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 ~1s after the Big Bang

This is the only phase transition that occurred in the early universe that can be recreated in the lab

SB (T) 2

30(Nbosons 7 /8 N fermions)T

4

Tc ~ 170 MeV; ~ 1 GeV/fm3

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:sNN

1/2=200 GeV

Also @ 130 GeV62 GeV56 GeV22 GeV

(10 GeV)

130 GeV 200 GeV

RHIC’s Two Major Discoveries

Strong Elliptic flowAgree with ideal hydrodynamicsLow viscosity/entropy (/s)

High pT suppressionEnergy loss of quark/gluonVery 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 v2

• Suppression at higher pT (up to 20 GeV/c)

– Constraining model parameter from RAA

• Modification of jet-correlations• J/ suppression• Heavy quark suppression and flow• Dileptons and photons

• Low pT hadron spectra

• Hadron ratios and thermal model

• Enhanced (anti-)baryons

• multi-strange baryons

• v1

Topics I don’t discuss due to time limitation

• v2/v4 scaling

• c/b • -jet correlation

• HBT and source imaging

• And more…

Elliptic flow

v2

Scaling of v2 of hadrons

• More data on v2(pT) of hadrons are accumulated• When v2/nq vs KET/nq (KET=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 followsthe number of quark (nq) scaling.

More on the scaling of v2: phi flow

PRL99, 052301 (2007)

v2 of Direct photon and J/e+e-

First ever at RHIC,

v2 - J/µ+µ- coming soon

J/Psi coalescence ?

PHENIX preliminary

Direct v2 Min Bias Au+Au 200 GeV (Run 4)

Sign of direct v2 (at high pT): – Positive == parton emission quenched– Negative == parton emission (Brems.)

enhancedAt high pT, photon v2 is consistent with zero

High p T suppression

RAA

π0 pT spectra at √sNN = 200 GeV

RUN2 Au+Au PRL91,072301

RUN4 Au+Au arXiv:0801.4020 [nucl-ex]

RAA measurements now extends to 20 GeV/c

RAA 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<pT<5 GeV/c

A factor of ~5suppression to~20 GeV !

Quantitative analysis: contrain density parameters

Comparison with GRV model: dNg/dy=1400 PRC77,064907

RAA beam energy dependence (Cu+Cu)

Cu+Cu 22,62,200 GeV (Run 5)

• Model calculations indicate quenching expected at sNN = 22 GeV, but Cronin effect dominates

• Species dependence to probe space/time of suppression

arXiv:0801.4555Accepted in PRL

Di-jet correlations

17

Dijet correlation

Back-to-back peakdue to di-jets is seenin two particle correlation

Reconstruction of jets is difficult in A+A @ 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

PRL97,052301 (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

224

2

/

)(

kurtosis

rms

nn

D

)(

)()(

)(

DG

DGG

J

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, 0805.0622

3-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 v2?

Screening by the QGP (An explicit test of deconfinement)

QCD potential at T=0

r -->

V(r

)

QCD potential at high T and

high density

r -->

V(r

)

Non-perturbative Vacuum

Perturbative Vacuum

cc

Color Screening

cc

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 RdAu 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 energyBreakup=4.2+/-0.5mb

• If Breakup is obtained separately in forward and central region, larger value is prefered in forward

PRC77_024912

+1.7-1.4

J/ RdAu 200 GeV

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

• Approx 2x 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 RdAu separately at y=0 and y > 1.2

• CNM from RdAu fit describes suppression well for Npart < 50.

J/ RAA 200 GeV PRL101,12301(2008)

RdAu 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 RAA and v2 of the electrons from heavy quark decays, the energy loss and the flow of heavy quarks are indirectly measured.

• So far, ce and be are not separated

c, b quark

D, B

e

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 pQCD theory (FONLL)

• The new data extends the pT reach to 9 GeV/c

• FONLL pQCD calculation agree with the data

• c e dominant in low pT

be is expected to be dominant in high pT

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 ce (dominant) and be, we cannot determine the

suppression or flow of be.• 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

RAA of b,c e v2 of b,c e

Strong suppression of electron from c and bLarge energy loss of heavy quark

Large elliptic flow of electrons from c and b!Heavy quark flows in the medium

Heavy flavor electron RAA and flow

Two models describes strong suppression and large v2

Rapp and Van Hee Moore and Teaney

From model comparison, viscosity to entropy ratio /s can be estimated

DHQ × 2πT = 4 - 6DHQ ~ 6 x /(+p) = 6 x /Ts

/s ~ (4/3 – 2)/4 The estimate of /s is close to the conjectured bound 1/4from AdS/CFT

PRL98,172301 (2007)

4/)8.30.1(/ s

S. Gavin and M. Abdel-Aziz: PRL 97:162302, 2006

pTfluctuations STAR

Comparison with other estimates

4/)2.12.01.1(/ sR. Lacey et al.: PRL 98:092301, 2007

v2 PHENIX & STAR

4/)4.24.1(/ s

H.-J. Drescher et al.: arXiv:0704.3553

v2 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 be/ce 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 pT where be dominates

• Silicon vertex tracker now under construction can answer this queston by separating be and ce in Au+Au collisions.

PRELIMINARYRun-4Run-7

Rapp & van Hees, PRC 71, 034907 (2005)

minimum-bias

Higher statistics electron v2 measurement b/c separation (so far only in pp)

Preliminary resultsSTAR 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-

pp and AuAu normalized to p0 Dalitz region (~ same # of particles)

p+p: agree with the expected background from hadron decays

Au+Au: large Enhancement in 0.15-0.75 GeV/c2

p+p NORMALIZED TO mee<100 MeV

submitted to Phys. Lett.B

arXiv: 0802.0050

submitted to Phys. Rev. Lett

arXiv:0706.3034

PHENIX low mass dielectrons

AuAu

pp

low mass

intermediate mass

J/

’

PT Dependence of Au+Au Mee

• Low Mass excess is a low pT enhancement– Huge excess at

lowest pT

– Excess reduced for higher pT

This suggests that the low mass enhancement is from later phase of the reaction

ee in later hadronic gas phase?

0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c

0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c

PHENIX Preliminary

Thermal(?) Photons from the hot matter

Decay photons (background)

nT

1

phard:

/ E Tethermal: If the dense matter formed at RHICThermailzed, 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<pT<5 GeV/c

p+p:• Good agreement of p+p data

and hadronic decay cocktail • Small excess in p+p at large

mee and high pT

Au+Au:• Clear enhancement visible

above for all pT

pp Au+Au (MB)

1 < pT < 2 GeV2 < pT < 3 GeV3 < pT < 4 GeV4 < pT < 5 GeV

arXiv:0804.4168

Determination of * fraction, r

r : direct */inclusive *

Direct */inclusive * is determined by fitting the following function for each pT bin.

eedirecteecocktaileedata mfrmfrmf 1

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 : fdirect 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 = 0.5pT

μ = 1.0pT

μ = 2.0pT

μ = 0.5pT

μ = 1.0pT

μ = 2.0pT

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 pT excess above scaled p+p expectations

• Excess is exponential in shape with inverse slope T~ 220MeV

• Thermal photons from hydrodynamical models with Tinit=300 – 600MeV at 0=0.6-0.15fm/c are qualitative agreement with the data (see next)

NLO pQCD (W. Vogelsang)

Fit to pp

exp + TAA scaled pparXiv:0804.4168

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=450-600MeV, 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 dataThery 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