Quantum Chromo many-body Dynamics probed in the hard sector at RHIC

Séminaire LAPP-Annecy

Annecy, Savoie - France, April 7, 2004

David d’Enterria

Nevis Labs, Columbia University, NY

Overview (“Status of QGP & CGC search”)

1. Introduction:

The goal: Study Quantum Chromo many-body Dynamics: QGP, CGC.

The means: Compare hard scattering production in diff. colliding had. systems.

2. “QCD vacuum” reference results – high pT in p+p

Baseline hard scattering data in free space.

3. “Hot QCD medium” highlights – high pT in central A+A

dN/dpT light hadrons (u,d,s): suppressed

s, pT, centrality, and meson-baryon dependence

dNpair/dφ azimuthal anisotropies:

dissapearence of away-side dijet correlations

dN/dpT colorless probes (g): unsuppressed

4. “Cold QCD medium” highlights – high pT in d+Au

 dN/dpT light hadrons (u,d,s):

enhanced at y ≤ 0 (midrapidity & high x2 in Au)

suppressed at y ≥ 1 (small x2 in Au)

5. What have we learnt ? Data vs. theory.

6. Summary

CGC ?

QGP ?

}

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

1. Learn about 2 basic properties of strong interaction: confinement, chiral symmetry breaking

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>

_

The "Little Bang" in the lab.

High-energy nucleus-nucleus collisions: fixed-target reactions (√s~17 GeV - SPS) or at 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 probe diff. reaction stages

Time

Penetrating

probes

t~0.1 fm/c

t ~ 10 fm/c

t ~ 107 fm/c

Penetrating probes

Final state probes

Hard QCD probes. Motivation (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. Motivation (II)

[3] Production yields theoretically calculable via:

perturbative-QCD or ...	classical-field QCD:

A

B

A

B

at small-x ...

Mueller diagram for

classical glue radiation

AB

Reference pQCD hard cross-sections in A+B

pQCD (factorization theorem) expectation for inclusive A+B hard cross-sections:

Independent scattering of “free” partons:

dAB → hard = A·B·dpp → hard

dNAB → hard (b) = TAB(b)·dpp → hard

Nuclear

Modification

Factor:

RAA = 1

A+A = “simple superposition of p+p

collisions” at high-pT where hard scattering dominates

AB

At imp. param. b:

geom. nuclear overlap at b

production is “shadowed”

TAB ~ # NN colls. (“Ncoll scaling”)

Hard scattering in A+A collisions

Hard scattering in A+A collisions

Leading-twist shadowing

or

Gluon saturation (CGC)

pT broadening

(Cronin enhancement)

[Experimental handle: p,d+A]

[Experiment. handle: e+A, p,d+A]

Initial-state effects

Hard scattering in A+A collisions

Leading-twist shadowing

or

Gluon saturation (CGC)

pT broadening

(Cronin enhancement)

medium-induced

parton energy loss:

“jet quenching” in QGP

possible hadronic

rescattering

[Experimental handle: p,d+A]

[Experiment. handle: e+A, p,d+A]

[Experimental handle: A+A]

Initial-state effects

Final-state effects

Hard scattering in A+A collisions

Leading-twist shadowing

or

Gluon saturation (CGC)

pT broadening

(Cronin enhancement)

medium-induced

parton energy loss:

“jet quenching” in QGP

possible hadronic

rescattering

Approach: Study modifs. (incl. spectra, partic. composition) of high pT production in A+A with respect to p+p, p+A to learn about QCD many-body dynamics:

“Quark Gluon Plasma” (final-state A+A) and/or

“Color Glass Condensate" (initial-state A).

Initial-state effects:	Final-state effects

[Experimental handle: p,d+A]

[Experiment. handle: e+A, p,d+A]

[Experimental handle: A+A]

Hard scatt. in A+A (large soft backgd) studied via high pT (leading) hadron = jet tag.

Initial-state effects

Final-state effects

Final-state QGP effects

Multiple final-state gluon radiation off the produced

hard parton induced by the traversed dense

colored medium:

Mean parton energy loss probes medium

properties:

DEloss ~ gluon (gluon density)

DEloss ~ DL2 (medium length)

Energy is carried away by gluonstrahlung

outside 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.

g

Jet

High- pT hadron

Gluon

bremsstrahlung

QGP

Initial-state CGC effects

Initial conditions at RHIC: high-energies + large nuclei

Values of small-x: xBj= 2pT/√s <<1

Large gluon densities

Colliding nuclei described via a colored

highly saturated gluonic wave-function

(“Color Glass Condensate”).

"Classical" approach valid around “sat. scale”: Qs~1.5 GeV/c

Particle production via glue-glue collisions:

Extension to pT > Qs (“geometric scaling”)

via quantum evolution.

Expected result: gluon fusion at low x leads to an effective depletion of the number of partonic scattering centers in the initial state.

RHIC ~ HERA x A1/ 3

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 @ √s = 200 GeV

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

p+p collisions @ 500 GeV

p+A collisions @ 200 GeV

4 experiments:

BRAHMS, PHENIX, PHOBOS, STAR

Run-1 (2000): Au+Au @ 130 GeV

Run-2 (2001-2): Au+Au, p+p @ 200 GeV

Run-3 (2002-3): d+Au, p+p @ 200 GeV

Run-4 (2004): Au+Au, p+p @ 200 GeV

Au+Au @ 62 GeV

BRAHMS

The 4 RHIC experiments

BRAHMS detector

The PHENIX collaboration

11 countries

51 institutions

400+ collaborators

Pioneering High-Energy Ion eXperiment

PHENIX @ RHIC

11 detector sub-systems

2 Arm central spectrometers:

|h|< 0.35, Df = p (e, g, hadrons)

Open geometry axial field

2 forward spectrometers:

1.2 < |h|< 2.5, Df = 2p (muons)

Radial magnetic field

3 global (inner) dets.: trigger, centrality

Designed to measure rare probes:

+ high rate capability & granularity

+ good mass resolution and PID

- limited acceptance

RHIC Au+Au luminosities

Luminosities

Au+Au collisions @ 200 GeV

~ 600 charged particles per unit rapidity at midrapidity (top 10% central)

High pT spectra in Au+Au @ 200 GeV

π0

Λ,Λbar

e± p,pbar

High pT p+p @ 200 GeV: “baseline” data

Good theoretical (NLO pQCD) description ...

KKP FF

Kretzer FF

(PDF: CTEQ6M)

PHENIX Collab.

PRL91, 241803

hep-ex/0304038

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

High quality data: sensitive to different

parametrizations of gluon FF

High pT p+p @ 200 GeV: “baseline” data

Good theoretical (NLO pQCD) description ...

... at variance with lower sqrt(s) results (factors of ~2-4 discrepancy):

non-perturbative effects (intrinsic kT), cured by NLL soft g resummation ?

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

High pT p+p @ 200 GeV: “baseline” data

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)

Au+Au vs. p+p @ 200 GeV (p0)

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/0309015

Nuclear modification factor (p0)

Discovery of

high pT suppression

(one of most significant

results @ RHIC so far)

PHENIX Collab.

PRL 91, 072303 (2003)

nucl-ex/0304022

Ncoll scaling

(“hard” production)

x4-5 suppression

Npart scaling

(“soft” production)

RAA << 1: well below pQCD (collinear

factorization) expectations for

hard scattering cross-section

Nuclear modification factor (p0): √sNN dependence

A.L.S.Angelis, PLB 185, 213 (1987)

WA98, EPJ C 23, 225 (2002)

(*) Reanalysis: D.d'E. nucl-ex/0403055

PHENIX, PRL 88 022301 (2002)

PHENIX, PRL 91, 072303 (2003)

CERN-SPS: Pb+Pb central (√sNN = 17.3 GeV): no suppression(*) (within errors)

CERN-ISR: a+a (√sNN = 31 GeV): Cronin enhancement.

RHIC: Au+Au (√sNN = 130, 200 GeV): x 4-5 suppression.

RAA ~ 0.4

RAA (p0) compilation in nucleus-nucleus collisions:

RAA ~ 1.0

RAA~ 1.5

RAA~0.2

High pT suppression: centrality dependence (I)

peripheral	central

Smooth evolution of suppression with respect to centrality:

PHENIX Collab.

to appear in PRC

nucl-ex/0308006

X.N.Wang,

nucl-th/0307036

in agreement with pQCD production +

parton energy loss in expanding plasma expectations

pQCD expectation

(w/o energy loss)

CGC prediction

RAANpart

Npart

High pT suppression: centrality dependence (II)

RAA using “soft” scaling factor (Npart) shows approx. Npart scaling: high pT production per participant pair ~const. in wide range of centralities

In accord with Color Glass Condensate predictions too ...

D.Kharzeev, E.Levin, L.McLerran

PLB 561, 93 (2003)

PHENIX Collab.

to appear in PRC

nucl-ex/0308006

High pT suppression. Particle dependence (I): h± vs. π0

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

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

... but less suppressed than π0

within pT = 2 – 5 GeV/c

High pT suppression - Particle depend. (II): baryons vs. mesons

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

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

2. Mesons: p0, k0s, η equally suppressed.

Particle composition inconsistent with known fragmentation functions.

Additional production mechanism for baryons in the intermediate pT range (quark recombination ?).

STAR Collab.

subm. to PRL,

nucl-ex/0306007

PHENIX Collab.

PRL91:172301(2003)

_ _

High pT suppression - Particle depend. (III): charged/neutral

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

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

Baryon enhancement limited to pT < 4.5 GeV/c (h±/ π ~ 1.6, perturbative ratio): charged hadron and π0 equally suppressed at pT > 5 GeV/c

PHENIX Collab.

PRL91:172301(2003)

nucl-ex/0305036

charged/neutral = 1.6

High pT azimuthal correlations: jet signals in Au+Au & p+p

dNpair/d for “trigger” (pT > 4GeV/c) & associated (pT = 2- 4 GeV/c) charg. hadrons:

Near-side peak: Au+Au = p+p.

Trigger hadrons (pT > 4GeV/c)

from jet fragmentation.

Away-side peak: Au+Au << p+p

Back-to-back jets suppressed

("mono-jet") in central Au+Au !

Periph.:

Central:

Red histogram: p+p (+flow)

Black points: Au+Au

Blue curve: flow contribution

High pT azimuthal correlations: Au+Au dijet signal disappearance

Ratio of Au+Au (- flow) over p+p azimuthal correlation “strengths”:

Increasing dissapearance of back-to-back correlation as a function of centrality.

Near-side peak: IAA~ 1

Away-side peak: IAA~ 0

High pT in d+Au (“control” experiment)

hot & dense medium	cold medium

(initial+final-state effects)	(initial- state effects only)

Au+Au collision

p,d+Au collision

d+Au nuclear modification factor (at y=0)

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

High pT production in d+Au not suppressed but enhanced ! RdAu > 1 as in p+A “Cronin enhancement”:

pT broadening due to initial-state soft & semihard scattering.

“pQCD” cross-sections (RAA~1) recovered at pT > 8 GeV/c

No Au shadowing effects in kinematic region probed (y = 0).

High pT azimuthal correlations: jets in d+Au and p+p

Near- side:

d+Au correlation strength and width similar to p+p (& Au+Au)

Increasing “underlying event”: p+p<d+Au(m.bias)<d+A(central)

Away-side: d+Au peak broadens but small centrality dependence

Back-to-back jets do not disappear in central d+Au !

Unquenched d+Au production at high pT

Suppression in central Au+Au not due to initial-state effects

Confirmation... unsuppressed hard colorless production in Au+Au central

“Control” observable: direct photons (clean, penetrating = directly coupled to partonic vertex, no fragmentation) non-hadronic hard probes.

Photons (insensitive to final-state effects) show collision scaling at high pT:

pQCD parton scattering holds for hard processes in central Au+Au !

g NLO pQCD x Ncoll Vogelsang/CTEQ6 (M = 0.5 - 2.0 pT)

preliminary AuAu 200 GeV 0-10%

[ J.Frantz QM'04 ]

“ g (direct+decay) / g decay ”

The quest for gluon saturation effects @ RHIC ...

RHIC kinematical regime:

High pT @ midrapidity:

y = 0, Q2 = 1-100 GeV2/c2

- pQCD collinear factorization

- DGLAP evolution (g splitting)

- small nuclear effects in PDFs

(LT shadowing).

Moderate pT, rapidities:

y ≈ 1-3, Q2 ≈ 10 GeV2/c2

- kT factorization

- linear BFKL evolution (g splitt.)

- “moderate” nuclear effects (LT

shadowing).

Low pT @ large rapidities:

y > 3, Q2 < Qs2 ≈ 5 GeV2/c2

- pQCD factorization breakdown

- non-linear evolution (g fusion)

- strong nuclear effects in the

initial-state

xT = pT /√s (e-y+ey)

(2 → 2)

x small: Look forward in rapidity !

d+Au nuclear modification factor (η = 3.2)

First time a large “shadowing” is seen at small-x and high pT in nuclear syst.

So far unexplored perturbative region of nuclear (x,Q2) plane.

Perturbative

non-perturbative

Factor ~2 suppression

pT = 1-3 GeV/c hadron

production at η = 3.2

(x2 ~ 10-4 in Au).

Is this “standard” nuclear shadowing ?

Take the predictions of your favourite leading-twist approach ...

Maximum gluon shadowing at x~10-4 (indirectly) constrained by available DIS data on nuclear targets is ~0.8

IF indeed RdAu(pT~2 GeV/c) ≈ 0.5 º RG(x=10-4) ≈ 0.5 this could be an evidence of extra higher-twist effects at small-x (breakdown of QCD factorization). BUT, soft physics effects can still be playing a role here ...

D. de Florian & R.Sassot

hep-ph/0311227

Q2=10 GeV2

RG(x)

NLO DGLAP global analysis

of nuclear PDFs

What hard scattering data at RHIC
tell us(*) about the properties of the
underlying QCD matter ...

Summary of possible physical scenarios:

1. Dense final-state partonic medium: Parton energy loss +

quark recombination.

2. Dense initial-state partonic medium: Gluon saturation.

3. Dense final-state hadronic medium: hadronic energy loss.

(*) via confronting data to theory

(at y=0)

(not at y=0, maybe at y=3)

(at y=0)

Final-state “QGP” effects vs. data (I)

Dense medium properties according to “jet quenching” 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 [BDMPS, F.Arleo ]

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 εcrit QCD in thermalized syst.

Final-state “QGP” effects vs. data (II)

Hwa & Yang

PRC 67, 034902

Greco, Ko, Levai

PRL 90, 202302

Fries, Mueller, Nonaka, Bass

PRL 90, 202303

Quark recombination (coalescence) mechanisms provide a simple explanation of anomalous baryon enhancement at interm. pT's (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

High quark densities in a thermal medium

are required.

However... is recomb. consistent with

(p+p-like) Au+Au dN/dφ near-side widths ?

Final-state effects in a dense hadronic medium ?

Energy loss of “pre-hadrons” inside a dense expanding hadronic fireball with εinit≈1 GeV/fm3

Pre-hadronic energy loss in dense

medium needed also in Dual-Parton-Model

based approaches

Cassing, Gallmeister, Bratkovskaya,

Greiner, Stoecker, nucl-th/0312049

Hadronic transport models (HSD,

UrQMD) produce suppression but not

enough to explain the observed

suppression factor at high pT

Capella, Ferreiro, Kaidalov, Sousa

hep-ph/0403081

Summary

 High pT central Au+Au vs p+p at midrapidity at RHIC:

 Observation 1: Light-flavor (u,d,s) spectra suppressed by a factor 4-5.

 Observation 2: Intermediate pT light-flavor composition inconsistent	with known fragmentation functions in free space.

 Observation 3: Disappearance of away-side jet correlations.

 Observation 4: Direct photon spectra unsuppressed.

 High pT d+Au vs p+p at midrapidity at RHIC:

 Observation 5: Spectra enhanced by a factor ~1.3

 “Explanation” (1,2 via 4,5): pQCD hard scattering + final-state parton	energy loss + parton recombination Dense thermal QCD medium

QGP ? : thermal γ ?, J/Ψ suppression ? (Run-4 @ RHIC)

 High pT in d+Au at forward rapidities at RHIC:

 Observation 6: Spectra suppressed by a factor ~2-3.

 “Explanation” (6): possible evidence of high twist effects at small-x.

backup slides ...

Unsuppressed (?) heavy-quark production

Indirect measurement via semileptonic open-charm decays: D e± X .

Within uncertainties, single electron Au+Au central spectra and total cross-section consistent with pQCD (Ncoll scaled p+p charm) production:

Strong(*) medium effects on heavy flavor production precluded so far.

(*) at least not as strong as for light-quark mesons.

Possible reduction (1σ) at high pT ?

factor ~2 less suppression expected

for D than for p in models of

medium-induced energy loss

Wait for results from hi-stat. Run-4.

see talks by M.Liu,

A. Dainese & N.Armesto

RAA vs. RdA (y = 0) : centrality dependence

PERIPHERAL Au+Au & d+Au

preliminary

RAA vs. RdA (y = 0) : centrality dependence

MID-PERIPHERAL Au+Au & d+Au

preliminary

RAA vs. RdA (y = 0) : centrality dependence

MID-CENTRAL Au+Au & d+Au

preliminary

RAA vs. RdA (y = 0) : centrality dependence

CENTRAL Au+Au & d+Au

preliminary

RAA vs. RdA (y = 0) : centrality dependence

Opposite centrality dependence of d+Au nuclear enhancement compared to Au+Au nuclear suppression.

(Model-independent) conclusion: Au+Au suppression at y = 0 not due to a “cold” (initial-state) nuclear matter effect: gluon saturation effects not relevant, final-state (QGP) interpretation favoured.

preliminary

High pT azimuthal correlations: Elliptic flow (I)

Elliptic flow = v2 second Fourier coefficient

Large v2 signal (saturating @ high pT):

Exhausting hydro limit for pT<1.5 GeV/c

 Strong (collective) pressure grads.

 Large and fast (t<1.0 fm/c) parton	rescattering: early thermalization.

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

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

2. “Hard” probe: develops exclusively in first instants of the reaction (t<3 fm/c).

see also talk by R.Snelling

High pT azimuthal correlations: Elliptic flow (II)

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

predicted by quark

recombination models:

v2 and pT normalized by #

of constituent quarks:

n = 2 mesons

n = 3 baryons

PHENIX Collab.

to appear in PRL

nucl-ex/0305013

High pT @ CERN-SPS: “Cronin” or “quenching” ?

New nuclear modification factor (better p+p → π0 ref. @ √sNN = 17.3 GeV)

[D.d'E. to be submitted]

No “Cronin” effect in central collisions (RAA ~ 1).

“Cronin” enhancement in peripheral ... and suppression in top central ?

Look for onset of suppression at RHIC Au+Au, p+p @ √sNN≈ 20 GeV ?

0-8% central

Peripheral

1% top central

D.Kharzeev, E.Levin, L.McLerran

PLB 561, 93 (2003)

pQCD expectation

CGC prediction

RAANpart

Centrality dependence of suppression (I): Npart scaling ?

Suppression (w.r.t. Ncoll scaling) increases smoothly with centrality:

peripheral	central

PHENIX

nucl-ex/0304022,

submitted to PRL

Approximate Npart scaling in

semiquantitative agreement with

Color Glass Condensate prediction

Foreword: central rapidity densities in Au+Au

dNch/dy constraints mechanisms of initial multi-particle production:

dNch/dy (per participant pair) increases faster than linearly with Npart :

Particle density at y=0 well described by pQCD- & CGC- based models alike:

 "Soft + hard" (string + pQCD “minijet”): increased hard contribution (Ncoll )

 Initial-state gluon saturation (CGC): dNch/dy ~ dNgluon/dy ~ 1/as ~ Npartln(Npart )

Au+Au 200 GeV

N.Armesto et al.

Int.J.Mod.Phys. A15, 2019

dNch/dy

centrality dependence:

David d'Enterria

2004-01-14T06:23:12