what did we learn at rhic?

What did we learn

at RHIC?Tetsufumi HiranoTetsufumi Hirano

Dept. of PhysicsDept. of Physics

The University of TokyoThe University of Tokyo

原子核・ハドロン物理：横断研究会

My Charge(?)

• 「原子核・ハドロン物理：横断研究会」におけるレビュー講演原子核の他分野の方々に当該分野の現状を報告

• ＳＰＩＲＥＳを使って引用数の多いＲＨＩＣ実験論文を理論屋（現象論屋）の立場から眺めてみよう。– 良い点：限られた時間で（客観的に）重要な

トピックスに絞れる– 悪い点：最新の結果に触れられない

• （私個人として、）何が分かって、何を目指すべきか。

What is RHIC?

Relativistic Heavy Ion Collider(2000-)

sideview

frontview

STAR

STAR

Purpose: Create a transient state of the QGPat high temperature and energy density ina laboratory and investigate its property.

これまでのＲＵＮRun Ions sNN Luminosity

1 Au+Au 130 GeV 1 b-1

2 Au+Au 200 GeV 24 b-1

p+p 200 GeV 0.15 pb-1

3 d+Au 200 GeV 2.74 nb-1

p+p 200 GeV 0.35 pb-1

4 Au+Au 200 GeV 241 b-1

Au+Au 62.4 GeV 9.1 b-1

p+p 200 GeV 324 nb-1

5 Cu+Cu 200 GeV 4.8 nb-1

Cu+Cu 62.4 GeV 190 b-1

p+p 200 GeV 3.8 pb-1

6 p+p 200 GeV 10.7 pb-1

p+p 62.4 GeV 0.1 pb-1

7 Au+Au 200 GeV 813 b-1

8d+Au

p+p

200 GeV

200 GeV予定

織田さん（ＣＮＳ）のトーク＠ RCNP （‘０７）から

•Size dependence•Energy dependence•Control experiment•(spin)

加速器側で制御できるのはせいぜい当てる原子核とエネルギー

FIND CN *** AND TOPCITE 100+*** = STAR, PHENIX, PHOBOS, BRAHMS (as of Nov.15,2007)

STAR PHENIX PHOBOS BRAHMSYields & Spectra 2 4 (1) 7 1

Elliptic flow 5 (3) 2(1)HBT 1 1

High pT 6 (4) 6 (3) 2Baryon 1

Etc. (incl. white paper) 4 (1) 3 (1) 1 (1) 1 (1)

Total 18 (8) 17 (6) 8 (1) 4 (1)

カッコ内はそのうち TOPCITE 250+

Dynamics of Heavy Ion Collisions

Time scale10fm/c~10-23sec<<10-4(early universe)

Temperature scale 100MeV~1012K

Freezeout

“Re-confinement”

Expansion, cooling

Thermalization

First contact (two bunches of gluons)

# of binary collisions

x

y

Thickness function:

Woods-Saxon nuclear density:Gold nucleus:0=0.17 fm-3

R=1.12A1/3-0.86A-1/3

d=0.54 fm

in = 42mb @200GeV

# of participants

1 －（ survival probability ）

Ncoll & Npart

Centrality （検出器側の制御）

PHENIX: Correlation btw. BBC and ZDC signals

•Npart and Ncoll as a functionof impact parameter•Categorize events with Npart or Ncoll

Au+Au 200 GeV






Total 18 (8) 17 (6) 8 (1) 4 (1)


Centrality Dependence of Multiplicity

PHENIX, PRL86,3500(‘01)Cited 194 times

•EKRT: final state saturation (Eskola et al.)•HIJING: event generator (string+jet) (Wang-Gyulassy)•Saturation Model: color glass condensate (Kharzeev-Levin)

PHOBOS,PRC65,061901(’02)Cited 105 times

•Nch ~ 5000(700) in very central collisions (at midrapidity)•~90% from soft particle production•Serves a severe test for models of particle production

Energy DensityBjorken formula (’83)

c from lattice

•Assuming =1fm/c, well above c from lattice in central collision at RHIC•Necessary condition to study the QGP at RHIC

Volume of cylinder

Sorry, I could not find the original paper.PHENIX, PRL87,052301(’01)Cited 121 times

Particle Relative Yield

STAR,NPA757,102(’05)Cited 408 times

•Tch = 157 +- 6 MeV, B = 22 +- 4 MeV•Really equilibrated in peripheral collisions?•Just fitting parameters?

STAR, PRL92,112301(’04)Cited 161 times

pT spectra

•Blue-shifted spectra are evidence of radial flow•Blast wave model gives = 0.48+-0.07 in 0-5% bin.•Quantitative interpretation needs dynamical model.

PHENIX, PRC69,034909(’04)Cited 268 times

Blast wave model (thermal+boost)






Total 18 (8) 17 (6) 8 (1) 4 (1)


What is Elliptic Flow?

How does the system respond to spatial anisotropy?

J.Y.Ollitrault,PRD46,229 (’92)

Hydro behavior

Spatial Anisotropy

Momentum Anisotropy

INPUTINPUT

OUTPUTOUTPUT

Interaction amongInteraction amongproduced particlesproduced particles

dN

/d

No secondary interaction

0 2

dN

/d

0 2

2v2

x

y

Multiplicity Dependence of v2

STAR, NPA757,184(’05)Cited 410 timesSTAR, PRC66,034904(’02)Cited 162 times

Particle Density

Re

sp

on

se

= (

ou

tpu

t)/(

inp

ut)

•Experimental data reach “hydrodynamic limit”for the first time at RHIC•But, only one data point?

P.F.Kolb et al.,PRC62,054909(’00)

pT dependence of v2

STAR, PRC72,014904(’05)Cited 127 times

PHENIX,PRL91,182301(’03)Cited 242 times

•A hydrodynamic model based on perfect fluidsreasonably describes pi, K, p, Lambda data in low pT region. •How small is viscosity?•How fragile/robust? See our recent papers

Hydro calculations: P.Huovinen

Hydrodynamic ModelFinal stage:Free streaming particles Need decoupling prescription

Intermediate stage:Solve energy-momentum conservation. Need EoS and/or transport coefficients

Initial stage:Particle production,pre-thermalization, instability?Instead, initial conditions for hydro simulations

0z

t






Total 18 (8) 17 (6) 8 (1) 4 (1)


Two-Particle Correlation FunctionC2

Bertsch-Pratt equation

•Inverse problem to get source information

Bertsch-Pratt parameterization

p1

p2

reaction planez

Rlong

KT

Rout

Rside

x

y

Two-pion correlation function

Bird’s eye view View from beam axis

q

1

C2

q

1/R

KT Dependence of HBT Radii

PHENIX, NPA757,184(’05)Cited 410 timesPHENIX, PRL88,192302(’02)Cited 128 timesSTAR, PRL87,082301(’01)Cited 193 times

•Hydro models do not give correct source sizes.•HBT puzzle in a narrow sense•Last interaction point Hydro description is not valid.

Hydro(-based) calculationSoff, Kolb,Hirano,…






Total 18 (8) 17 (6) 8 (1) 4 (1)


Tomography

* 平野哲文、浜垣秀樹、「ジェットで探るクォークグルーオンプラズマ」、日本物理学会誌 2004 年 12 月号

CT (computed tomography) scan

“Tomography”1. Known probes: Spectra reliably calculable via pQCD2. Good detector: RHIC experiments!3. Interaction btw. probes and unknowns: Recent development in this field

Jet Tomography

g g

g

Tool 1. Jet quenching

High “density” matter

Tool 2. Jet acoplanarity

180 deg. correlation?

Bjorken(’82)Gyulassy,PlümerWang (’90)

Bjorken(’82)Appel (’86)Blaizot & McLerran (’86)

pT

RAA 1

binary collision scaling

Au+Au 0-10% central•b=2.8 fm

•Ncoll = 978•Npart = 333

•Npart/Ncoll = 0.341

participant scaling0.341

Nuclear Modification Factor

(null result)

High pT Spectrum in pp Collisions

PHENIX, PRL91,241803(’03)Cited 193 times

•NLO pQCD works at RHIC•Important for AA collisions•Serves a reference spectrum

Nuclear Modification Factor

PHENIX, PRL88,022301(‘02)Cited 402 times

•RAA < 1 for the first time at RHIC•Significant suppression in central collisions

char

gedSTAR, PRL89,202301(’02)Cited 292 times

Di-hadron Distribution

STAR, PRL90,082302(’03)Cited 341 times

•Disappearance of away-side peaks in central collisions•Away side jet may be gone.

4<pT,trigger<6 GeV/c2 GeV/c<pT,associate<pT,trigger

???

Initial vs. Final

Importance of pA (dAu) collisions

D.Kharzeev et al., PLB561, 93(‘03).

•Saturation Npart scaling(Later, ∃Cronin peak even within CGC)•ggg (no back-to-back)

I.Vitev and M.Gyulassy,PRL89, 252301(‘02)

•Jet quenching•dNg/dy =500-1200 @ RHIC

Results from d-Au Collisions

STAR,PRL91,072304(’03)Cited 314 times


Results from d-Au Collisions

PHOBOS, PRL91,072302(’03)Cited 169 times

BRAHMS, PRL91,072305(’05)Cited 204 times

•Neither suppression nor disappearance in d+Au•Jet quenching scenario turns out to be favored.•This does not mean saturation models are killed.

High pT at forward


=0

=2.2

•More suppression at forward, or initial state effect?

Kinematic EffectJet quenchingSpectrum shift

pT

(1/p

T)d

N/d

p T “parallel shift”

pT(1

/pT)d

N/d

p T Different slope,but same shift

00

Ratio at a fixed pT.

TH and Y.Nara, PRC68,064902(’03)

•R can be less than unity.•But, insufficient to explain data

Saturation effect in d+Au?


•Forward rapidity Small x in a nucleus•Manifestation of Color Glass Condensate?

Cronin Peak Suppression

D.Kharzeev et al.,PRD68,094013(’03);PLB599,23(’04).

•Cronin peak disappears as moving away from midrapidity.•Qualitatively consistent with data

preliminary

H.Fujii, talk at RCNP workshop(’07)

y=0,1,2,3

Di-Hadron spectra revisited


2.5<pT,trigger<4.0 GeV/c1.0<pT,associated<2.5 GeV/c

4.0<pT,trigger<6.0 GeV/c0.15<pT,associated<4.0 GeV/c

STAR,PRL95,152301(’05)Cited 131 times

•Where does the lost energy go?•Mechanism? (Mach cone? Deflected jets?)






Total 18 (8) 17 (6) 8 (1) 4 (1)


No Suppression for Baryons

C.Seife, Science298,718(2002)

Baryon Enhancement


PHENIX, PRC69,034909(’04)Cited 268 times

•Baryons are not suppressed in intermediate pT

•Mechanism of enhancement?

Recombination as a Third Component

Low pT (bulk, hydro)

High pT (fragmentation)

““Third Component”Third Component”Intermediate pIntermediate pTT (~2-5 GeV/c) (~2-5 GeV/c)

RecombinationRecombination

Soft-Hard Soft-Hard CoalescenceCoalescence

Soft-Softrecombination

Elliptic Flow Scaling

STAR, PRC72,014904(’05)Cited 127 times

qq qbarqbar+

= mesonmeson

qq qq

qq

+

+

= baryonbaryon

meson(n=2)

baryon(n=3)

Voloshin(’02), Molnar, Lin(’03), Fries,Nonaka,Muller,Bass(’03),Hwa,Yang(’04),Pratt,Pal(’05)…

Summary1. Ｏｎｅ　ｄａｙ　ｅｘｐｅｒｉｍｅｎｔで、粒子生成メカニズムに制

限をつけることができる。素朴な２成分モデルでは、ソフトな生成は９０％近い。

2. 中心衝突では、エネルギー密度は十分格子ＱＣＤから予言されている臨界値を超えている。～５ＧｅＶ／ｆｍ３　＠　 τ ＝１ｆｍ／ｃ

3. 化学平衡が切れる温度は１６０ＭｅＶ程度。擬臨界温度に近い。4. 光速の５０％近い横膨張速度が得られている。大きな圧力（勾配）。5. 完全流体ＱＧＰを仮定したダイナミクスと楕円型フローの実験結果が

よく合う。6. ＨＢＴ半径は流体模型では合わない。7. ミニジェットの収量は中心衝突では２０％程度まで減っており、終状

態相互作用の結果である。8. 前方ラピディティでは、作られたミニジェットが減っており、ＣＧＣ

の発展方程式の振る舞いとコンシステント。9. 消えた後方ジェットは、マッハ錐状（？）に出ている。10. バリオンの収量は中間横運動量領域（２－６ＧｅＶ／ｃ）で減らない。

楕円型フローのスケーリング的振る舞いから、クォーク再結合模型が有力。 •個々のモデルはそれなりにうまくいっている。

•しかし、統一的な記述は未だに得られず。

Talk by T.Hallman @ ICHEP04

contd.

Inconsistency• Some hydro model does not reproduce relative yields.

Need chemical potential for hadrons

Importance of viscosity at a hadronic level (hadronic corona)• The number of partons obtained from jet tomography is smaller than that

from hydro.

Need chemical non-equilibrium process in QGP fluids.• Energy per particle from CGC is larger than data.

Need pdV work (e.g., hydrodynamic approach)• CGC gives a large eccentricity at initial state and overshoot elliptic flow

data as an initial condition for ideal hydro.

Need viscosity in QGP fluids• Twice larger radial flow to understand data in intermediate pT via

recombination than the one obtained from hydro simulations.

Need quantitative and dynamical anaysis of recombination mechanism• …

Current Status of Dynamical Modeling

Pro

per

tim

e

Transverse momentum

CGC Geometric Scaling

Shattering CGC

Hydrodynamics•viscosity•non chem. eq.

Parton energy loss•Inelastic (light)•Elastic (heavy)

Hadroniccascade

Low pT High pT

RecombinationCoalescence

“DGLAP region”

(N)LOpQCDBefo

re

colli

sions

Part

on

pro

duct

ion

Pre

-equili

bri

um

“Perf

ect

” fluid

QG

P o

r G

P

Dis

sipati

ve

hadro

ngas

Fragmentation

Interaction

Intermediate pT

fluctuationInstability?Equilibration?

Our studies reduce model ambiguities

• TH,PRL86,2754(’01);PRC65,011901(’02).

First realistic full 3D hydro. No Bjorken boost invariant ansatz.• TH and K.Tsuda, PRC65,061902(’02).

Introduce chemical freezeout in hydro. Get correct particle ratios and spectra simultaneously for the first time.

• TH and Y.Nara, PRC66,041901(’02);PRL91,082301(’03); PRC68,064902(’03);PRC69,034908(’04).

Jet quenching in QGP fluids. Realistic matter profile for jet quenching analysis.

• TH and Y.Nara, NPA743,305(’04).

CGC initial condition in hydro. Remove some issues for initial conditions in hydro simulations.

• TH and M.Gyulassy, NPA769,71(’06); TH et al., PLB636,299(’06).

Hadronic cascade after QGP evolution. Remove issues for final decoupling.

(Incomplete) Reference List• RHIC white papers (BRAHMS,PHOBOS,STAR,PHENIX)

– Nucl.Phys.A757,1(‘05)

• Review papers– Quark Gluon Plasma 3, Eds. R.C.Hwa and X.-N. Wang (World

Scientific, Singapore, 2004)• Hydro: P.F.Kolb and U.W.Heinz, P.Huovinen

• Jet quenching: M.Gyulassy,I.Vitev,X.-N.Wang,B.-W.Zhang

• CGC: E.Iancu and R.Venugopalan

• …

– Hydro• TH, proceedings of Quark Matter 2004

(J.Phys.G30.S845(’04)),ISMD04(Acta.Phys.Pol.B36,187(’05)), YKIS06(Prog.Theor.Phys.Supplement168,347(’07))

– Femtoscopy• M.A.Lisa, S.Pratt,R.Soltz,U.Wiedemann,nucl-ex/0505014

– ReCo• R.Fries, proceedings of Quark Matter 2004,nucl-th/0403036

what did we learn at rhic?

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