Overview of Experimental results from RHIC Y. Akiba (RIKEN Nishina Center) ATHIC08 Tsukuba October 13, 2008

2 QCD Phase Transition The colliding nuclei at RHIC energies would melt from protons and neutrons into a collection of quarks and gluons A QCD phase transition that the universe last went through ~1  s after the Big Bang This is the only phase transition that occurred in the early universe that can be recreated in the lab T c ~ 170 MeV;  ~ 1 GeV/fm 3

The RHIC Experiments RHIC Approx 500 tracks result from a Au+Au ion collision

RHIC runs ( ) Beam species: p+p (polaized) d+Au Cu+Cu Au+Au Energy: s NN 1/2= 200 GeV 130 GeV 62 GeV 56 GeV 22 GeV (10 GeV) 130 GeV 200 GeV

RHIC’s Two Major Discoveries Strong Elliptic flow  Agree with ideal hydrodynamics  Low viscosity/entropy (  /s) High p T suppression  Energy loss of quark/gluon  Very dense matter STAR PRL86,402 (2001) PHENIX PRL88,022301(2002) Based on these two major discoveries and other evidence, RHIC experiments concluded that that state of dense partonic matter is formed in A+A collisions at RHIC

Highlights from more recent RHIC results Scaling of v 2 Suppression at higher p T (up to 20 GeV/c) –Constraining model parameter from R AA Modification of jet-correlations J/  suppression Heavy quark suppression and flow Dileptons and photons Low p T hadron spectra Hadron ratios and thermal model Enhanced (anti-)baryons multi-strange baryons v 1 Topics I don’t discuss due to time limitation v 2 /v 4 scaling c/b    -jet correlation HBT and source imaging And more…

Elliptic flow v 2

Scaling of v2 of hadrons More data on v2(pT) of hadrons are accumulated When v2/nq vs KE T /nq (KE T =transverse kinetic enery), all data points are on a universal curve, suggesting that v2 developed in partonic stage PRL98,162301(2007)

Phi meson (small interaction cross section) also follows the number of quark (nq) scaling. More on the scaling of v2: phi flow PRL99, (2007)

v 2 of Direct photon and J/   e + e - First ever at RHIC, v 2 - J/   µ + µ - coming soon J/Psi coalescence ? PHENIX preliminary Direct  v 2 Min Bias Au+Au 200 GeV (Run 4) Sign of direct  v 2 (at high p T ): –Positive == parton emission quenched –Negative == parton emission (Brems.) enhanced At high pT, photon v2 is consistent with zero

High p T suppression R AA

π 0 p T spectra at √s NN = 200 GeV RUN2 Au+Au PRL91, RUN4 Au+Au arXiv: [nucl-ex] RAA measurements now extends to 20 GeV/c

R AA of hadrons and direct photon (AuAu 200GeV) A factor of ~5 suppression of  0 to ~20 GeV/c Ncoll scaling for direct  Same suppression pattern for  0 and  : Consistent with parton energy loss and fragmentation in the vacuum Smaller suppression for the  meson for 2<p T <5 GeV/c A factor of ~5 suppression to ~20 GeV !

Quantitative analysis: contrain density parameters Comparison with GRV model: dNg/dy=1400PRC77,064907

R AA beam energy dependence (Cu+Cu)   Cu+Cu 22,62,200 GeV (Run 5) Model calculations indicate quenching expected at  s NN = 22 GeV, but Cronin effect dominates Species dependence to probe space/time of suppression arXiv: Accepted in PRL 

Di-jet correlations

17 Dijet correlation Back-to-back peak due to di-jets is seen in two particle correlation Reconstruction of jets is difficult in RHIC In central Au+Au collisions, the peak in the far side (  ~  ) is suppressed, consistent with energy loss of the recoil jet. Trigger Recoil jet

Modification of jet correlation PRL97, (2006) Au+Au This is another big surprise: two particle of two high pT track (jet correlation) is modified in central Au+Au collisions. Many theory attempts to explain this effect

Origin of the modification of jets? An interesting interpretation of the modification is that it is Mach cone in the medium Scattered parton travels faster than the speed of sound in the medium, causing a shock-wave If this is the case, the opening angle can be related to the speed of sound in the medium…

More detailed study of jet correlation D PRL98_232302

Reaction plane dependence of di-jet correlation Shape of the near-side peak is unchanged Far-side shape strongly depends on the angle from the reaction plane o Stronger modification for longer pathlength in the dense matter Shortest path length longest path length

Conical emission? PHENIX Preliminary  *=   *=  STAR, particle correlation analysis shows that the data is consistent with conical emission Consistent with conical emission;

More surprize: the Ridge? RidgeTrigger Jet Bulk Medium In QM2006, STAR shows that there is “Ridge”, Enhancement in small  and large  of leading particle This is the latest surprise in jet correlation in Au+Au and becomes a hot topics STAR QM2006

Is there “Ridge”? Apparently… In QM2008, both PHENIX and PHOBOS shows that they also see “Ridge” So far there is no consensus on the origin of this effect. It is difficult to imagine that information can propagate for a wide rapidity gap. My Speculation: Effect can be due to non-linear correlation between jets and v 2 ?

Screening by the QGP (An explicit test of deconfinement) In normal vacuum, J/  particle is formed In QGP, J/  is destroyed by color screening If QGP is formed, J/  production is suppressed

J/  suppression in Au+Au High statistics measurement of J/  in AuAu in wide rapidity range –Mid-rapidty J   ee –Forward rapidty J/    Strong suppression of J/  is observed –Consistent with the prediction that J/  s are destroyed in de- confined matter Surprisingly, the suppression is stronger at forward rapidity than in mid-rapidity –J/  formation by recombination of charm pairs in deconfined matter? But…we need to look the cold nuclear matter effect PRL98_172301

J/  in d+Au: Cold Nuclear Matter effect Nuclear suppression factor R dAu of J/  in d+Au is measured and compared with models of CNM Result: CNM = Shadowing(EKS)+  Breakup  Breakup = 2.8 mb This is consistent with the J/  break up cross section at lower energy  Breakup =4.2+/-0.5mb If  Breakup is obtained separately in forward and central region, larger value is prefered in forward PRC77_ J/  R dAu 200 GeV As SQM participants are aware of it, PHENIX is revisiting the systematic error in the break-up cross section.

J/  R AA Cu+Cu and Au+Au Approx 2x more J/  in Cu+Cu sample than Au+Au sample –More precise N part <100 info Curves show R AA prediction from ad hoc CNM fit to R dAu separately at y=0 and y > 1.2 CNM from R dAu fit describes suppression well for N part < 50. J/  R AA 200 GeV PRL101,12301(2008) R dAu constraints are not sufficient to say if suppression beyond cold nuclear matter is stronger at forward rapidity New Au+Au data (x4 statistics) and d+Au data (x30 statistics) obtained in 2007 and 2008 run can determine if the suppression really stronger beyond CNM in forward region.

Heavy quark (charm and bottom) probe Study medium effect in open charm and bottom production Ideally, D or B meson should be measured, but for technical reason most of the measurement so far is done through electron decay channel. From R AA and v2 of the electrons from heavy quark decays, the energy loss and the flow of heavy quarks are indirectly measured. So far, c  e and b  e are not separated c, b quark D, B e

Heavy flavor production in pp (base line) Phys. Rev. Lett 97, (2006) Single electrons from heavy flavor (charm/bottom) decay are measured and compared with pQCD theory (FONLL) The new data extends the p T reach to 9 GeV/c FONLL pQCD calculation agree with the data c  e dominant in low pT b  e is expected to be dominant in high p T

Large energy loss and flow of heavy quarks These results require very strong interaction between the dense matter and heavy quarks. Since the observed electron is mixture of c  e (dominant) and b  e, we cannot determine the suppression or flow of b  e. Theoretical expectation is that the medium-quark interaction becomes weaker for heavier quark. Large energy loss and/or flow of b quark would be very interesting R AA of b,c  ev 2 of b,c  e Strong suppression of electron from c and b  Large energy loss of heavy quark Large elliptic flow of electrons from c and b!  Heavy quark flows in the medium

Heavy flavor electron R AA and flow Two models describes strong suppression and large v 2 Rapp and Van Hee Moore and Teaney From model comparison, viscosity to entropy ratio  /s can be estimated D HQ × 2πT = D HQ ~ 6 x  /(  +p) = 6 x  /Ts   /s ~ (4/3 – 2)/4  The estimate of  /s is close to the conjectured bound 1/4  from AdS/CFT PRL98, (2007)

S. Gavin and M. Abdel-Aziz: PRL 97:162302, 2006 p T fluctuations STAR Comparison with other estimates R. Lacey et al.: PRL 98:092301, 2007 v 2 PHENIX & STAR H.-J. Drescher et al.: arXiv: v 2 PHOBOS conjectured quantum limit Estimates of  /s based on flow and fluctuation data indicate small value as well close to conjectured limit significantly below  /s of helium (  s ~ 9)

Bottom Measurement Charm and bottom spectra are both by a factor  above FONLL pQCD calculations (but within the uncertainty) STAR studied b  e/c  e ratios in pp and obtained similar b/c ratios p+p 200 GeV Charm and bottom extracted via e-h mass analysis

Next steps in Heavy quark measurements Does b quark also have large energy loss and/or flow? Recent data show large v2 at high p T where b  e dominates Silicon vertex tracker now under construction can answer this queston by separating b  e and c  e in Au+Au collisions. PRELIMINARY Run-4 Run-7 Rapp & van Hees, PRC 71, (2005) minimum-bias Higher statistics electron v2 measurementb/c separation (so far only in pp) Preliminary results STAR and PHENIX

Electromagentic probes (photon and lepton pairs) Photons and lepton pairs are cleanest probes of the dense matter formed at RHIC These probes has little interaction with the matter so they carry information deep inside of the matter   e+e+ e-e-

pp and AuAu normalized to p 0 Dalitz region (~ same # of particles) p+p: agree with the expected background from hadron decays Au+Au: large Enhancement in GeV/c 2 p+p NORMALIZED TO m ee <100 MeV submitted to Phys. Lett.B arXiv: submitted to Phys. Rev. Lett arXiv: PHENIX low mass dielectrons AuAu pp low mass intermediate mass J/  ’’  

P T Dependence of Au+Au M ee Low Mass excess is a low p T enhancement –Huge excess at lowest p T –Excess reduced for higher p T This suggests that the low mass enhancement is from later phase of the reaction   ee in later hadronic gas phase? 0 < p T < 8 GeV/c0 < p T < 0.7 GeV/c 0.7 < p T < 1.5 GeV/c1.5 < p T < 8 GeV/c PHENIX Preliminary

Thermal(?) Photons from the hot matter Decay photons (background) hard: thermal: If the dense matter formed at RHIC Thermailzed, it should emit “thermal radiation”. The temperature of the matter can directly measured from the spectrum of thermal photon. Measurement is difficult since the expected signal is only 1/10 of photons from hadron decays

Enhancement of almost real photon Low mass e + e - pairs (m<300 MeV) for 1<p T <5 GeV/c p+p: Good agreement of p+p data and hadronic decay cocktail Small excess in p+p at large m ee and high p T Au+Au: Clear enhancement visible above for all p T ppAu+Au (MB) 1 < p T < 2 GeV 2 < p T < 3 GeV 3 < p T < 4 GeV 4 < p T < 5 GeV arXiv:

Determination of  * fraction, r r : direct  * /inclusive  * Direct  * /inclusive  * is determined by fitting the following function for each pT bin.  the mass spectrum follows the expected 1/m behavior of photon internal conversion  Determine the fraction r of the “direct photon” component from the fit Reminder : f direct is given by Eq.(1) with S = 1.

Fraction of direct photons Fraction r of direct photons p+p: Consistent with NLO pQCD favors small μ Au+Au: Clear excess above pQCD μ = 0.5p T μ = 1.0p T μ = 2.0p T p+p Au+Au (MB) NLO pQCD calculation is provided by Werner Vogelsang

Direct photon in p+p, Au+Au The p+p data agrees with NLO pQCD predictions For Au+Au there is a significant low p T excess above scaled p+p expectations Excess is exponential in shape with inverse slope T~ 220MeV Thermal photons from hydrodynamical models with T init =300 – 600MeV at  0 = fm/c are qualitative agreement with the data (see next) NLO pQCD (W. Vogelsang) Fit to pp exp + T AA scaled pp arXiv:

Theory comparison Hydrodynamical models are compared with the data D.d’Enterria &D.Peressounko T=590MeV,  0 =0.15fm/c S. Rasanen et al. T=580MeV,  0 =0.17fm/c D. K. Srivastava T= MeV,  0 =0.2fm/c S. Turbide et al. T=370MeV,  0 =0.33fm/c J. Alam et al. T=300MeV,  0 =0.5fm/c Hydrodynamical models are in qualitative agreement with the data Thery compilation by D. d’Enterria and D. Peressounko EPJC46, 451 (2006)

Summary Huge amount of data are accumulated from RHIC in the past 8 years Many interesting phenomena are observed –Strong elliptic flow of light hadrons and heavy quarks –Strong suppression of high pT jets –Modification of jet correlation –Strong suppression of J/  –Energy loss and flow of heavy quarks –Enhanced production of lepton pairs and photons These observations are consistent with formation of thermalized, high temperature, high density partonic fluid