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Jets at RHIC

Jets at RHIC. Jiangyong Jia Columbia University, Nevis Lab. Jets in p-p, d-Au jT,kT Cronin effect Jets in Au-Au Away side quenching, centrality dependence path length dependence. What is a jet. Jets arise from hard-scattering of gluons and quarks

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Jets at RHIC

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  1. Jets at RHIC Jiangyong Jia Columbia University, Nevis Lab Jets in p-p, d-Au jT,kT Cronin effect Jets in Au-Au Away side quenching, centrality dependence path length dependence Jiangyong Jia

  2. What is a jet • Jets arise from hard-scattering of gluons and quarks • Outgoing partons fragment into sprays of hadrons –Jets • Jets are back-to-back – Momentum conservation • Dominate hadron yield at pT>2 GeV/c and Rate calculable Leading hadron Jiangyong Jia

  3. jT and kT jet • jT = Momentum perpendicular to jet axis: jT= p sins • <jT> is related to the non-perturbative scale. • Typical value is 500 MeV/c, weakly depends on pT and s. • Jets are not exactly back-to-back in transverse direction • kT = Intrinsic + radiative transverse momentum of the initial partons. Jiangyong Jia

  4. Calculate jT, kT from jet width • snear of Df distribution from same jet is determined by jT. • sfar of Df distribution from opposite jet is determined by both jT and kT. Jiangyong Jia

  5. Medium modification of jet Au Au p+p p+A A+A • Jets is an ideal probe for medium • Cold medium – pA • Hot dense medium – AA • Presence of medium can broaden the kT • Presence of medium can attenuate the jet yield (absorption or quenching) Au d Jiangyong Jia

  6. Jet production in pA collisions Enhancement at intermediate pT RAB pT • Multiple soft collisions broaden high pT spectrum and modify the back-to-back correlation strength • Two methods to study the jet modification in cold medium • Study the nuclear modification factor • Study modification of the back-to-back correlation kT broaden is reflected in the increase of the away side width 1.1-1.5 1 Suppression at low pT Df Jiangyong Jia

  7. Cronin enhancement in dAu • Cronin enhancement at 2<pt<6 GeV/c • Disappear at pT>8 GeV/c • Different models including multiple scattering gives consistent results. • Accardi, Wang, Vitev, Boris, Fai Small additional kT: C~ 0.2-0.4 Accardi <kT2>=0.52 Jiangyong Jia

  8. Two particle azimuth correlation d+Au 1.5<pT<2.0 d+Auh 1.0<pT<1.5 • Jet signal jet(Df) are measured statistically using two particle correlation method. • In ideal acceptance, foreground pair distribution is : c + jet(Df). • Foreground pair distribution is modulated by pair acceptance function. • Pair acceptance function can be determined from event mixing technique 5.0<pTtrigg <16.0 GeV/c Jet function assumed to be Gaussian Fit = const + Gauss(0)+Gauss() Jiangyong Jia

  9. jT kT results • jT values are consistent between pp and dAu • jT drops for low associated pT: Seagull effect • Mean of absolute value is related to RMS value via: • So 2D jT value is: • kT values are also consistent between pp and dAu • kT value seems increases with pT • 2D value is(assuming <z>=0.76): STAR: 615 ± 60sysMeV/c Jiangyong Jia

  10. Centrality dependence • jT kT have no apparent centrality dependence!! Jiangyong Jia

  11. Comparison to outside world PHENIX preliminary L. Apanasevich et al., Phys. Rev. D59, (1999) A.L.S. Angelis et al , Phys. Lett. B97, 163 (1980) Jiangyong Jia

  12. Summarize jet production in pp & dAu • Characteristics of jet correlation are similar between pp, dAu • Similar snear, sfar similar jT, kT. • Very weak centrality dependence • Value consistent between PHENIX and STAR • Apparent Cronin effect in 2-6 GeV/c in pion Rcp • Reproduced including multiple soft scattering. • Is there a problem? NO • Probably, the intrinsic <kT> in pp is so big, that dAu measurement is not sensitive to the additional small kT broadening which is enough to produce the observed Cronin effect. Jiangyong Jia

  13. Jet production in AuAu • Jet loss energy through gluon bremsstrahlung –suppression of high pt yield • Gluon radiation soften the jet fragmentation and broadens jet cone *** • Away side jet width broadened. • Decrease in the associated yield. • Path length dependence – jet tomography *** • v2 or reaction plane dependence of jet correlation. Azimuthal anisotropy, v2 Large v2 at high pT di-jet correlation, DAA Suppression of away side yield hadron yield, RAA Suppression in central collisions Jiangyong Jia

  14. Medium effect on away side correlation • Energy loss • kT broadening • Gluon radiationjT broadening 3 T.Hirano, Y,Nara. Nucl-th 0301042 1 2 Jiangyong Jia

  15. STAR away-side quenching STAR • Yield integrated in a limited Df window shows suppression on away-side. • Most likely all three effects contributes Jiangyong Jia

  16. PHENIX away-side broadening in AuAu (2.5pTtrigg4.0)(1.0pTassoc2.5) pp <z><|kTy|> pp <|jTy|> • Near side jT consistent with pp • Consistent with surface emission • Away side “kT” broadens for central collisions • This kT includes both the true kT and jT broadening Jiangyong Jia

  17. PHENIX away side associated yields (2.5pTtrigg4.0)(1.0pTassoc2.5) GeV/c Jet Func. = Correl Func - flow Note pT is rather low; associated particle yields increase with centrality Jiangyong Jia

  18. STAR away side associated yield p+p Au+Au 5% (1/Ntrig) dN/d(Df) stat. mom. conserv.Borghini et al. stat. mom. conserv.Borghini et al. Df Df Fit to near side: const. + gaussian + Borghini-cos(fixed) STAR, PRL90, 082302 (2003) (1/Ntrig) dN/d(Df) Au+Au 5% STAR Au+Au 5% Df Df • Broadening of away side correlation(energy loss+jT broadening) • Energy from initial parton converts to lower pT particles • Modification of away side jet fragmentation function • Reminiscent of energy loss predictions Fuqiang Wang QM04 Jiangyong Jia

  19. Away side jT broadening • Both STAR and PHENIX data indicates jT broadening Baier,et.al XinNian,et.al Nucl-th 0305010 Is hep-ph/0310079 consistent with data Very weak centrality dependence of correlation width! Jiangyong Jia

  20. Path length dependence of medium effect Au Au • Energy loss(thus the jT broadening) depends on the orientation relative to reaction plane. Thus • Suppression and away side jet yield also depends on the orientation. • Their centrality dependences to first order depends only on the collision geometry. Jiangyong Jia

  21. Simple treatment: Jet absorption • expanding source absorption  L2 • static source absorption  L • static source absorption  L2 • expanding source absorption  L • Glauber gives the medium profile in transverse • Jet are absorpted in medium: Jiangyong Jia

  22. RAA and dijet correlation Near side Away side • Absorption strength k fixed by RAA in central • Centrality dependence reflects mostly collision geometry • Some sensitivity on path length dependence Jiangyong Jia

  23. Centrality dependence of v2 • Largest v2 is 5-9% • Sensitive to energy loss assumption • Sensitive to Au nuclear profile Hard sphere 5-13% Cylindrical 7-18% v2 at pT>5 GeV is still puzzling Jiangyong Jia

  24. Path length dependence of away side jet di-hadron, 20-60% Central STAR Preliminary Out-of-plane In-plane Measured Reflected STAR Suppression larger out of plane However, threshold effect between in plane and out plane? Why jet width not modified for in plane? Difference in surfaces? Jiangyong Jia

  25. Path length dependence of away side jet di-hadron, 20-40% Central excess in limited angular range on top of a wide gauss. PHENIX Out-of-plane In-plane Jiangyong Jia

  26. Summarize jet production in AuAu • Medium effect on away side jet correlation • Result consistent with energy loss + jT broadening. Towards more central collisions: • Jet width increase • Jet yield in fixed Df window decrease • Jet energy converted to low pT partons. • Path length dependence of medium effect • Can be studied from centrality and reaction plane dependence • Centrality dependence of Raa and dijets correlation mostly refects collision geometry • Centrality dependence of v2 qualitatively consistent with energy loss • Reaction plane dependence of away side dijet correlation qualitatively consistent with energy loss. • However, energy loss model • can not be reproduced magnitude of v2 at high pT. • Seems hard to produce the strong RP dependence of dijet correlation Jiangyong Jia

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