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Recent Results from RHIC Recent Results from RHIC

Workshop on Physics at Hadron Collider

KIAS, June 24th, 2005

Ju Hwan KangYonsei University

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OverviewOverview

Introduction:

Why high-energy A+A collisions ? RHIC and Experiments at RHIC

Major Findings at RHIC:

Reaches chemical equilibrium at (or before) hadronization Collective flow from nearly ideal (hydrodynamical) fluid which

is consistent with strongly coupled “perfect fluid” Suppression of high pT hadrons (Jet Quenching) Disappeareance of back-to-back (di)jet correlations Degrees of freedom consistent with constituent quarks

Summary and Outlook:

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High-energy heavy-ion physics program (in 4 plots)High-energy heavy-ion physics program (in 4 plots)

1. Learn about 2 basic properties of strong interaction: (de)confinement,

2. chiral symm. breaking (restoration)

s(Q2) ~1/ln(Q2/), MeV

3. Probe quark-hadron phase transition of the primordial Universe (few μsec after the Big Bang)

2. Study the phase diagram of QCD matter: esp. produce & study the QGP

4. Study the regime of non-linear (high density) many-body parton dynamics at small-x (CGC)

s=g2/4) <qq>_

/T4

T/Tc

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The "Little Bang" in the lab.

High-energy nucleus-nucleus collisions: fixed-target reactions (√s=20 GeV, SPS) or colliders (√s=200 GeV, RHIC. √s=5.5 TeV, LHC)QGP expected to be formed in a tiny region (~10-14 m) and to last very short times (~10-23 s).Collision dynamics: Diff. observables sensitive to diff. react. stages

Tim

e

Penetratingprobes

t~0.1 fm/c

t ~ 10 fm/c

t ~ 107 fm/c

Penetrating probes

Final state probes

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Relativistic Heavy-Ion Collider (RHIC) @ BNL

STAR

PHOBOS

PHENIX

Specifications: 3.83 km circumference 2 independent rings:

120 bunches/ring106 ns crossing time

A + A collisions @ √sNN = 200 GeV

Luminosity: 2·1026 cm-2 s-1 (~1.4 kHz)

p+p collisions @ √smax

= 500 GeV

p+A collisions @ √smax

= 200 GeV

4 experiments: BRAHMS, PHENIX, PHOBOS, STAR

Runs 1 - 5 (2000 – 2005):

Au+Au @ 200, 130, 62.4 GeV p+p @ 200 GeV d+Au @ 200 GeV Cu+Cu @ 200, 62.4 GeV

BRAHMS

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The 4 RHIC experiments

BRAHMS detector

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RHIC Au+Au luminosities

Luminosities

maximumprojection

physics target

minimumprojection

● RHIC (Au+Au) is currently running at ~2x design luminosity

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Energy density (Au+Au Energy density (Au+Au @@ 200 GeV, y=0) 200 GeV, y=0)

Bjorken ~5.0 GeV/fm3

R2

0 ~ 1 fm/c

Au+Au @ 200 GeV

Bjorken estimate:

(longitudinally expanding plasma)

dET/dη at mid-rapidity measured by calorimetry (using PHENIX EMCal as

hadronic calorimeter: EThad = (1.17±0.05) ET

EMCal)

<dET/d> ~ 650 GeV (top 5% central)

(~70% larger than at CERN-SPS)

> QCD critical density (~1 GeV/fm3)

PHENIX Collab.PRL 87, 052301 (2001)nucl-ex/0104015

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Soft particle spectra

Bulk ±, K±, p(pbar) spectra reproduced by hydro w/ QGP EOS at 0= 0.6

fm/c

Strong radial collective flow built-up at freeze-out: <

T> 0.6

D.d'E. & Peressounkonucl-th/0503054

Au+Au central (b = 2.6 fm)

Hydro (quenched) pQCD

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Ratios of particle yields

Ratios of hadron yields consistent w/ system at chemical equilibrium at hadronization (T

chem.freeze-out ~ T

crit) :

Hadron composition (even for strange had., s=1) “fixed” at

hadronization

PBM, Redlich, Stachelnucl-th/0304013Kaneta, Xunucl-th/0405068

Assume all distrib. described by one T and one :

1 ratio (e.g. p/p) determines /T

2nd ratio (e.g. K/pi) provides T,.

Then predict all other hadronic yields and ratios

dN ~ e - (E- )/T d3p

p/p ~ e – (E+ )/T/e –(E- )/T = e - 2 /T

_

_

157 MeV9.4 MeV

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Elliptic flow

Initial anisotropy in x-space in non-central collisions (overlap) translates into final azimuthal asymmetry in p-space (transverse to react. plane)

1. Truly collective effect (absent in p+p collisions).

2. Early-state phenomenon: develops only in 1st instants of reaction. Strongly self-quenches after t~1 fm/c

Time evolution of ellipsoid eccentricity:

Hydro calculationsKolb, Sollfrank, Heinz PRC62, 054909 (2000)

Elliptic flow = v2

2nd Fourier coefficientof dN/d

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Elliptic flow at RHIC

Large v2 signal at RHIC:

Exhausts hydro limit for pT<1.5 GeV/c

Strong (collective) pressure grads. Large & fast parton rescattering: early thermalization.

Mass dependence of v2

consistent w/ hydrodynamics too:

PHENIX .PRL 91, 181301 (2003)nucl-ex/0305036

PHENIX . PRLnucl-ex/0411040

√s-dependence of v2:

~50% increase from CERN-SPS (apparent saturation within 62-200 GeV)

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Anisotropic FlowAnisotropic FlowSame phenomena observed in gases of strongly interacting atoms

M. Gehm, S. Granade, S. Hemmer, K, O’Hara, J. ThomasScience 298 2179 (2002)

weakly coupled

strongly coupled

The RHIC fluid behaves like this,

that is, a strongly coupled fluid

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VV22 requires ultra-low viscosity requires ultra-low viscosity

( )

0 with

( trace)P u

T

uT uu Pg

D. Teaney

Elliptic flow from hydro with early thermalization requires /s 0.1 ( D.Teaney; nucl-th/0301099)

Quantum lower bound on /s : /s = 1/4 (P.K. Kovtun, D.T. Son, A.O. Starinets; hep-th/0405231)

RHIC data suggest that the fluid is “as perfect as it can be”, that is, it approaches the (conjectured) quantum mechanical limit

Realized in strongly coupled N = 4 SUSY YM theory, also in QCD ?

QGP(T≈Tc) = sQGP

Relativistic viscous fluid dynamics:

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Remove your organic prejudicesDon’t equate viscous with “sticky”

Think instead of a not-quite-ideal fluid:“not-quite-ideal” “supports a shear stress”Viscosity then defined as

Dimensional estimate:Viscosityincreases withtemperature

Large cross sections small viscosity

Viscosity PrimerViscosity Primer

yv

AF xx

σ

mkTη

Nσ

Vpath freemean ,

m

kTvNkT,PVThen

path)free /(meanv

Pressure

y/v

Area)Unit/Δ(Momentumη

x

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RHIC Scientists Serve Up “Perfect” LiquidRHIC Scientists Serve Up “Perfect” Liquid

New state of matter more remarkable than predicted -- raising many new questionsApril 18, 2005TAMPA, FL -- The four detector groups conducting research at the Relativistic Heavy Ion Collider (RHIC) -- a giant atom “smasher” located at the U.S. Department of Energy’s Brookhaven National Laboratory -- say they’ve created a new state of hot, dense matter out of the quarks and gluons that are the basic particles of atomic nuclei, but it is a state quite different and even more remarkable than had been predicted. In peer-reviewed papers summarizing the first three years of RHIC findings, the scientists say that instead of behaving like a gas of free quarks and gluons, as was expected, the matter created in RHIC’s heavy ion collisions appears to be more like a liquid.

High-Energy Physics: An emptier emptiness?Nature 435, 152-153 (May 12, 2005) by Frank WilczekAt the RHIC, collisions between heavy ions create a fireball in which temperature exceeding 1.5x1012K are achieved. Impressive evidence has accumulated that a qualitative new state of matter has been created, a liquid-like plasma of quarks and gluons. Could something even more dramatic - a qualitative change in the properties of empty space - be occurred as well? Theoretical calculations indicate that at such temperatures the pairs that make up the chiral condensate will break apart. When the condensate vaporizes, the full underlying chiral symmetry of QCD becomes operative.

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Hard QCD probes

Production yields theoretically calculable via perturbative-QCD:

A

B

AB

dAB → hard = A·B·dpp → hard

dNAB → hard (b) = T

AB(b)·dpp → hard

Nuclear Modification Factor:

AB

At impact parameter b:

geom. nuclear overlap at bproduction is “shadowed”

TAB

~ # NN collisions (“Ncoll

scaling”)

“Factorization theorem”:

Independent scattering of “free” partons:

A+B = “simple superposition of p+p collisions”

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Suppressed high pSuppressed high pTT hadroproduction in Au+Au @ RHIC ! hadroproduction in Au+Au @ RHIC !

Au+Au 0 X (peripheral) Au+Au 0 X (central)

Peripheral data agree well with Strong suppression inp+p (data & pQCD) plus N

coll-scaling central Au+Au

collisions

D.d'E, nucl-ex/0401001

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Suppressed high pSuppressed high pTT hadroproduction @ RHIC hadroproduction @ RHIC

Discovery of

high pT suppression

(one of most significant results @ RHIC so far)

D.d'E., HP'04nucl-ex/0504001

Ncoll

scaling

(“hard” production)

x5 suppression

Npart

scaling (surface emission)

RAA

<< 1: well below pQCD (collinear factorization) expectations for

hard scattering cross-sections

PHENIX Collab.PRL 88, 022301 (2002)nucl-ex/0109003

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Nucleus- nucleuscollision

Proton/deuteron nucleuscollision

• Jet Quenching interpretation; interaction with medium produced in final state suppresses jet. • Gluon Saturation interpretation, gluons are suppressed in initial state resulting in suppression of initial jet production rate. • If these initial state effects are causing the suppression of high-PT hadrons in Au+Au collisions, we should see suppression of high-PT hadrons in d+Au collisions.

P+A (or d+A): The control ExperimentP+A (or d+A): The control Experiment

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Unquenched d+Au production at high pUnquenched d+Au production at high pTT

• Conclusion: High pT suppression in central Au+Au due to final-state effects (absent

in “control” d+Au experiment)

D.d'E.,nucl-ex/0401001

PHENIX.PRL91, 072303(2003)

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““NN scaling” in Au+Au @ 200 GeV: Direct PhotonsNN scaling” in Au+Au @ 200 GeV: Direct Photons

• Direct photon production in Au+Au (all centralities) consistent w/

p+p incoherent scattering (“NN-scaled” pQCD) predictions:

Submitted to PRLnucl-ex/0503003

Direct photon production in Au+Au

unmodified by QCD medium.

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“Jet quenching” predictions

Multiple final-state non-Abelian (gluon) radiation off the produced hard parton induced by the traversed dense medium.

Parton energy loss medium properties:

Eloss

~ gluon

(gluon density)

Eloss

~L2 (medium length)

Energy is carried away by gluonsstrahlung inside jet cone: dE/dx ~ s k2

T

Correction for expanding (1-D) plasma : E1-D =(20/RA) · Estatic ~ 15·Estatic (0=0.2 fm/c, RA=6 fm)

“gluonstrahlung”

Prediction I: Suppression of high pT leading hadrons

Prediction II: Disappeareance of back-to-back (di)jet correlations

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Parton exiting on the periphery of the collision zone should survive while partner parton propagating through the collision zone is more likely to be absorbed if Jet-Quenching is the correct theory.

60-90%60-90%

PHENIX Preliminary

“Near Side Jet” Escapes

“Far Side Jet” Lost

d+Aud+Au Au+AuAu+Au

NearNear Far Far

Min BiasMin Bias 0-10%0-10%

PHENIX Preliminary

Far-side Jet is suppressed in Central Au+Au : Further indication of suppression by produced medium.

Jet Correlations: 2-particle Correlations

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C2(Au Au)C2(p p) A*(1 2v22 cos(2))

STAR azimuthal correlation function shows ~ complete absence of “away-side” jet

Surface emission only (?)That is, “partner” in hard scatter is absorbed in the dense medium

GONE

GONE

Pedestal&flow subtracted

Further evidence

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Unsuppressed baryon productionUnsuppressed baryon production

Rc p

(ratio central/peripheral) at intermediate pT = 2 – 4 GeV/c:

Particle composition inconsistent with known (universal) fragmentation functions.

Additional production mechanism for baryons in the intermediate pT range

STAR Collab.subm. to PRL,nucl-ex/0306007

PHENIX Collab.PRL91:172301(2003)

_ _

Mesons: 0, K0s, η,

equally suppressed.

Baryons: p, p, Λ, Λ NOT (or much less) suppressed in central Au+Au.

D.d'E.J.Phys. G30, S677 (2004)

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Enhanced (anti)proton/pion ratioEnhanced (anti)proton/pion ratio

Central Au+Au: p/π ~ 0.8 (at pT = 2 - 4 GeV/c) at variance with perturbative production mechanisms (favour lightest mesons).

Periph. Au+Au: p/π ~ 0.2 as found in p+p (ISR,FNAL) & e+e- jet fragmentation

(ant

i)pro

ton/

pion

PHENIX Collab.PRL91:172301(2003)

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Enhanced baryonic elliptic flow

• Different v2 saturation for mesons and baryons:

v2meson > v

2baryon at low p

T

v2meson ≈ v

2baryon at p

T≈ 2 GeV/c

v2meson < v

2baryon at higher p

T

• Simple v2 scaling behaviour

if v2 and p

T are normalized by

number of constituent quarks:

n = 2 mesons n = 3 baryons

(“universal” parent quark flow ?)

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““Quark recombination” models vs. dataQuark recombination” models vs. data

Anomalous baryon enhancement & quark number scaling of v2 at p

T= 2--5 GeV/c

explained by “quark recombination” (coalescence) in dense (thermal) medium:

recombining partons:

p1+p2=ph

fragmenting parton:

ph = z p, z<1

Rethink hadronization at interm. pT at RHIC !

Phase space filled with partons Recombine quarks into hadrons

Greco, Ko, LevaiPRL 90, 202302

Fries, MuellerNonaka, Bass PRL 90, 202303

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SummarySummary

1. Energy densities: Maximum dE

T/dη ~ 600 GeV at midrapidity consistent w/ initial ε > 5 GeV/fm3 > ε

crit

2. Elliptic flow: Strong elliptic flow v

2 consistent w/ short thermalization times

0 ~ 1 fm/c

3. Soft particle spectra: Shapes & yields consistent w/ hydrodyn. (thermal+coll. velocity) source emission Particles ratios consistent w/ chemically equilibrated system before hadronization

4. Hard particle spectra: Strong high p

T suppression in central A+A (compared to p+p, p+A & pQCD)

consistent w/ final-state partonic energy loss in dense system: dNg/dy~1100

5. Intermediate pT spectra:

Enhanced baryon yields & v2 (compared to meson) consistent w/ quark

recombination mechanisms in a thermal and dense system

All observations consistent with formation of thermalized dense partonic matter in central Au+Au collisions

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Open QuestionsOpen QuestionsIs the quark-gluon plasma being formed in RHIC collisions? To be determined:

Does charmonium show the expected suppression from (color) Debye screening?

Is there direct (photon) radiation from the plasma?Do the suppression effects extend to the highest pT’s?

What are the properties of the produced matter (sound speed, heat capacity, viscosity, etc.)What are the gluon and sea-quark contributions to the proton spin? (polarized proton running)

RHIC