Overview of Experimental results from RHIC Y Akiba

Overview of Experimental results from RHIC Y. Akiba (RIKEN Nishina Center) ATHIC 08 Tsukuba October 13, 2008

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 ms after the Big Bang Tc ~ 170 Me. V; e ~ 1 Ge. V/fm 3 This is the only phase transition that occurred in the early universe that can be recreated in the lab 2

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

RHIC runs (2001 -2008) Beam species: p+p (polaized) d+Au Cu+Cu Au+Au Energy: s. NN 1/2=200 Ge. V Also @ 130 Ge. V 62 Ge. V 56 Ge. V 22 Ge. V (10 Ge. V) 130 Ge. V 200 Ge. V

RHIC’s Two Major Discoveries STAR PRL 86, 402 (2001) Strong Elliptic flow Agree with ideal hydrodynamics Low viscosity/entropy (h/s) PHENIX PRL 88, 022301(2002) High p. T suppression Energy loss of quark/gluon Very dense matter Based on these two major discoveries and other evidence, RHIC experiments concluded 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 Ge. V/c) – Constraining model parameter from RAA • • Modification of jet-correlations J/ suppression Heavy quark suppression and flow Dileptons and photons Topics I don’t discuss due to time limitation • • • Low p. T hadron spectra Hadron ratios and thermal model Enhanced (anti-)baryons multi-strange baryons v 1 • • • v 2/v 4 scaling c/b m -jet correlation HBT and source imaging And more…

Elliptic flow v 2

Scaling of v 2 of hadrons PRL 98, 162301(2007) • More data on v 2(p. T) of hadrons are accumulated • When v 2/nq vs KET/nq (KET=transverse kinetic enery), all data points are on a universal curve, suggesting that v 2 developed in partonic stage

More on the scaling of v 2: phi flow PRL 99, 052301 (2007) Phi meson (small interaction cross section) also follows the number of quark (nq) scaling.

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

High p T suppression RAA

π0 p. T spectra at √s. NN = 200 Ge. V RUN 4 Au+Au ar. Xiv: 0801. 4020 [nucl-ex] RUN 2 Au+Au PRL 91, 072301 RAA measurements now extends to 20 Ge. V/c

RAA of hadrons and direct photon (Au. Au 200 Ge. V) A factor of ~5 suppression to ~20 Ge. V ! • A factor of ~5 suppression of 0 to ~20 Ge. V/c • Ncoll scaling for direct • Same suppression pattern for 0 and h: Consistent with parton energy loss and fragmentation in the vacuum • Smaller suppression for the f meson for 2<p. T<5 Ge. V/c

Quantitative analysis: contrain density parameters Comparison with GRV model: d. Ng/dy=1400 PRC 77, 064907

RAA beam energy dependence (Cu+Cu) Cu+Cu 22, 62, 200 Ge. V (Run 5) ar. Xiv: 0801. 4555 Accepted in PRL • Model calculations indicate quenching expected at s. NN = 22 Ge. V, but Cronin effect dominates • Species dependence to probe space/time of suppression

Di-jet correlations

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

Modification of jet correlation PRL 97, 052301 (2006) Au+Au • This is another big surprise: two particle of two high p. T 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 PRL 98_232302

Reaction plane dependence of di-jet correlation Shortest path length longest path length • 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

Conical emission? Df*= 0 Dq*= p PHENIX Preliminary Consistent with conical emission; STAR, 0805. 0622 3 -particle correlation analysis shows that the data is consistent with conical emission

More surprize: the Ridge? Trigger Jet STAR QM 2006 Ridge Bulk Medium In QM 2006, STAR shows that there is “Ridge”, Enhancement in small Df and large Dh of leading particle This is the latest surprise in jet correlation in Au+Au and becomes a hot topics

Is there “Ridge”? Apparently… • • In QM 2008, 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) If QGP is formed, J/ production is suppressed In normal vacuum, J/ particle is formed In QGP, J/ is destroyed by color screening

J/ suppression in Au+Au PRL 98_172301 • High statistics measurement of J/ in Au. Au in wide rapidity range – Mid-rapidty J/ ee – Forward rapidty J/ mm • Strong suppression of J/ is observed – Consistent with the prediction that J/ s are destroyed in deconfined 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

J/ in d+Au: Cold Nuclear Matter effect J/ Rd. Au 200 Ge. V PRC 77_024912 • Nuclear suppression factor Rd. Au of J/ in d+Au is measured and compared with models of CNM • Result: CNM = Shadowing(EKS)+ Breakup +1. 7 Breakup = 2. 8 -1. 4 mb • This is consistent with the J/ break up cross section at lower energy Breakup=4. 2+/-0. 5 mb • If Breakup is obtained separately in forward and central region, larger value is prefered in forward As SQM participants are aware of it, PHENIX is revisiting the systematic error in the break-up cross section.

J/ RAA Cu+Cu and Au+Au J/ RAA 200 Ge. V PRL 101, 12301(2008) • Approx 2 x more J/ in Cu+Cu sample than Au+Au sample – More precise Npart<100 info • Curves show RAA prediction from ad hoc CNM fit to Rd. Au separately at y=0 and y > 1. 2 • CNM from Rd. Au fit describes suppression well for Npart < 50. Rd. Au constraints are not sufficient to say if suppression beyond cold nuclear matter is stronger at forward rapidity New Au+Au data (x 4 statistics) and d+Au data (x 30 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 e • • D, B • c, b quark • 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 RAA and v 2 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

Heavy flavor production in pp (base line) Phys. Rev. Lett 97, 252002 (2006) • Single electrons from heavy flavor (charm/bottom) decay are measured and compared with p. QCD theory (FONLL) • The new data extends the p. T reach to 9 Ge. V/c • FONLL p. QCD calculation agree with the data • c e dominant in low p. T b e is expected to be dominant in high p. T

Large energy loss and flow of heavy quarks RAA of b, c e v 2 of b, c e Strong suppression of electron from c and b Large elliptic flow of electrons from c and b! Large energy loss of heavy quark Heavy quark flows in the medium • 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

Heavy flavor electron RAA and flow n Two models describes strong suppression and large v 2 l Rapp and Van Hee l Moore and Teaney n From model comparison, viscosity to entropy ratio h/s can be estimated DHQ × 2πT = 4 - 6 DHQ ~ 6 x h/(e+p) = 6 x h/Ts h/s ~ (4/3 – 2)/4 estimate of h/s is close to the conjectured bound 1/4 from Ad. S/CFT n The PRL 98, 172301 (2007)

Comparison with other estimates R. Lacey et al. : PRL 98: 092301, 2007 H. -J. Drescher et al. : ar. Xiv: 0704. 3553 S. Gavin and M. Abdel-Aziz: PRL 97: 162302, 2006 p. Tfluctuations STAR v 2 PHENIX & STAR v 2 PHOBOS Estimates of h/s based on flow and fluctuation data indicate small value as well close to conjectured limit significantly below h/s of helium (4 h/s ~ 9) conjectured quantum limit

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

Next steps in Heavy quark measurements Higher statistics electron v 2 measurement minimum-bias Run-4 PRELIMINARY Run-7 b/c separation (so far only in pp) Preliminary results STAR and PHENIX Rapp & van Hees, PRC 71, 034907 (2005) • Does b quark also have large energy loss and/or flow? Recent data show large v 2 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.

Electromagentic probes (photon and lepton pairs) e+ * e- • 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

PHENIX low mass dielectrons p+p NORMALIZED TO mee<100 Me. V low mass w f Au. Au intermediate mass J/ ’ pp submitted to Phys. Lett. B ar. Xiv: 0802. 0050 pp and Au. Au normalized to p 0 Dalitz region (~ same # of particles) submitted to Phys. Rev. Lett ar. Xiv: 0706. 3034 p+p: agree with the expected background from hadron decays Au+Au: large Enhancement in 0. 15 -0. 75 Ge. V/c 2

PT Dependence of Au+Au Mee 0 < p. T < 8 Ge. V/c 0. 7 < p. T < 1. 5 Ge. V/c 0 < p. T < 0. 7 Ge. V/c 1. 5 < p. T < 8 Ge. V/c PHENIX Preliminary • 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?

Thermal(? ) Photons from the hot matter thermal: Decay photons (background) 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. hard: Measurement is difficult since the expected signal is only 1/10 of photons from hadron decays

Enhancement of almost real photon ar. Xiv: 0804. 4168 pp Au+Au (MB) 1 < p. T < 2 Ge. V 2 < p. T < 3 Ge. V 3 < p. T < 4 Ge. V 4 < p. T < 5 Ge. V Low mass e+e- pairs (m<300 Me. V) for 1<p. T<5 Ge. V/c p+p: • Good agreement of p+p data and hadronic decay cocktail • Small excess in p+p at large mee and high p. T Au+Au: • Clear enhancement visible above for all p. T

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

Fraction of direct photons Au+Au (MB) p+p μ = 0. 5 p. T μ = 1. 0 p. T μ = 2. 0 p. T • Fraction r of direct photons p+p: • Consistent with NLO p. QCD • favors small μ Au+Au: • Clear excess above p. QCD NLO p. QCD calculation is provided by Werner Vogelsang

Direct photon in p+p, Au+Au ar. Xiv: 0804. 4168 exp + TAA scaled pp Fit to pp NLO p. QCD (W. Vogelsang) • The p+p data agrees with NLO p. QCD 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~ 220 Me. V • Thermal photons from hydrodynamical models with Tinit=300 – 600 Me. V at t 0=0. 6 -0. 15 fm/c are qualitative agreement with the data (see next)

Theory comparison • Hydrodynamical models are compared with the data D. d’Enterria &D. Peressounko T=590 Me. V, t 0=0. 15 fm/c S. Rasanen et al. T=580 Me. V, t 0=0. 17 fm/c D. K. Srivastava T=450 -600 Me. V, t 0=0. 2 fm/c S. Turbide et al. T=370 Me. V, t 0=0. 33 fm/c J. Alam et al. T=300 Me. V, t 0=0. 5 fm/c • Hydrodynamical models are in qualitative agreement with the data Thery compilation by D. d’Enterria and D. Peressounko EPJC 46, 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 p. T 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