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Measurement of Jet Modification at RHIC

Measurement of Jet Modification at RHIC. Fuqiang Wang Purdue University. – THE PLAN – Physics motivation Results on high p T inclusive yield Results on high p T angular correlation Results on high p T + low p T angular correlation Summary and open question.

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Measurement of Jet Modification at RHIC

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  1. Measurement of Jet Modification at RHIC Fuqiang Wang Purdue University – THE PLAN – Physics motivation Results on high pT inclusive yield Results on high pT angular correlation Results on high pT + low pT angular correlation Summary and open question thanks tothe Collaboration

  2. Lattice QCD prediction at mB = 0: T Quark-GluonPlasma F. Karsch, Nucl. Phys. A698, 199c (2002). ~200 MeV r/r0 5-10 TC ~ 170 15 MeV eC ~ 0.5 GeV/fm3 Physics motivation QuantumChromodynamicspredicts phase transition between hadrons and Quark-Gluon Plasma (a state of thermalized partons) at high energy density. Fuqiang Wang – Moriond QCD 2004

  3. Physics motivation – cont’d What medium are we creating in heavy ion collisions: QGP or hadron gas? Want to probe the density of the medium. self-generated, penetrating probe: large pT hadrons, jets • they are generated early (by initial hard-scatterings); • they need some time to escape from the collision zone – the size of nuclei; • during this time frame, a QGP (or whatever medium) is expected to form; • they interact with the medium, giving us information about the medium. Fuqiang Wang – Moriond QCD 2004

  4. Large pT hadrons are mainly from jets produced by initial hard-scattering. Small cross-sections, scale with number of binary collisions. If no nuclear effect, AA = binary-scaled pp; AA binary-scaled pp  nuclear effect. Inclusive high pT yield – suppressed Fuqiang Wang – Moriond QCD 2004

  5. Jet-like angular correlation – disappeared Peripheral Au + Au near-side well-described peripheral collision: similar to p+p away side hadrons are suppressed! Central Au + Au central collision: strong absorption in medium Fuqiang Wang – Moriond QCD 2004

  6. Phys. Rev. Lett. 91, No. 7, August 15, 2003 d+Au control experiment Yes, there is nuclear effect. Energy loss  suppression of high pT hadron yield. But, were jets initially there? Maybe the initial hard scatterings in AA were different from pp: e.g. modified PDF’s. Fuqiang Wang – Moriond QCD 2004

  7. High pT: High pT particle STAR Preliminary: Quark M 2004 High pT particle p+p Jet-like structures Au+Au p+p (1/Ntrig) dN/d(Df) Low pT: Signal Au+Au top 5% 1/Ntrigger dN/d(Df) background Df D f (radians) Reconstructing those jets… Fuqiang Wang – Moriond QCD 2004

  8. p+p Jet charge multiplicity and “energy” STAR Preliminary: QM 2004 With the same final leading particle, we are selecting a larger energy jet in central AA than in pp. Estimate of energy loss scale. Fuqiang Wang – Moriond QCD 2004

  9. 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 converted to lower pT particles energy loss in medium! syst. error Away Associated particles pT distributions STAR Preliminary: Quark Matter 2004 Near softening in spectra: equilibration with medium Fuqiang Wang – Moriond QCD 2004

  10. syst. error away side energy from theinitial parton hasbeen converted tolower pT particlesby final stateinteractions dense mediumin AuAu collisions approachingthermalization 1/Ntrigger dN/d(Df) pT (GeV/c) D f (radians) Summary Large pT hadrons are suppressed in AuAu relative to pp. Large pT angular correlation disappeared at away side. Low pT angular correlation is there and stronger than pp. Fuqiang Wang – Moriond QCD 2004

  11. Conclusion / Open question • Have we created a new form of matter? Definitely! It’s dense, strongly interacting, dissipative. • Is it a QGP (i.e. a state of deconfined, thermalized quarks and gluons)? Fuqiang Wang – Moriond QCD 2004

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