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Nuclear effects in DIS at HERMES

Nuclear effects in DIS at HERMES. Erika Garutti (NIKHEF) On behalf of the HERMES collaboration. Inclusive DIS on nuclei ( 14 N and 84 Kr) Semi-inclusive DIS on nuclei Nuclear Attenuation  Hadron Formation Times Link to heavy-ion physics Conclusion and Outlook.

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Nuclear effects in DIS at HERMES

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  1. Nuclear effects in DISat HERMES Erika Garutti (NIKHEF) On behalf of the HERMES collaboration • Inclusive DIS on nuclei (14N and 84Kr) • Semi-inclusive DIS on nuclei • Nuclear Attenuation  Hadron Formation Times • Link to heavy-ion physics • Conclusion and Outlook Antwerpen, 16-19 Sep. 2002

  2. Medium modification of Parton Distributions Late ’60s:Inclusive high energy DIS of leptons on nucleons Parton Distributions ’80s:Inclusive lepton DIS on nuclei Medium effects (shadowing,EMC effect) Antwerpen, 16-19 Sep. 2002

  3. … the Q2 dependence Antwerpen, 16-19 Sep. 2002

  4. Medium modification ofParton Fragmentation DF FF Semi-Inclusive DIS of leptons on nucleons (LO) Fragmentation functions from SIDIS multiplicities Semi-Inclusive DIS of leptons on nuclei • Nuclear Attenuation: • Reduction of multiplicity of the • fast forward hadrons due to: • Hard partonic interactions • Soft hadron interactions. • Formation time: tf = lf / c Antwerpen, 16-19 Sep. 2002

  5. Motivation • Why is formation time of interest? • Dependence on hadron type • Comparison to model predictions • Link to Heavy Ion physics • Formation time in Heavy Ion physics: • Initial energy density in Quark Gluon Plasma plasma formation time  Estimate t0 ~ tf (?) • Impact of tpf in dilepton spectrum (P.Filip,J.Pisut Phys. Lett. B 411 (1997) 187) Predictions for in Pb-Pb (LHC, @ 160 GeV) depend on tpf mT = transverse mass A = collision area of two nuclei dN/dy = rapidity density Antwerpen, 16-19 Sep. 2002

  6. Hadron Attenuation 14N Multiplicity ratio for A/D Multiplicity ratio for A/D z > 0.8 Unexplored region • Data Selection • Kr: p, K, (p) from RICH • 2.5 (4) < Ph < 15 GeV • x > 0.06 , y < 0.85 • Q2 > 1 GeV2 , W2 > 4 GeV2 N: h, p from Cherenkov • Ph < 1.4, 4 < Pp < 13.5 GeV Antwerpen, 16-19 Sep. 2002

  7. Gluon Bremsstrahlung Model • (B.Kopeliovich et al., hep-ph/9511214) • pQCD based model for high-z mesons: • Highly virtual quark looses energy via gluon • emission till final meson is formed • induced radiation + • nuclear suppression • (dominant) •  •  cp = 1.35 [fm/GeV c] • Suppression of high-z mesons • high-z: quarks emit only few gluons • correspond to small tf • long time for hadron-Nucleus interaction tf = ch n(1-z) Antwerpen, 16-19 Sep. 2002

  8. Direct Production on 14N (HERMES Coll., EPJ C20 (2001) 479) • Phenomenological inter-nuclear • reinteraction model: • DIRECT PRODUCTION • (1-time scale model) • (Bialas & Chmaj, PLB 133(1983) 241) • tf = ch n(1-z) • s*= 0 • sh fixed for diff. h-type Multiplicity ratio for 14N/D Antwerpen, 16-19 Sep. 2002

  9. Hadron Separation vs z 1-time scale model fit: • leading hadrons (z > 0.5) cp consistent with 14N • 14N: 1.37+/-0.18 fm/Gev • 84Kr: 1.49+/-0.10 fm/GeV • cp consistent with gluon brems. model • Th: 1.35fm/GeV • cp > cp/K • att(z)|p ~ att(z)|K •  • att|p >1 for low z: • rescattering • att|p at high z: • information on Dpq(z) • (Guo & Wang , PRL 85 (2000) 3592) Antwerpen, 16-19 Sep. 2002

  10. Hadron Separation vsn • Observations: • att|p+/p- ~ att|K+/K- •  tfp+/p- ~ tfK+/K- • att|p> att|p : • s(pd) @ 3s(pd) @ E = 1 GeV (?) • att|p< att|p/K •  tfp> tfp/K (?) • Average kinematics Antwerpen, 16-19 Sep. 2002

  11. Hadron Separation vsn Enhancement at high n due to rescattering of produced hadron on target protons Leading protons have Ratt < 1 Antwerpen, 16-19 Sep. 2002

  12. Fragmentation Function Modification Model (Guo & Wang , PRL 85 (2000) 3592) Modification of Dhf(z) in medium due to multiple parton scattering (in QCD)  Dhf(z) modification increases for small n  DDhf(z) scales with A2/3 due to LPM interference Antwerpen, 16-19 Sep. 2002

  13. Parton Energy Loss (X.N. Wang, hep-ph/0111404, X.N. Wang, hep-ph/0202105) Compared to hadron yield in Au-Au collisions: average over expanding system (QGP) Dhf(z) Modificationequivalent toDhf(z) Rescaling by parton Energy loss (DE/E):  dE/dx ~ 0.25GeV/fm • expansion corr.  hot,dens system • dE/dx|stat~dE/dx|exp(RA/2t0) = 4.5 GeV/fm • RA ~ 6fm, t0 ~ 0.2 fm • Gluon density in initial stage of Au-Au QGP is ~ 15 times that of cold nuclear matter Calculated from DIS data: cold, static system  dE/dx ~ 0.3GeV/fm Antwerpen, 16-19 Sep. 2002

  14. Cronin Effect In Heavy Ion Collisions: Enhancement at high pt in AA collisions relative to pp collisions. In DIS on nuclei: Multiplicity ratio for A/D Agreement with multiple parton scattering model (E.Wang and X.N.Wang nucl-th/0104031)  Transition at pt ~ 1-2 GeV/c • Similar pt2-enhancement • Clean information on quark • transport in cold nuclear matter Antwerpen, 16-19 Sep. 2002

  15. Conclusions & Outlook • Strong nuclear attenuation at high z and low n • Similar attenuation for p and K • Observed: att|p< att|p/K • Interpretation: tfp> tfp/K (?) • Nuclear modification of Dhf(z) unambiguous info • Hadronization times on quark transport • Parton energy loss and Cronin effect in cold matter more to come: • additional HERMES data on 4He and 20Ne • pt broadening for p, K, p • comparison to theory for separate hadron type Antwerpen, 16-19 Sep. 2002

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