Bilan de la recherche du QGP au RHIC
- Une perspective de PHENIX (*) -

Institut de Recherches Subatomiques

Strasbourg, 14 janvier, 2005

David d’Enterria

Nevis Labs, Columbia University, NY

(*) Basé sur “PHENIX White Paper”: http://www.phenix.bnl.gov/phenix/WWW/info/comment/

Overview/Summary

0. Introduction:

Goal: Study Quantum Chromo (many-body) Dynamics (QGP, CGC) in high-energy A+A collisions by comparing to p+A (“cold” QCD medium), p+p (QCD vacuum).

(1) Energy densities:

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

(2) Elliptic flow:

Strong elliptic flow v2 consistent w/ short thermalization times t0 ~ 1 fm/c

(3) Soft particle spectra:

Shapes & yields consistent w/ hydrodynam. (thermal+coll. velocity) source emission

Particles ratios consistent w/ chemical equilibrium 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 dense thermal system

6. Summary & open questions: “QGP” ? Observations @ lower √s ?

Future measurements.

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~17 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 GeV

BRAHMS

The 4 RHIC experiments

BRAHMS detector

RHIC Au+Au luminosities

Luminosities

maximum
projection

physics target

minimum
projection

Au+Au collisions @ 200 GeV

~ 700 charged particles per unit rapidity at midrapidity (top 10% central)

(1) Energy densities at RHIC

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> ~ 600 GeV (top 5% central)

(~70% larger than at SPS)

> QCD critical density (~1 GeV/fm3)

1 fm/c thermalization time ?

Not unrealistic at RHIC... (for the 1st time: ttherm > tcross = 2R/g = 0.13 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)

(2) Elliptic flow at RHIC

Elliptic flow

Elliptic flow = v2

(2nd Fourier coefficient)

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~2 fm/c

Time evolution of ellipsoid eccentricity:

Hydro calculations

Kolb, Sollfrank, Heinz

PRC62, 054909 (200)

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.

But(!) no significant increase

between 62 and 200 GeV

√s-dependence of v2:

~50% increase from SPS

Apparent saturation within 62-200 GeV

Mass dependence of v2

consistent w/ hydrodynamics too:

(3) Soft particle production at RHIC

Soft particle spectra

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

pions:	protons:

Solid lines: QGP+HG

Dashed lines: HG

Radial collective flow

built-up at freeze-out:

<bT> » 0.6

Kolb&Heinz

nucl-th/0305084

Ratios of particle yields

Ratios of hadron yields consistent w/ system at chemical equilibrium (strangeness saturation factor gs=1) at hadronization (Tchem.freeze-out ~ Tcrit) :

PBM, Redlich, Stachel

nucl-th/0304013

Kaneta, Xu nucl-th/0405068

(4) Hard QCD production at RHIC

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)

“ g (direct+decay) / g decay ”

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

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

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

[ J.Frantz QM'04 ]

Preliminary

--- “Ncoll-scaled” p+p pQCD

“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)

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

Suppressed high pT hadroproduction @ RHIC : RAA (po)

Discovery of

high pT suppression

(one of most significant

results @ RHIC so far)

PHENIX Collab.

PRL 91, 072303 (2003)

Ncoll scaling

(“hard” production)

x4-5 suppression

Npart scaling

(“soft” production)

RAA << 1: well below pQCD (collinear

factorization) expectations for

hard scattering cross-sections

“Jet quenching” predictions

Multiple final-state gluon radiation off the produced

hard parton induced by the traversed dense

colored medium.

Mean parton energy loss µmedium properties:

DEloss ~ gluon (gluon density)

DEloss ~ DL2 (medium length)

Energy is carried away by gluonstrahlung

inside jet cone: dE/dx ~ s k2T

Formalisms: BDMPS (thick plasma), GLV (thin plasma),

Correction for expanding plasma (1-D):

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

Expected result: Suppression of high pT leading hadrons due to non-Abelian final-state gluon radiation.

Jet

High- pT hadron

Gluon

bremsstrahlung

QGP

[Armesto-Salgado-Wiedemann]

“Jet quenching” model vs. data (I)

Dense medium properties according to 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> ~ 3.5 GeV/fm2

Plasma temperatures:

T ~ 0.4 GeV [G. Moore]

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. data (II)

Excitation function of high pT suppression:

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 , Hard Probes'04]

Unquenched d+Au production at high pT

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

(5) Hadron production at intermediate pT

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

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.

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

“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)

Enhanced baryonic elliptic flow

Particle species

hierarchy of flow values:

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 ?)

PHENIX Collab.

to appear in PRL

nucl-ex/0305013

“Quark recombination” models vs. data

Quark recombination (coalescence) mechanisms provide a simple explanation of anomalous baryon enhancement & v2 at pT= 2—5 GeV/c:

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 ?)

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

OK ... but didn't we hear the same at CERN-SPS in 2000 ?

Summary (2)

1. Energy densities:

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

Seen at SPS ? Yes. ε > 2.9 GeV/fm3 > εcrit (Note, however tcross=1.6 fm/c >t0 = 1 fm/c)

2. Elliptic flow:

Strong elliptic flow v2 consistent w/ short thermalization times t0 ~ 1 fm/c

Seen at SPS ? No. Not as large v2. Described by hydro only with too short freezeout.

3. Soft particle spectra:

Shapes & yields consistent w/ hydrodynam. (thermal+coll. velocity) source emission

Particles ratios consistent w/ chemically equilibrated system before hadronization

Seen at SPS ? Yes.

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

Seen at SPS ? Probably. Suppression (not that strong) w.r.t. peripheral.

5. Intermediate pT spectra:

Enhanced baryon yields & v2 (compared to meson) consistent w/ quark	recombination mechanisms in a thermal and dense system

Seen at SPS ? No.

We need a few additional observables to claim “QGP” at RHIC ...

thermal photons (temperature of system), J/Ψ (deconfined system)

Summary (2)

1. Energy densities:

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

Seen at SPS ? Yes. ε > 2.9 GeV/fm3 > εcrit (Note, however tcross=1.6 fm/c >t0 = 1 fm/c)

2. Elliptic flow:

Strong elliptic flow v2 consistent w/ short thermalization times t0 ~ 1 fm/c

Seen at SPS ? No. Not as large v2. Described by hydro only with too short freezeout.

3. Soft particle spectra:

Shapes & yields consistent w/ hydrodynam. (thermal+coll. velocity) source emission

Particles ratios consistent w/ chemically equilibrated system before hadronization

Seen at SPS ? Yes.

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

Seen at SPS ? Probably. Suppression (not that strong) w.r.t. peripheral.

5. Intermediate pT spectra:

Enhanced baryon yields & v2 (compared to meson) consistent w/ quark	recombination mechanisms in a thermal and dense system

Seen at SPS ? No.

We need a few additional observables to claim “QGP” at RHIC ...

thermal photons (temperature of system), J/Ψ (deconfined system)

Stay tuned !

backup slides ...

“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

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.

David d'Enterria

2004-01-14T06:23:12