Jet quenching at RHIC:
experimental perspective

Workshop on Parton Propagation

through Strongly Interacting Matter

ECT*, Trento, October 2nd, 2005

David d’Enterria

LLR – Ecole Polytechnique, IN2P3, Paris

Overview

 Physics motivation:

Jet production in QCD medium (AA) vs. QCD vacuum (pp)

as a signature of QGP formation at RHIC.

“Jet physics”@RHIC (w/o full jet reco): Inclusive high pT spectra, 2-hadron correls.

 High pT (leading) hadron suppression data in central AA

confronted to non-Abelian radiative energy loss “paradigm”:

1. Magnitude ÞVery dense medium: dNg/dy ~ 1000 (~ dNch/dη). OK.

2. Transverse momentum dependence: flat pT . OK.

3. Centrality dependence. OK.

4. Light-meson species dependence (π0 vs. η). OK.

5. Center-of-mass energy dependence (SPS-20 GeV, RHIC-62,-200 GeV). OK.

6. Non-Abelian radiation. OK.

7. Path-length dependence. OK ?

8. System-size (CuCu vs. AuAu) dependence. OK ?

9. Baryon vs. meson suppression. OK ?

10. Heavy vs. light quark suppression. OK ?

 Summary

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 collective dynamics of q&g

(QCD phase diagram): 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

Jet production in the “QCD vacuum” (pp collisions)

Jet : Collimated spray of hadrons in a cone ( ~ 0.7) with 4-momentum of original fragmenting parton

Leading hadron takes away large fraction (<z> ~0.6 –0.8 @ RHIC) of parent parton pT

Jet balanced back-to-back by other hard-scattered "parton" (jet, direct g, ...) A (small) acoplanarity appears due to intrinsic kT (parton Fermi motion).

Jet production in “QCD media” (pA, AA collisions)

kT broadening

(Cronin enhancement)

(Leading-twist) shadowing

or gluon saturation (CGC)

Final-state effects

Parton energy loss due to medium-induced

gluon-strahlung in hot & dense environment

Initial-state effects

(accessible via pA colls.):

(accessible in AA colls.):

“Jet quenching” as a QGP signal

Multiple final-state non-Abelian (gluon)

radiation off the produced hard parton

induced by the dense QCD medium

Parton energy loss µmedium properties:

Energy is carried away by gluons emittted

inside (broader) jet cone:

Different energy losses:

“gluonsstrahlung”

Prediction I: Suppression of high pT leading hadrons: dN/dpT

Prediction II: Modification of (di)jet correlations: d2Npair/dφdη

Predition III: Modified energy- & particle- flow within full jet

~collinear but broadened

“Softened” fragmentation

dE/dx ~ s k2T

DEloss(g) > DEloss(q) > DEloss(Q)

(color factor) (mass effect)

SPS,RHIC,LHC

RHIC,LHC

LHC

DEloss ~ gluon (gluon density)

DEloss ~ DL2 (medium length)

Jet physics at RHIC: full jet reconstruction ?

Full jet reconstruction w/ standard algorithms is unpractical at RHIC due to huge soft background (large “underlying event”):

Feasible at LHC for Ejet >~ 50 GeV

p+p ®jet+jet [Ös = 200 GeV]

Au+Au ® X [ÖsNN = 200 GeV]

“Jet physics” at RHIC: single inclusive high pT spectra

Alternative I : Study the energy modifications suffered by the highest pT hadron in the event (“leading” hadron of the jet) in AA (compared to pp):

Many interesting results obtained from this “first-order” approach !

p+p ® h+ X [Ös = 200 GeV]

Au+Au ® h+X [ÖsNN = 200 GeV]

“Jet physics” at RHIC: di-hadron azimuthal correlations

Alternative II : Study the azimuthal correlations in AA w.r.t. pp between the highest pT hadron (“trigger”) & any other “associated” hadron:

Many interesting results obtained from this “2nd-order” approach !

p+p ® h1+h2+ X [Ös = 200 GeV]

Au+Au ® h1+h2+ X [ÖsNN = 200 GeV]

“Jet physics” at RHIC: di-hadron azimuthal correlations

Alternative II : Study the azimuthal modifications in AA w.r.t. pp between the highest pT hadron (“trigger”) & any other “associated” hadron:

Many interesting results also obtained from this “2nd-order” approach !

p+p ® h1+h2+ X [Ös = 200 GeV]

Au+Au ® h1+h2+ X [ÖsNN = 200 GeV]

Jet production in AA : (a few) theoretical expectations

Mono-jets:

Jet broadening in η:

X.N.Wang&M.Gyulassy PRL 68, 1480 (1992)

Armesto et al

hep-ph/0405301

Leading hadron suppression:

Valuable diagnostic tools of QCD medium properties (dNg/dy, <q0>, cs, ...)

Medium-modified FFs:

"Mach cone":

X.N.Wang;

A. Majumder,

Salgado&Wiedem.

Arleo, ...

Bjorken, 1982

dNpair/dφ

dNpair/dη

dN/dpT

M

Trigger

Mach cone:

dN/dzT

Stoecker et al. hep-ph/0505245.
Casalderrey, Shuryak, hep-ph/0411315

nucl-th/0302077

Nchjet increases

<zjet> decreases

High pT leading hadron spectra at RHIC
& jet-quenching models:
Good agreement data  theory

Inclusive single spectra at high pT (AA, dA, pp)

High quality large-pT data (up to ~20 GeV/c) available in pp, dA and AA collisions:

Au+Au – 200 GeV

d+Au – 200 GeV

p+p – 200 GeV

How to compare high pT spectra in AA and pp ?

High pT particles issue from hard scatterings describable by pQCD:

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”

Leading hadron spectra in free space: pp @ 200 GeV

High pT p0,h± spectra up to ~15 GeV/c. Good theoret. (NLO pQCD) description

KKP FF

Kretzer FF

(PDF: CTEQ6M)

PHENIX Collab.

PRL 91, 241803

hep-ex/0304038

High quality data: sensitive to different

parametrizations of gluon FF

p+p  p0 X	p+p  h± X

Well calibrated (experimentally & theoret.)

p+p baseline spectra at hand.

Leading hadron spectra in AuAu @ 200 GeV

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 in central AuAu

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)

Very strong suppression (RAA ~ 0.2 !) up to

pT ~ 10 GeV/c for π0 h±, well below pQCD

expectations for hard cross-sections

PHENIX Collab.

PRL 88, 022301 (2002)

nucl-ex/0109003

[Run-2 data]

Hadrons are suppressed. Photons are not.

Colorless hard probes (direct g insensitive to final-state) are unsuppressed.

Confirms that AuAu collision = incoherent sum of pp collisions

(i.e. “Ncoll scaling” expectation is valid) for perturbative probes.

D.d'E, HP'04

EPJ C to appear

(nucl-ex/0504001)

PHENIX.

PRL 94, 232301

Hadrons are suppressed in AuAu. Not in dAu.

Initial-state cold nuclear matter effects (shadowing, Cronin) are small at RHIC mid-rapidity.

High pT suppression in central AuAu is due to final-state effects

(absent in “control” dAu experiment)

D.d'E.,

nucl-ex/0401001

PHENIX.

PRL 91, 072303 (2003)

Magnitude of the suppression: medium properties

Data vs. models (pQCD+ non-Abelian parton energy loss) comparison:

Initial gluon densities:

dNg/dy ~ 1000 [Vitev & Gyulassy]

Opacities:

<n> = L/≈ 3 – 4 [Levai et al.]

Transport coefficients:

<q 0> ~ 14 GeV2/fm

Medium-induced radiative

energy losses:

dE/dx ≈ 0.25 GeV/fm (expanding)

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

Very large gluon densities: dNg/dy~1000 consistent w/ measured dNch/dη ~700

All transport & thermodynam. values imply energy densities well above ecrit QCD

[BDMPS ]

[Salgado-Wiedemann]

[Dainese, Loizides ...]

High pT suppression: pT-dependence

Flat pT- dependence described by parton energy loss models:

The matter is so opaque that even a 20 GeV 0 is stopped.

Underlying LPM interference for single gluon bremsstrahlung would give: DEloss ~ log(pT)

Combination of different effects (convolution w/ realistic gluon energy distribution, local parton pT slope, ...) yields constant suppression factor.

Question ... What about running as ?

PQM

PQM – A. Dainese, C. Loizides, G. Paic

EPJ C 38, 461(2005)

GLV – I. Vitev 2005

High pT suppression: “Universal” for all light mesons

Common suppression pattern (magnitude, pT, centrality, ... dependence) for 0 and :

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

The matter is so opaque that even a 20 GeV 0 is stopped.

Same flat RAA ~ 0.2 up to 10 GeV/c

Universal suppression for light mesons indicates it is at partonic level before q,g fragments into leading meson according to vacuum FFs.

High pT suppression: Ös-dependence

Ös- dependence in agreement with parton energy loss in increasingly dense (expanding) medium:

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

D.d'E., HP'04

EPJ C to appear

nucl-ex/0504001

RAA ~ 1 @ s ~ 20 GeV Þ dNg/dy ~ 400 , <q0> ~ 3.5 GeV/fm2

RAA ~ 0.3 @ s = 62 GeV Þ dNg/dy ~ 650 , <q0> ~ 7 GeV/fm2

RAA ~ 0.2 @ s = 200 GeV Þ dNg/dy ~ 1100 , <q0> ~ 14 GeV/fm2

[Note: RAA @ SPS

uses “revised” pp ref.]

Initial gluon density:

Medium transport coeff.:

SPS

RHIC

RHIC

High pT meson suppression in AA @ 17.3 GeV ?

Revised pp reference: high pT π0 production in (0-10%) central PbPb at SPS is slighted suppressed or consistent w/ “Ncoll-scaling” :

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

NA57, PLB62, 317 (2005)

Confirmed by NA57 (& NA49) recent high pT results in central PbPb at SPS:

E loss

(Wang)

NA49 (A. Lazslo, QM'05)

High pT suppression: non-Abelian nature

Excitation function (Ös-dependence) & non-Abelian nature of energy loss in agreement w/ parton energy loss calculations:

(i) rising initial parton density with Ös

(ii) increasing relative fraction of hard-scattered gluons (at fixed pT) with Ös

“Jet quenching” model + 2-D longitudinal plasma expansion

D.d'E. EJP C to appear

nucl-ex/0504001

Gluon: CA = Nc = 3

Quark: CF =(Nc2-1)/2Nc = 4/3

Relative fraction of q,g at pT = 5 GeV/c:

QCD radiation probability:

CA /CF=2.25

}

High pT suppression: centrality dependence

Increasing centrality Þ increased Npart Þ increased suppression

Agreement data  models OK

“Jet quenching” model + 2-D longitudinal plasma expansion

PQM – A. Dainese, C. Loizides, G. Paic

EPJ C 38, 461(2005)

X.N. Wang, PLB 2003

High pT leading hadron spectra at RHIC
& jet-quenching models:
Less good agreement data  theory ?

High pT suppression: system-size dependence

Smaller CuCu system adds significant precision at intermediate Npart~100:

Theory predicts: ln(RAA) µNpart-2/3

Both PHENIX & STAR preliminary data seem to exclude = -2/3

Fit to STAR Npart prefers “shallower” ~ -1/3 (circumf./area ~ A-1/3 ~ Npart-1/3 ?)

PHENIX data seems to indicate a “steeper” slope at low Npart.

Differences STAR  PHENIX and PQM  GLV still unclear at this point.

M. van Leeuwen, QM'05

C.Klein-Boesing, QM'05

High pT suppression: path-length dependence

2 times more suppression out-of-plane (“long” direction)

than in-plane (“short” direction).

Glauber parton energy loss model predicts only ~50% increased “out-of-plane” vs “in-plane” π0 emission

Azimuthal anisotropy stronger than “canonical” L2 (or L) path-length dependence.

Source of extra azim. anisotropy above pT ~ 4 GeV/c ?

 = 0°

 = 90°

PRELIMINARY

[B. Cole, S. Mioduszewski HP'04]

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

RAA vs φ w/ respect to reaction plane :

PQM – Dainese, Loizides, Paic EPJ C 38, 461(2005)

Intermediate pT mesons suppressed. Baryons are not

Strongly enhanced baryon (p, L) production at pT ~ 2 – 4 GeV/c

Strong centrality dependent baryon/meson: ratio well above “perturbative” (pp) ratios.

Clear deviation from std. vacuum fragmentation functions (large non-pQCD effects) calls for extra baryon production mechanism: recombination.

Above pT ~ 6 GeV/c: Recovery of “vacuum” fragmentation ratio. Baryons suppressed too.

/K0s

Au+Au 0-5%

p+p

Au+Au 0-10%

p+p

Lamont, QM'05

O.Barannikova, QM'05

Baryon vs. meson “fragmentation functions”

However ... Associated yields similar for meson & baryon triggers.

Magnitude and centrality-dependence of associated (near- & away- side) hadron pT spectra for baryon & meson triggers show small differences.

Jet-like production but different suppression for leading baryons and mesons !?

PRC 71, 051902 (2005)

Near-side

Away-side

pT trigg= 2.5 – 4 GeV/c

pTassoc = 1.7 – 2.5 GeV/c

Heavy quark suppression via non-photonic electrons (I)

Semi-leptonic decays of open charm and bottom mesons = main source

of high pT (“non-photonic”) electrons.

proton-proton baseline:

Au+Au suppression

(Note: state-of-the-art theory underpredicts data)

Heavy quark suppression via non-photonic electrons (II)

Latest single e± RAA indicates large suppression in central AuAu:

Note: STAR – PHENIX RAA agrees,

but the pp refs are different by ~ 50%.

Peripheral AuAu

Central AuAu

Heavy quark suppression via non-photonic electrons (III)

Theory expectations :

Quantitative predictions:

Note: Using larger medium densities: dNg/dy=3500, <q> = 14 GeV2/fm

than for light mesons RAA ! Unclear consistency w/ dNch/dy ~ 600

M. Djordjevic et al., nucl-th/0507019

Armesto et al. QM'05

Charm RAA = 0.2 - 0.3

Beauty RAA = 0.4 - 0.6

DEloss(g) > DEloss(q) > DEloss(Q)

(color factor) (mass effect)

Gluonsstrahlung probability

“Dead cone”: g rad.

suppressed at  < mQ/EQ

Heavy quark suppression via non-photonic electrons (IV)

Models need too dense medium to account for observed suppression in data:

Possible resolutions of the disagreement (or a combination of them ?):

(1) Larger suppression of beauty …or charm dominance up to electron pT ≈ 10 GeV?

(2) Extra jet-fragmentation production of charm which will be affected by energy loss ? (supported by PHENIX proton-proton data itself ?)

(3) Hadronic (rather than partonic) energy loss ?

(4) Radiative + collisional energy loss ? Other ... ?

Summary

Large amount of high precision large-pT hadron production data at RHIC after 5 years of operation allows to quantitatively address jet physics in

QCD medium (w/o full jet reco).

Details of suppressed hadro-production in central Au+Au provide:

- stringent constraints on underlying physics.

- direct access to the density and transport properties of the QCD medium.

Is jet quenching due to radiative energy loss in a QGP ?

Good agreement with calculations on:

Magnitude, √s, pT, centrality, (light) species dependence

Some tests are weak at present:

Few details missing in system-size dependence

no sharp test of L2 dependence yet.

unsuppressed (but jet-like) baryon production points to (sthg. more than) recombination ?

heavy quark energy loss larger than expected

LHC will provide enormous reach and qualitatively new observables (full jet reco, jet-jet, jet-g,Z correlations ...)

backup slides ...

High pT suppression: system-size dependence

RAA for Cu+Cu @ √sNN = 200 GeV

Suppression observed for central Cu+Cu

Models scale density from central Au+Au

All models show reasonable to good agreement

STAR preliminary

Charged hadrons

M. van Leeuwen (STAR), QM'05

M. Shimomura (PHENIX), QM'05

High pT suppression: charm quark (theory)

(1) Slow clock for formation time

Djordjevic & Gyulassy

Zhang & XNW

Armesto,Dainese, Salgado & Wiedemann

(2) Color factor

(3) Dead cone effect

EQ < Eq < Eg

David d'Enterria

2004-01-14T06:23:12