F2LP=rLP*F2 (ZEUS-S parametrization used) F2LP: same dependence on x and Q2 as F2 x

  10. Leading proton vs leading neutron data Very similar behaviour xL<0.85 LP cross section almost twice LN In particle exchange model: expected from isovector: LP=1/2LN Other exchanges needed (isoscalars) Slopes comparable 0.7<xL<0.8 where p exchange dominates

  11. Comparisons to Reggeon exchange model Very good decription in shape xL slighty underestimated b-slope slightly overestimated

  12. Summary and conclusions LP production measured in DIS: • Cross section vs xL flat below the diffractive peak • LP cross section vs pT2 falls exponentially with a mean slope ~6.7 GeV-2, approximately independent on xL • the rates rLP(2) and rLP(3) show no dependence on x and Q2 and LP structure functions proportional to F2 • No evidence of vertex factorization violation • LP and LN: agreement with expectations from particle-exchange models • A Regge-inspired model reproduces quite well the LP features

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