1Carl Gagliardi – QNP 2004 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-p T ” regime – In the “intermediate-” and “high-p T ” regimes What did we see in d+Au collisions?

2Carl Gagliardi – QNP 2004 Lattice QCD at finite temperature F. Karsch, hep-ph/ Critical energy density: T C ~ 175 MeV  C ~ 1 GeV/fm 3 Ideal gas (Stefan- Boltzmann limit )

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

4Carl Gagliardi – QNP 2004 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” x z

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

6Carl Gagliardi – QNP 2004 p T distribution of charged particles Systematic Errors not shown nucl-ex/

7Carl Gagliardi – QNP 2004 Bulk particle production in Au+Au dN ch /d   19.6 GeV 130 GeV200 GeV PRL 91, Central Peripheral Central collisions at 200 GeV: 5000 charged particles (!) Energy density: boost invariant hydrodynamics (Bjorken) (R ~ A 1/3,  = 1 fm/c) Central Au+Au at 130 GeV:  GeV/fm 3 (PHENIX, PRL 87, )

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

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

10Carl Gagliardi – QNP 2004 HBT interferometry versus reaction plane Geometrical analog of v 2 – confirms source anisotropy. Note: Magnitudes of R l and R o /R s smaller than predicted by hydrodynamic calculations. nucl-ex/ STAR

11Carl Gagliardi – QNP 2004 Chemical equilibrium? Thermal model: Partition fn with params T,  B,  s,  I3 Fit to ratios of antiparticle/particle: , K, p, , , , K * 0, ….

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

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

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

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

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

17Carl Gagliardi – QNP 2004 Suppression of inclusive hadron yield central Au+Au collisions: factor ~4-5 suppression p T > 5 GeV/c: suppression ~ independent of p T PRL 91, Au+Au relative to p+p Au+Au central/peripheral R AA R CP

18Carl Gagliardi – QNP 2004 PHENIX observes a similar effect with π 0 PRL 91, Binary scaling Factor of 4- 5 PRL 91, p-p Au-Au

19Carl Gagliardi – QNP 2004 So do PHOBOS (  ~ 0.8) and BRAHMS (  ~ 2.2) PHOBOS: PL B578, 297 and PRL 91, BRAHMS: PRL 91, Also have  = 0 BRAHMS

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

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

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

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

24Carl Gagliardi – QNP 2004 Other effects that might change R AA and R CP 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

25Carl Gagliardi – QNP 2004 Final-state “pre-hadronic” plus hadronic rescattering May also have difficulty explaining high-p T v 2 and near-side angular correlations Cassing, Gallmeister, and Greiner, hep- ph/

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

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

28Carl Gagliardi – QNP 2004 Constituent quark scaling at intermediate p T At intermediate p T, different baryon and meson v 2 values may reflect a common underlying partonic flow.

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

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

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

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

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

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

35Carl Gagliardi – QNP 2004 Azimuthal distributions 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 PRL 91,

36Carl Gagliardi – QNP 2004 The strong suppression of the inclusive yield and back-to-back correlations at high p T observed in central Au+Au collisions are due to final- state interactions with the dense medium generated in such collisions.

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

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

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

40Carl Gagliardi – QNP 2004 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:

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

42Carl Gagliardi – QNP 2004 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 There is still work to do We have yet to show: hydrodynamics can describe spectra, v 2, and HBT simultaneously direct evidence the system is thermalized chiral restoration a transition occurs: can we turn the effects off ? This represents significant progress in our understanding of QCD matter

43Carl Gagliardi – QNP 2004

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

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

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

47Carl Gagliardi – QNP 2004 Suppression of inclusive yield at 130 GeV PHENIX, PRL 88, STAR, PRL 89, Both STAR and PHENIX see significant suppression Limitation: Ambiguities in the reference spectra at 130 GeV p T (GeV/c) 1 2 R AA

48Carl Gagliardi – QNP 2004 R AA different for charged hadrons and  0 ? STAR PRL 91, PHENIX PRL 91, For p T less than ~5 GeV/c, charged hadrons are less suppressed in central Au+Au collisions than  0 Charged Hadrons 00

49Carl Gagliardi – QNP 2004 Baryons vs. mesons – yet another view K * (and maybe  ) follow K 0 S -- Ξ (and maybe Ω) follow Λ

50Carl Gagliardi – QNP 2004 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” t f. The interaction cross section grows over the same period. tftf Calculations use:

51Carl Gagliardi – QNP 2004 Modified gluon density: “Gluon saturation” Mid Rapidity Forward Rapidity CTEQ6M Saturation sets in when gluons start to overlap

52Carl Gagliardi – QNP 2004 Relative charge dependence Strong dynamical charge correlations in jet fragmentation  Compare ++ and -- charged azimuthal correlations to +- azimuthal correlations STAR PRL 90, % most central Au+Au p+p minimum bias 4<p T (trig)<6 GeV/c 2<p T (assoc.)<pT(trig) System(+-)/(++ & --) p+p % Au+Au Jetset Same correlated particle production for p T >4 GeV/c in pp and central Au+Au Au+Au p+p

53Carl Gagliardi – QNP 2004 High p T correlations: Au+Au vs p+p PRL 90, (2003) Central Au + Au Peripheral Au + Au Back-to-back jets are suppressed in central collisions near side away side peripheral central

54Carl Gagliardi – QNP 2004 Collision timescale Evolution time (fm/c) Hydrodynamical model Blast-wave fit R long Sinyukov fit cc  eq HBT(  ) Buda-Lund

55Carl Gagliardi – QNP 2004 Baryon production in d+Au nucl-ex/ STAR Central Au+Au d+Au p+p Baryon enhancement in central Au+Au is final state effect