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Recent Results from RHIC

This article discusses the recent findings from RHIC regarding relativistic heavy ion collisions, including insights from Au+Au and d+Au collisions, lattice QCD at finite temperature, bulk particle production, and more.

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Recent Results from RHIC

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  1. Recent Results from RHIC Carl Gagliardi Texas A&M University Outline Introduction to relativistic heavy ion collisions What have we learned in Au+Au collisions? In the “low-pT” regime In the “intermediate-” and “high-pT” regimes What did we see in d+Au collisions? Carl Gagliardi – QNP 2004

  2. Lattice QCD at finite temperature Ideal gas (Stefan-Boltzmann limit) F. Karsch, hep-ph/0103314 Critical energy density: TC ~ 175 MeV  eC ~ 1 GeV/fm3 Carl Gagliardi – QNP 2004

  3. RHIC: the Relativistic Heavy Ion Collider Carl Gagliardi – QNP 2004

  4. z x Geometry of heavy ion collisions Number of participants: number of incoming nucleons in the overlap region Number of binary collisions: number of equivalent inelastic nucleon-nucleon collisions Both depend on impact parameter or “centrality” Carl Gagliardi – QNP 2004

  5. Paddles/BBC ZDC ZDC Au Au Paddles/BBC Central Multiplicity Detectors 5% Central Paddle signal (a.u.) STAR Experimental determination of centrality Carl Gagliardi – QNP 2004

  6. pT distribution of charged particles nucl-ex/0401006 Systematic Errors not shown Carl Gagliardi – QNP 2004

  7. Bulk particle production in Au+Au PRL 91, 052303 130 GeV 200 GeV 19.6 GeV Central dNch/dh Peripheral h • Central collisions at 200 GeV: 5000 charged particles (!) • Energy density: boost invariant hydrodynamics (Bjorken) (R ~ A1/3, t = 1 fm/c) Central Au+Au at 130 GeV: e = 4.6 GeV/fm3 (PHENIX, PRL 87, 052301) Carl Gagliardi – QNP 2004

  8. z y x Azimuthal anisotropy: “elliptic flow” py px Initial coordinate-space anisotropy Final momentum-space anisotropy Elliptic term Carl Gagliardi – QNP 2004

  9. Kolb and U. Heinz (2002) Hydrodynamic fits Radial flow Elliptic flow Obtain reasonable simultaneous description with the QGP equation of state predicted by lattice QCD. Carl Gagliardi – QNP 2004

  10. STAR HBT interferometry versus reaction plane nucl-ex/0312009 • Geometrical analog of v2 – confirms source anisotropy. • Note: Magnitudes of Rl and Ro/Rs smaller than predicted by hydrodynamic calculations. Carl Gagliardi – QNP 2004

  11. Chemical equilibrium? Thermal model: Partition fn with params T, mB, ms, mI3 Fit to ratios of antiparticle/particle: , K, p, , , , K*0, …. Carl Gagliardi – QNP 2004

  12. Phase diagram at chemical freeze-out Carl Gagliardi – QNP 2004

  13. Hard partonic collisions and energy lossin dense matter Bjorken, Baier, Dokshitzer, Mueller, Pegne, Schiff, Gyulassy, Levai, Vitev, Zhakarov, Wang, Wang, Salgado, Wiedemann,… Multiple soft interactions: Gluon bremsstrahlung Opacity expansion: • Strong dependence of energy loss on gluon density glue: • measure DE color charge density at early hot, dense phase Carl Gagliardi – QNP 2004

  14. jet parton nucleon nucleon Jets at RHIC Find this……….in this p+p jet+jet (STAR@RHIC) Au+Au  ??? (STAR@RHIC) Carl Gagliardi – QNP 2004

  15. Binary collision scaling p+p reference Partonic energy loss via leading hadrons Energy loss  softening of fragmentation  suppression of leading hadron yield Carl Gagliardi – QNP 2004

  16. Au+Au and p+p: inclusive charged hadrons PRL 91, 172302 Carl Gagliardi – QNP 2004

  17. Suppression of inclusive hadron yield Au+Au relative to p+p RAA Au+Au central/peripheral RCP PRL 91, 172302 • central Au+Au collisions:factor ~4-5 suppression • pT > 5 GeV/c: suppression ~ independent of pT Carl Gagliardi – QNP 2004

  18. Binary scaling Factor of 4-5 PHENIX observes a similar effect with π0 PRL 91, 241803 PRL 91, 072301 p-p Au-Au Carl Gagliardi – QNP 2004

  19. BRAHMS So doPHOBOS (h ~ 0.8) and BRAHMS (h ~ 2.2) PHOBOS: PL B578, 297 and PRL 91, 072302 BRAHMS: PRL 91, 072305 Also have h = 0 Carl Gagliardi – QNP 2004

  20. M. Gyulassy, I. Vitev and X.N. Wang Azimuthal anisotropy and partonic energy loss Anisotropy at high pT is sensitive to the gluon density of the medium. Carl Gagliardi – QNP 2004

  21. STAR Anisotropy vs. pT at 200 GeV Preliminary Preliminary Significant anisotropy in Au+Au at least up to 7~8 GeV/c Carl Gagliardi – QNP 2004

  22. trigger Jets and two-particle azimuthal distributions p+p  di-jet • trigger: track with pT>4 GeV/c • Df distribution: 2 GeV/c<pT<pTtrigger • normalize to number of triggers Phys Rev Lett 90, 082302 Carl Gagliardi – QNP 2004

  23. ? Azimuthal distributions in Au+Au Au+Au peripheral Au+Au central pedestal and flow subtracted Phys Rev Lett 90, 082302 Near-side: peripheral and central Au+Au similar to p+p Strong suppression of back-to-back correlations in central Au+Au Carl Gagliardi – QNP 2004

  24. Other effects that might change RAA andRCP • Hadronic re-interactions • Change in particle production mechanisms • Initial- or final-state multiple scattering (“Cronin effect”) • Nuclear modifications of the parton distributions (“shadowing” and “anti-shadowing”) • Gluon saturation at high energy and low x Carl Gagliardi – QNP 2004

  25. Final-state “pre-hadronic” plus hadronic rescattering Cassing, Gallmeister, and Greiner, hep-ph/0311358 May also have difficulty explaining high-pT v2 and near-side angular correlations Carl Gagliardi – QNP 2004

  26. Baryons vs. mesons PHENIX: PRL 91, 172301 In central Au+Au collisions, baryons are substantially overproduced relative to mesons at intermediate pT Carl Gagliardi – QNP 2004

  27. Hadronization via constituent quark recombination/coalescence From R. Fries, QM’04 Coalescence of partons into hadrons: Where does coalescence win over fragmentation? Assume Wab(1;2) = wa(1)wb(2) Exponental: fragmenting parton: ph = z p, z<1 recombining partons: p1+p2=ph Power law: Carl Gagliardi – QNP 2004

  28. Constituent quark scaling at intermediate pT At intermediate pT, different baryon and meson v2 values may reflect a common underlying partonic flow. Carl Gagliardi – QNP 2004

  29. STAR Limits of recombination/coalescence STAR: PRL 92, 052302 PHENIX: PRL 91, 172301 The “baryon enhancement” ends around pT ~ 5-6 GeV/c Carl Gagliardi – QNP 2004

  30. Theory vs. data pQCD-I: Wang, nucl-th/0305010 pQCD-II: Vitev and Gyulassy, PRL 89, 252301 Saturation: KLM, Phys Lett B561, 93 PRL 91, 172302 RCP Final state Initial state pT>5 GeV/c: well described by gluon saturation model (up to 60% central) and pQCD+jet quenching Carl Gagliardi – QNP 2004

  31. partonic energy loss Is suppression an initial or final state effect? Initial state? Final state? gluon saturation How to discriminate? Turn off final state d+Au collisions! Carl Gagliardi – QNP 2004

  32. Inclusive spectra RAB If Au+Au suppression is final state 1.1-1.5 1 If Au+Au suppression is initial state (KLM gluon saturation: 0.75) ~2-4 GeV/c pT High pT hadron pairs broadening? pQCD: no suppression, small broadening due to Cronin effect saturation models: suppression due to mono-jet contribution? 0 suppression? /2  0  (radians) d+Au vs. p+p: Theoretical expectations Carl Gagliardi – QNP 2004

  33. STAR d+Au inclusive yields relative to binary-scaled p+p STAR: PRL 91, 072304 • d+Au : enhancement • Au+Au: strong suppression Suppression of the inclusive yield in central Au+Au is a final-state effect Carl Gagliardi – QNP 2004

  34. PHENIX, PHOBOS, BRAHMS find similar results PHOBOS PHENIX BRAHMS PRL 91, 072302/3/5 Carl Gagliardi – QNP 2004

  35. Azimuthal distributions PRL 91, 072304 pedestal and flow subtracted Near-side: p+p, d+Au, Au+Au similar Back-to-back: Au+Au strongly suppressed relative to p+p and d+Au Suppression of the back-to-back correlation in central Au+Au is a final-state effect Carl Gagliardi – QNP 2004

  36. Thestrong suppressionof the inclusive yield and back-to-back correlations at high pT observed in central Au+Au collisions are due to final-state interactions with the dense mediumgenerated in such collisions. Carl Gagliardi – QNP 2004

  37. Phys Rev Lett 91, 072302/3/4/5 Carl Gagliardi – QNP 2004

  38. 1 + (g pQCD direct x Ncoll) / gphenix backgrd Vogelsang NLO 1 + (g pQCD direct x Ncoll) / gphenix backgrd Vogelsang, mscale = 0.5, 2.0 1 + (g pQCD direct x Ncoll) / (gphenix pp backgrd x Ncoll) Direct photons preliminary AuAu 200 GeV Central 0-10% Direct photons aren’t suppressed in central Au+Au collisions. Another demonstration that suppression is a final-state effect Carl Gagliardi – QNP 2004

  39. STAR Back-to-back correlations vs. reaction plane preliminary 20-60% central Near-side correlations: independent of orientation Back-to-back correlations: suppressed more strongly when the path length is longer Carl Gagliardi – QNP 2004

  40. Mid-rapidity vs. forward rapidity Mid-rapidity spectra tend to sample quarks and gluons in the incident nuclei at: In contrast, forward spectra tend to sample quarks at large x and gluons around: Carl Gagliardi – QNP 2004

  41. BRAHMS RdAu at different pseudo-rapidities Statistical errors dominate over systematic at =2 and 3 Systematic error (not shown) ~15% nucl-ex/0403005 and preliminary Possible evidence that gluon saturation is important forforward physics at RHIC Carl Gagliardi – QNP 2004

  42. Have we found the Quark Gluon Plasma at RHIC? We now know that Au+Au collisions generate a medium that: • is dense (pQCD theory: up to ~100 times cold nuclear matter) • is dissipative • exhibits strong collective behavior This represents significant progress in our understanding of QCD matter We have yet to show: • hydrodynamics can describe spectra, v2, and HBT simultaneously • direct evidence the system is thermalized • chiral restoration • a transition occurs: can we turn the effects off ? There is still work to do Carl Gagliardi – QNP 2004

  43. Carl Gagliardi – QNP 2004

  44. Rapidity: Invariant cross section: Kinematics for colliders Pseudo-rapidity: for m/p << 1 Transverse momentum (pT)and(pseudo-)rapidity (y or ) provide a convenient description Carl Gagliardi – QNP 2004

  45. Zhang, Gyulassy, Ko, PL B455 (1999) 45 Free-streaming does not convert spatial anisotropy to momentum anisotropy Developed early (t < 2 fm/c) Why is elliptic flow interesting? Sensitive to the physics of constituent interactions at early time Carl Gagliardi – QNP 2004

  46. BRAHMS Particle spectra and radial flow • Fit Au+Au spectra to blast wave model: • S (surface velocity) drops with dN/d • T (temperature in GeV) almost constant. Blue y=0 Red y=1 y= 0 T 1 and 3  contours preliminary An explosive expansion? pT (GeV/c) Carl Gagliardi – QNP 2004

  47. RAA 2 1 0 0 2 4 pT (GeV/c) Suppression of inclusive yield at 130 GeV STAR, PRL 89, 202301 PHENIX, PRL 88, 022301 Both STAR and PHENIX see significant suppression Limitation: Ambiguities in the reference spectra at 130 GeV Carl Gagliardi – QNP 2004

  48. RAA different for charged hadrons and p0 ? Charged Hadrons p0 PHENIX PRL 91, 072301 STAR PRL 91, 172302 For pT less than ~5 GeV/c, charged hadrons are less suppressed in central Au+Au collisions than p0 Carl Gagliardi – QNP 2004

  49. Baryons vs. mesons – yet another view K* (and maybe ) follow K0S -- Ξ (and maybe Ω) follow Λ Carl Gagliardi – QNP 2004

  50. Final state “pre-hadronic” and hadronic scattering • Calculations based on the idea of “color transparency” • Hard-scattered parton neutralizes its color, then propagates as a color-neutral object • The object grows into a well-defined hadron during a “formation time” tf. The interaction cross section grows over the same period. tf Calculations use: Carl Gagliardi – QNP 2004

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