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STAR Measurement of Jet Modification in Au+Au Collisions

STAR Measurement of Jet Modification in Au+Au Collisions. Fuqiang Wang Purdue University for the Collaboration. – OUTLINE – motivation analysis results summary. Physics motivation. The goal of RHIC is to create QGP –

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STAR Measurement of Jet Modification in Au+Au Collisions

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  1. STAR Measurement of Jet Modification in Au+Au Collisions Fuqiang Wang Purdue University for the Collaboration – OUTLINE – motivation analysis results summary Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  2. Physics motivation The goal of RHIC is to create QGP – a state of deconfined, thermalized quarks and gluons Lattice QCD prediction: F. Karsch, Nucl. Phys. A698, 199c (2002) Two prerequisites: (1) High enough energy density (2) Parton thermalization TC ~ 170 15 MeV eC ~ 0.5 GeV/fm3 Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  3. t~1 fm/c 30xr0 PRL 87 (01) 52301 nucl-ex/0311017 (1) energy density: Bjorken estimate Boost invariant hydrodynamics: Bjorken Estimate of Initial Energy Density Cold nuclear matter: r0~ 0.16 GeV/fm3 Low bound: t likely is smaller at RHIC. ET drops with time. Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  4. (1) energy density: “measuring” it? Jet quenching: • Large pT partons/jets are generated early (initial hard-scatterings) • d+Au ~ p+p: hard-scatterings are similar in Au+Au and p+p • Partons / jets need time to escape the collision zone, during which a QGP (or whatever medium) is formed. • Partons/jets lose energy when traversing and interacting with the medium (final state interactions)  modifications to jets Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  5. (1) energy density: inferred from models X.-N. Wang, PLB 579 (04) 299 pQCD calculations: x30 gluon density x100 energy density in central Au+Au collisions ~ Bjorken estimate Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  6. Measuring the lost energy? possible… by going to low pT. S. Pal, S. Pratt, PLB574 (2003) 21. X.-N. Wang, PLB 579 (2004) 299, nucl-th/0307036. C.A. Salgado, U.A. Wiedemann, hep-ph/0310079. M. Gyulassy, I. Vitev, X.-N. Wang, B.-W. Zhang, nucl-th/0302077. …… Pal, Pratt, PLB 574 (2003) 21 • How is energy distributed? • amount of energy loss? • contribution from medium? Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  7. (2) thermalization • Thermalization in the final state: • Final state low pT hadron distributions look thermal. • Event-by-event <pT> variation is small: every event looks thermal. • Hadron compositions described by thermal models. • TChemical ~ 160 MeV ~ TC. • Necessary but not sufficient condition for early thermalization. NA49, PRC 68 (2003) 34903 • Early state thermalization? • First time at RHIC elliptic flow at low pT described by hydro:zero mean free path  max. possible v2. • SPS v2 lower than hydro, however energy density may be not much lower. Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  8. (2) thermalization at work? • yes… by putting two sources of particles together: • one from jet fragmentation that are initially hard. • the other from bulk medium that are soft. going to low pT. jet medium Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  9. Are leading particles from jets? PHENIX, PRL 91, 172301 (2003) non-frag. p / p ~ 0.6 non-frag. p / Nch ~ 0.3 pT=3-4 GeV/c: ~30% are probably from other sources. p / p ~ 0.9 in central p / p ~ 0.3 in peripheral QM’04 p+p @ ISR B. Alper, NPB 87 (1975) 41 Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  10. Coalescence / recombination models Greco et al, PRC 68 (03) 34904 Fries et al, PRC 68 (03) 44902 Hwa et al, nucl-th/0401001 Coalescence / recombination models predict a range of non-fragmentation contributions. All predict a rapid drop of non-fragmentation contribution above 4 GeV/c. pT>4 GeV/c: may mainly come from jets, or related to jets. Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  11. High pT particle STAR Preliminary High pT particle p+p Jet-like structures Au+Au p+p (1/Ntrig) dN/d(Df) Signal Au+Au top 5% background Df Reconstructing low pT associated particles • Select a leading particle 4<pT<6 GeV/c, |h|<0.75. • Associate other particles(0.15<pT<4 GeV/c,|h|<1.1)with the leading particle.FormDf,Dh correlation. • Background from mix- events. v2 modulation on background. Normalize in0.9<|Df|<1.3. • Efficiency corrections are applied to associated particles. • Take difference and normalize per trigger. Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  12. High pT: M.G. Albrow et al. NPB145, 305 (1978) Low pT: 1/Ntrigger dN/d(Df) D f (radians) Azimuth angular correlations near side: |Df|<1.1, |Dh|<1.4away side: |Df-p|<2, |h|<1.1 Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  13. STAR Preliminary s RMS “Jet” sizes near: |Df|<1.1, |Dh|<1.4 away: |Df-p|<2, |h|<1.1 bkgd subt. Au+Au top 5% (1/Ntrig) dN/d(Df) Df near (1/Ntrig) dN/d(Dh) • With increasing centrality: • Near side broadens in h but not f. • Away side modest increase in size. Dh Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  14. p+p “Jet” charge multiplicity and “energy” STAR Preliminary With the same final leading particle, we are selecting a larger energy jet in central AA than in pp. Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  15. Medium contribution? Total scalar pT:Initial parton energy + medium contribution? TPC acceptance of away side partner? Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  16. } DE = 1.4 – 2.2 GeV Jet quenching model X.-N. Wang, PLB 579 (2004) 299, nucl-th/0307036 with energy loss without energy loss Caution: cannot be readily compared to data yet. Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  17. Thermal-shower recombination Hwa, Yang, nucl-th/0401001 In this model, the thermal- shower recombination is the largest contribution to high pT particles. One mechanism for energy contribution from medium. Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  18. Near side: overall enhancement from pp to AA larger initial parton energy (and modest energy loss)? Away side: energy from the initial parton has been convertedto lower pT particles energy loss in medium! syst. error Away Associated particles pT distributions STAR Preliminary Near Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  19. away side associated particle <pT> decreases with centrality, approaching medium hadron <pT> in central collisions equilibration between the two sources of particles Away side <pT> Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  20. STAR Preliminary Fit to near side: const. + gaussian + Borghini-cos(fixed) p+p Au+Au 5% free fit (1/Ntrig) dN/d(Df) Borghini et al. PRC 62, 034902 (2000): stat. mom. conserv.Borghini et al. stat. mom. conserv.Borghini et al.  Df Df Cannot distinguish: (1) the full event participates in momentum balance. (2) Only a handful particles: e.g. jet-jet production. Broadened distribution and thermalization the away excess has a similar shape to a stat. distr. from momentum conservation. the away side excess approaching equilibrium with the medium! Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

  21. Summary • high enough energy density?Models require x30 normal nuclear gluon density to describe suppression data.Statistical reconstruction of jets in pp and AA collisions.Potential possibility of experimental measure of energy loss.Same pT leading particles come from larger jet energy in central AA than in pp. • Near side: overall increase in multiplicity. • Away side: increase in multiplicity and softening in pT. (2) parton thermalization? • Away side: towards thermalization in more central collisions. • May imply high degree of thermalization in medium itself. Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT

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