Bilan de la recherche du Plasma de Quarks et Gluons (QGP) au RHIC
- A PHENIX perspective (*) -

Séminaire IPN-Orsay

Orsay, Paris, Mai 26th, 2005

David d’Enterria

Nevis Labs, Columbia University, NY

(*) Basé sur “PHENIX White Paper”: Nucl. Phys. A (à apparaitre), nucl-ex/041003

Overview/Summary

Introduction:

Goal of high-energy A+A collisions ?

Study/characterize Quantum Chromo (many-body) Dynamics

(By comparing A+A to: p+A = “cold QCD medium”, p+p = “QCD vacuum” colls.)

Head-on Au+Au collisions @ RHIC energies (Ös=200 GeV) produce a strongly interacting system:

with the highest energy densities ever achieved at the lab: ε > 5 GeV/fm3

with a strong degree of collectivity at very short time-scales: t0 < 1 fm/c

that behaves like an nearly ideal (hydrodynamical) fluid: dN/dpT (pT<2 GeV/c)

that reaches chemical equilibrium at (or before) hadronization: Tchem~ Tcrit

with the largest initial gluon densities ever measured: dNg/dy ~ 1000

with degrees of freedom consistent with constituent quarks

Summary & open questions

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

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

as(Q2) ~1/ln(Q2/L2), L~200 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)

(as=g2/4p)

<qq>

_

e /T4

T/Tc

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

Time

Penetrating

probes

t~0.1 fm/c

t ~ 10 fm/c

t ~ 107 fm/c

Penetrating probes

Final state probes

Relativistic Heavy-Ion Collider (RHIC) @ BNL

STAR

PHOBOS

PHENIX

Specifications:

3.83 km circumference

2 independent rings:

120 bunches/ring

106 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

The 4 RHIC experiments

BRAHMS detector

RHIC Au+Au luminosities

Luminosities

maximum
projection

physics target

minimum
projection

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

Au+Au collisions @ 200 GeV

~ 700 charged particles per unit rapidity at midrapidity (top 5% central)

(1) Energy densities at RHIC

The highest energy densities ever achieved at the lab: ε > 5 GeV/fm3

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

Bjorken ~5.0 GeV/fm3

pR2

t0

~ 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) ETEMCal)

<dET/dh> ~ 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

1 fm/c thermalization time ?

Not unrealistic at RHIC... (for the 1st time: ttherm > tcross = 2R/g ~ 0.15 fm/c)

Time evolution of energy density in longitud. expanding system: e ~1/t

tcross=2R/g = 0.13 fm/c

ttherm~ 0.6 - 1 fm/c

tsecondaries formation = hbar/E ~ hbar/<mT>

= 0.35 fm/c

<mT> = (dET/dh)/(dN/dh) ~

(dET/dh)/(3/2*dNch/dh)

= 0.57 GeV

(from elliptic flow & hydrodynamics, see next)

PHENIX Collab.

To appear in NPA

nucl-ex/041003

(2) Elliptic flow at RHIC

Strong degree of collectivity at very short time-scales: t0 < 1 fm/c

Elliptic flow

Initial anisotropy in x-space in non-central collisions (overlap) translates into final azimuthal asymmetry in p-space (w.r.t. reaction plane)

Elliptic flow = v2

2nd Fourier coefficient

of dN/df

PHENIX Collab.

PRL 89, 212301 (02)

nucl-ex/0204005

Elliptic flow

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

“Elliptic flow” in low -T (strongly coupled) Li atoms:

K.M.O'Hara,

Sci. 289, 2179

(2002)

T. Bourdel et al.

PRL 91, 020402

(2003)

Elliptic flow = v2

2nd Fourier coefficient

of dN/df

PHENIX Collab.

PRL 89, 212301 (02)

nucl-ex/0204005

Elliptic flow

Initial anisotropy in x-space in non-central collisions (overlap) translates into final azimuthal asymmetry in p-space (w.r.t. 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 calculations

Kolb, Sollfrank, Heinz

PRC62, 054909 (2000)

Elliptic flow = v2

2nd Fourier coefficient

of dN/df

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 . PRL

nucl-ex/0411040

√s-dependence of v2:

~50% increase from CERN-SPS

(apparent saturation within 62-200 GeV)

(3) Soft particle production at RHIC

A system that behaves like an nearly ideal (hydrodynamical) fluid and reaches chemical equilibrium at (or before) hadronization: Tchem~ Tcrit

Soft particle spectra

Bulk p±, K±, p(pbar) spectra reproduced by hydro w/ QGP EOS at t0= 0.6 fm/c

Solid lines: QGP+HG

Dashed lines: HG

Strong radial collective flow

built-up at freeze-out: <bT> » 0.6

Soft particle spectra

Bulk p±, K±, p(pbar) spectra reproduced by hydro w/ QGP EOS at t0= 0.6 fm/c

Solid lines: QGP+HG

Dashed lines: HG

Strong radial collective flow

built-up at freeze-out: <bT> » 0.6

D.d'E. & Peressounko

nucl-th/0503054

Au+Au central (b = 2.6 fm)

Hydro (quenched) pQCD

Ratios of particle yields

Ratios of hadron yields consistent w/ system at chemical equilibrium at hadronization (Tchem.freeze-out ~ Tcrit) :

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

PBM, Redlich, Stachel

nucl-th/0304013

Kaneta, Xu

nucl-th/0405068

Assume all distrib. described

by one T and one m:

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

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

Then predict all other hadronic yields and ratios

dN ~ e - (E- m )/T d3p

p/p ~ e – (E+m )/T/e –(E- m )/T

= e - 2m /T

_

_

157 MeV

9.4 MeV

(4) Hard QCD production at RHIC

The largest initial gluon densities ever measured: dNg/dy ~ 1000

Hard QCD probes (I)

Hard probes: High-pT, jets, direct , heavy-quarks (D, B), ...

1. Early production ( ~ 1/pT< 0.1 fm/c) in parton-parton scatterings with large Q2:

Closest experimental probes to underlying QCD (q,g) degrees of freedom.

2. Direct probes of partonic phase(s)  Sensitive to QCD medium properties:

q: fast color triplet

g: fast color octet

Q: slow color triplet

QQbar: slow color

singlet/octet

Virtual photon: colorless

Real photon: colorless

Unknown QCD Medium

Induced

gluon

radiation ?

Energy

Loss ?

Dissociation ?

Controls

QCD probe in

QCD medium

(possible quark-gluon plasma)

Modification?

QCD probe out

Hard QCD probes (II)

3. Production yields theoretically calculable via perturbative-QCD:

A

B

AB

dAB → hard = A·B·dpp → hard

dNAB → hard (b) = TAB(b)·dpp → hard

Nuclear Modification Factor:

AB

At impact parameter b:

geom. nuclear overlap at b

production is “shadowed”

TAB ~ # NN collisions (“Ncoll scaling”)

“Factorization theorem”:

Independent scattering of “free” partons:

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

High pT p+p baseline data well described by pQCD

Good theoretical (NLO pQCD) description:

KKP FF

Kretzer FF

(PDF: CTEQ6M)

PHENIX Collab.

PRL91, 241803

hep-ex/0304038

Well calibrated (experimentally &

theoretically) p+p references at hand

p+p  p0 X	p+p  h± X (non singly diffractive)

“NN 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 PRL

nucl-ex/0503003

Direct photon production in Au+Au

unmodified by QCD medium.

Suppressed high pT hadroproduction in Au+Au @ RHIC !

Au+Au p0 X (peripheral) Au+Au p0 X (central)

Peripheral data agree well with Strong suppression in

p+p (data & pQCD) plus Ncoll-scaling central Au+Au collisions

D.d'E, nucl-ex/0401001

Suppressed high pT hadroproduction @ RHIC

Discovery of

high pT suppression

(one of most significant

results @ RHIC so far)

D.d'E., HP'04

nucl-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

Unquenched d+Au production at high pT

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)

“Jet quenching” predictions

Multiple final-state non-Abelian (gluon)

radiation off the produced hard parton

induced by the traversed dense medium.

Parton energy loss µmedium properties:

DEloss ~ gluon (gluon density)

DEloss ~ DL2 (medium length)

Energy is carried away by gluonsstrahlung

inside jet cone: dE/dx ~ s k2T

Correction for expanding (1-D) plasma :

E1-D =(20/RA) · Estatic ~ 15·Estatic (0=0.2 fm/c, RA=6 fm)

Jet

High- pT hadron

Gluon

bremsstrahlung

QGP

“gluonstrahlung”

Prediction I: Suppression of high pT leading hadrons

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

“Jet quenching” model vs. data (I)

Dense medium properties from pQCD+ final-state parton energy loss models:

Initial gluon densities:

dNg/dy ~ 1100 [Vitev & Gyulassy]

Opacities:

<n> = L/≈ 3 – 4 [Levai et al.]

Transport coefficients:

<q 0> ~ 14 GeV/fm2

Medium-induced radiative

energy losses:

dE/dx ≈ 0.25 GeV/fm (expanding)

dE/dx|eff ≈ 14 GeV/fm (static source) [X.N.Wang]

Large opacities imply fast thermalization.

All these values imply energy densities well above ecrit QCD (in thermalized syst.)

[BDMPS, F.Arleo ]

[Salgado-Wiedemann]

“Jet quenching” model vs. high pT suppression (II)

sqrt(s)-dependence:

RAA ~ 1 @ s~20 GeV (dNg/dy~400)

RAA ~ 0.3 @ s~62 GeV (dNg/dy~650)

RAA ~ 0.2 @ s~200 GeV (dNg/dy~1100)

“Jet quenching” model + 1-D longitudinal plasma expansion

D.d'E., HP'04

nucl-ex/0504001

peripheral	central

PHENIX Collab.

PRC 69, 034910 (2004)

nucl-ex/0308006

Centrality dependence:

Reaction-plane (in-medium path

length) dependence:

Non-Abelian energy loss:

“Jet quenching”: modified (di)jet structure

PHENIX Preliminary

Strongly modified dNpair/d j high pT correlations in central Au+Au:

Strongly non-Gaussian

away-side (“dip”) peak.

Standard back-to-back

di-jet topology:

Au+Au periph

Au+Au central

(5) Hadron production at intermediate pT

Degrees of freedom consistent with constituent quarks

Unsuppressed 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: p0, 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)

Enhanced (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

(anti)proton/pion

PHENIX Collab.

PRL91:172301(2003)

Enhanced baryonic elliptic flow

PHENIX Collab.

to appear in PRL

nucl-ex/0305013

Different v2 saturation

for mesons and baryons:

v2meson > v2baryon at low pT

v2meson ≈ v2baryon at pT≈ 2 GeV/c

v2meson < v2baryon at higher pT

Simple v2 scaling behaviour

if v2 and pT are normalized by

number of constituent quarks:

n = 2 mesons

n = 3 baryons

(“universal” parent quark flow ?)

“Quark recombination” models vs. data

Anomalous baryon enhancement & quark number scaling of v2 at pT= 2--5 GeV/c explained by “quark recombination” (coalescence) in dense (thermal) medium:

Via quark momenta addition, recombination

dominates for pT ~ 1- 4 GeV/c:

pT(baryons)> pT(mesons)> pT(quarks)

Fragmentation dominates for pT > 5 GeV/c:

pT(hadrons)= z pT(partons) , with z<1

Constituent-quark scaling of v2 naturally

explained

However ... Near-side azimuthal correlations

of high pT trigger (leading) baryons is jet-like

No pure thermal + thermal parton

recombination produces such

jet-like correlations ... (thermal+hard ?)

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, Levai

PRL 90, 202302

Fries, Mueller

Nonaka, Bass

PRL 90, 202303

Summary

1. Energy densities:

Maximum dET/dη ~ 600 GeV at midrapidity consistent w/ initial ε > 5 GeV/fm3 > εcrit

2. Elliptic flow:

Strong elliptic flow v2 consistent w/ short thermalization times t0 ~ 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 pT 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

Outlook: What do we need to claim “QGP !” at RHIC ?

Somebody called us “overly conservative” but we (experimentalist @ RHIC) would like at least to know ...

(1) What is the temperature of the produced system?

Outlook: What do we need to claim “QGP !” at RHIC ?

Somebody called us “overly conservative” but we (experimentalist @ RHIC) would like at least to know ...

(1) What is the temperature of the produced system?

= “do we see thermal photons from the radiating plasma” ?

Au+Au central

Au+Au periph

D.d'E. & D.Peressounko

nucl-th/0503054

Outlook: What do we need to claim “QGP !” at RHIC ?

Somebody called us “overly conservative” but we (experimentalist @ RHIC) would like at least to know ...

(1) What is the temperature of the produced system?

= “Do we see thermal photons from the radiating plasma” ?

D.d'E. & D.Peressounko

nucl-th/0503054

Current experimental

upper limits in

“interesting” region

pT = 1 – 3 GeV/c

preclude a quantitative

answer ...

Wait for Run-4 data

Outlook: What do we need to claim “QGP !” at RHIC ?

Somebody called us “overly conservative” but we (experimentalist @ RHIC) would like at least to know ...

(1) What is the temperature of the produced system?

(2) Is the system deconfined ?

Outlook: What do we need to claim “QGP !” at RHIC ?

Somebody called us “overly conservative” but we (experimentalist @ RHIC) would like at least to know ...

(1) What is the temperature of the produced system?

(2) Is the system deconfined ?

= “Do we see the predicted melting of quarkonia bound states (seen at CERN-SPS) ?

Pb+Pb --> J/

Outlook: What do we need to claim “QGP !” at RHIC ?

Somebody called us “overly conservative” but we (experimentalist @ RHIC) would like at least to know ...

(1) What is the temperature of the produced system?

(2) Is the system deconfined ?

= “Do we see the predicted melting of quarkonia bound states (seen at CERN-SPS) ?

Pb+Pb --> J/

PHENIX .

PRC 69, 014901 (2004)

nucl-ex/0305030

Outlook: What do we need to claim “QGP !” at RHIC ?

Somebody called us “overly conservative” but we (experimentalist @ RHIC) would like at least to know ...

(1) What is the temperature of the produced system?

(2) Is the system deconfined ?

= “do we see the predicted melting of quarkonia bound states (seen at CERN-SPS) ?

Again the answer is ...

Wait for Run-4 data !

Pb+Pb --> J/

Stay tuned !

PHENIX .

PRC 69, 014901 (2004)

nucl-ex/0305030

backup slides ...

Jet production in hot&dense QCD: modified (di)jet structure (I)

Discovery of “mono-jet” like topologies (away-side disappearance):

Pedestal&flow subtracted

Suppressed high pT hadroproduction @ RHIC: h± vs po

Inclusive charged hadrons suppressed by a factor ~ 4 – 5 at pT> 5 GeV/c

Universal (PID-wise) suppression above pT = 5 GeV/c

... but less suppression than π0

within pT = 2 – 5 GeV/c

“Anomalous” particle composition: hadron/meson ratio

p+p collisions: hadron/meson ~ 1.6

RHIC

Au+Au (central): hadron/meson ~ 2.5

at pT = 1 - 4 GeV/c (inconsistent w/

known fragmentation functions).

Baryon enhancement limited to

pT < 5 GeV/c (h±/ π ~ 1.6, perturb. ratio):

h±, π0 equally suppressed

ISR

PHENIX Collab.

PRL91:172301(2003)

Unsuppressed baryon production: not a mass effect !

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)

_ _

Heavy f as suppressed

as other mesons (p0, k0s, η)

Baryons: p, p, Λ, Λ NOT (or much less)

suppressed in central Au+Au.

“Ncoll scaling” in Au+Au @ 200 GeV: Total charm

Open-charm indirect measurement via semi-leptonic channel: D e± +X

Single e± Au+Au spectra & total cross-section consistent w/ Ncoll -scaled p+p charm production:

pQCD parton scattering holds for hard processes in Au+Au (all centralities).

“Ncoll scaling” in A+A @ 17, 31 GeV: High pT hadrons

High pT π0 production in (0-10%) central A+A at SPS (and a+a @ ISR) energies consistent w/ “Ncoll-scaling” (or Cronin enhancement):

D.d'E. PLB 596, 32 (2004)

“Ncoll scaling” in d+Au @ 200 GeV: High pT hadrons

d+Au @ √sNN = 200 GeV	p+A @ √sNN = 20 - 40 GeV

Enhanced high pT production in d+Au (RdAu > 1) also found in p+A at lower Ös (“Cronin enhancement”): pT broadening due to initial-state soft & semihard scattering.

Expected pQCD behaviour (RpA,dA~1) recovered for pT > 8 GeV/c

Energy loss in a dense hadronic medium ?

Hadronic transport models (HSD, UrQMD) or DPM-based models do not produce enough suppression. Additional pre-hadronic energy loss needed.

Cassing, Gallmeister, Bratkovskaya,

Greiner, Stoecker, nucl-th/0312049

Capella, Ferreiro, Kaidalov, Sousa

hep-ph/0403081

David d'Enterria

2004-01-14T06:23:12