“Jet quenching”:
Latest news from RHIC

CERN Heavy-Ion Forum

CERN, April 13th, 2005

David d’Enterria

Nevis Labs, Columbia University, NY

Overview

Physics motivation.

Latest experimental results on

AA single (leading) hadron spectra at high pT:

Excitation function of leading hadron suppression: RAA( Ös)

Reaction-plane dependence of hadron suppression: RAA(φ)

High pT single electrons & heavy-quark energy loss: RAA ® Q(pT)

Strong modif. ("dip") in away-side di-hadron azim. corrs.: dNpair/dφ

Enhanced di-jet acoplanarity. “Thermalized” away-side dNpair/dφ

Di-jet pseudo-rapidity broadening: dNpair/dη

Summary

Latest experimental results on

AA di-hadron φ-,η-correlations at high pT:

Physics motivation

Multiple final-state non-Abelian (gluon)

radiation off the produced hard parton

induced by the dense QCD medium.

Parton energy loss µmedium properties:

DEloss ~ gluon (gluon density)

DEloss ~ DL2 (medium length)

Energy is carried away by gluonsstrahlung

inside (broader) jet cone (modified multiplicity& energy flow):

“gluonsstrahlung”

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

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

Leading high- pT hadron

~collinear but broadened

“Softened” fragmentation

dE/dx ~ s k2T

High pT leading hadron pT spectra
in high-energy AA collisions

Suppressed high pT hadroproduction in AuAu (central) colls.

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)

“Universal” (pid-wise) suppression: RAA~0.2

up to pT ~ 14 GeV/c for π0, η, h± well below

pQCD expectations for hard cross-sections

PHENIX Collab.

PRL 88, 022301 (2002)

nucl-ex/0109003

Unquenched high pT hadroduction in dAu collisions

High pT suppression in central AuAu due to final-state effects (absent in “control” dAu experiment)

D.d'E.,

nucl-ex/0401001

STAR.

PRL91, 072304 (2003)

PHENIX.

PRL91, 072303 (2003)

Unquenched direct photons in AuAu collisions

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

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

Submitted to PRL

nucl-ex/0503003

pQCD parton scattering holds

for hard (colorless) QCD processes

in AuAu (all centralities).

High pT suppression: sqrt(s)-dependence (I)

sqrt(s)- (and pT) 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

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 g density:

Medium transport coeff.:

High pT suppression: sqrt(s)-dependence (II)

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

(i) rising initial density w/ Ös

(ii) increasing relative fraction of hard-scattered gluons (for fixed pT) w/ Ös

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

D.d'E., HP'04

nucl-ex/0504001

Gluon: CA = Nc = 3

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

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

QCD radiation probability:

CA /CF=2.25

}

High pT suppression: Reaction-plane dependence (I)

Less suppression in-plane (“short” direction).

More suppression out-of-plane (“long” direction).

Associated path-length dependence not reproduced by “canonical” L2 (or L) behaviour (in “simple” Glauber model).

 = 0°

 = 90°

PRELIMINARY

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

High pT suppression: Reaction-plane dependence (II)

RAA(φ) versus parton energy loss model:

Data: factor of ~2 more suppression “out-of-plane” than “in-plane”.

Model: predicts only ~50% increased “out-of-plane” vs “in-plane” π0 emission

D.d'E., HP'04

nucl-ex/0504001

Dainese et al Eur.Phys.J. C38 (2005) 461

Charm production in AuAu: “NN scaling”

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

Single e± AuAu spectra (pT ~ 0.3 – 2 GeV/c) & total cross-section consistent w/ Ncoll -scaled pp charm production:

PHENIX.

PRL94, 082301 (2005)

NcharmAuAu = (Ncoll)a A

Note: a = (0.906 – 1.042) Þ

Ncolla » (0.6 – 1.2) Ncoll

Charm quark suppression at high pT ?

Latest single e± RAA at higher pT < 4.5 GeV/c (large uncertainties still @ low pT):

New kinematic domain accessible with heavy-Q: Hard production at low pT

RAA(lo pT) ~1 >> RAA(hi pT) ~ RAA(p0): Energy loss for fast heavy Q shifts them down to low pT? No en. loss effect for slow Q (flatter charm dN/dpT at low pT) ?

See next talks

by A. Dainese

& R. Turrisi

Preliminary

Suppressed charm

production above

pT~2 GeV/c ?

High pT di-hadron φ correlations
in high-energy pp, dAu collisions

3-5 GeV/c

d+Au

Near-side

away-side

Dijets via high pT dihadron azimuthal correlations: pp, dAu

Two-particle correlations: h±- – h±, p0,± – h±. Trigger: highest pT (leading) hadron.

Associated  distribution (e.g. ”assorted”: 2 GeV/c < pTassoc < pTtrigger)

Normalized to number of triggers:

Clear near- ( ~ 0) and away- ( ~ p) side jet signals:

trigger

Phys Rev Lett 90, 082302

STAR Preliminary

Jt = 500 ± 40 ± 150 MeV/c

kt= 1.9 ± 0.2 ± 0.3 GeV/c /<z>

4.5 < Ettrig < 6.5 GeV/c

6.5 < Ettrig < 8.5 GeV/c

ptassoc > 2.0 GeV/c

d+Au (cent, MB)

p+p

2-3 GeV/c

p+p

Preliminary

d+Au

p+p

pTassoc = 2-3 GeV/c

pTassoc = 3-5 GeV/c

Jet properties from dihadron correlations

Jet “width” jT :

Di-jet acoplanarity kT:

(1) 2-hadron correlation function:

(2) Fit to 2-gaussians:

near-side sN, far-side sF widths

(3) Extraction of jT, kT from sN, sF via

[*], [**] (and dN/dxE from YieldN,F)

Þ

[*]

[**]

near-side far-side

[details in J.Jia, nucl-ex/0409024]

Mean transverse momentum of jet hadrons (jT): pp, dAu

Jet (near-angle) “width” jT :

jT

p+p

Pythia + GEANT

< jT> ~ 500 MeV/c (from full jet reco)

p+p

< jTy> ~350 MeV/c º < jT> ~ 500 MeV/c

< jT> ~ 500 MeV/c: Agreement between RHIC and ISR data.

No apparent difference	between dAu and pp.

Fragmentation not affected by cold QCD medium.

[J.Rak, QM'04]

Di-jet acoplanarity (“intrinsic” kT) : pp, dAu

Intrinsic kT (di-jet acoplanarity):

Jet 2

Jet 1



z = 0.75 taken as constant

Non-negligible kT broadening in pp: <kTy> ~ 1.1 GeV/c

In dAu:

Non-null (but small) <kT>nuclear (constraints models of multiple scattering in cold nuclear medium)

R=0.35

d+Au

= +

=0.22±0.02±0.06

=0.31±0.05±0.06

(from full jet reco: ET~13 GeV)

[J.Jia, HQ'04, J.Rak, QM'04]

Excitation function of pp di-jet acoplanarity (“intrinsic” kT)

Dihadron in pp @ RHIC

<kT> ~ 1.8 GeV/c

sqrt(s)-dependence of <kT>pair:

(Logarithmic) increase with sqrt(s) consistent with growing gluon radiation contribution (not just intrinsic parton Fermi motion).

High pT di-hadron φ-,η- correlations
in high-energy AA collisions

Dijets in AuAu via high pT dihadron azimuthal correlations

Same dNpair/dφ analysis as in pp (dAu) but 2 extra “complications”:

(1) Increased “underlying event” background: a0

(2) Elliptic flow subtraction: H(Dj)

Ajitanand, ICPAQGP'04

and nucl-ex/0501025

Complex procedure (esp. in finite acceptances) under control now

(see e.g. Ajitanand et al: nucl-ex/0501025)

High pT AuAu dihadron azimuthal correlations

Near-side: Unmodified (AuAu ~ dAu ~ pp)

Away-side: Disappearance of high pT correlations (“monojet”-like topologies) in central AuAu.

STAR, PRL90, 082302 (2003)

Away-side:

Broadened distribution of

associated low pT hadrons

1/Ntrigger dN/d()

 (radians)

pT trigg = 4 – 6 GeV/c

pT assoc = 0.15 – 4 GeV/c

High pT :

Lower pT :

STAR, submitted to PRL

nucl-ex/0501016

pT trigg = 4 – 6 GeV/c

pT assoc > 2 GeV/c

Centrality dependence of AuAu jet properties: jT, kT

The near-side width independent of centrality.

The away-side width increases with centrality.

2.5 < pTtrigg< 4.0 GeV/c

1.0 <pT Assoc.< 2.5 GeV/c

Jets widths:

Preliminary

pp <z><|kTy|>

pp <|jTy|>

pTtrigg =2.5 – 4.0 GeV/c , pTassoc = 1.0 – 2.5 GeV/c

<jT>AuAu » <jT>pp: Near-side fragmentation unaffected by QCD medium.

<kT>AuAu » 3 GeV/c: Significant kT broadening (strongly centrality dependent) indicating substantial final-state rescattering of away-side fragmenting parton.

[J.Rak, QM'04]

Centrality dependence of |jTy| & z |kTy| :

Away-side AuAu dihadron azimuthal correlations (I)

W.Holtzman, RIKEN-Corrs. Workshop '05

Excess of energy on the away side most prominent at  1.

Strongly modified away-side Dj correlations in central AuAu:

Away-side () “dip” Û excess of energy “double peak” at  1.

Broader & double peaked at large pTassoc (more pronounced for lower pTtrig)

F.Wang, RIKEN-Corrs. Workshop '05

“peak”

“dip”

Std.

away-side

Gaussian

PHENIX Preliminary

Away-side AuAu dihadron azimuthal correlations (II)

Same PHENIX dNpair/dDj result now in polar coords:

Strongly non-Gaussian

away-side “peak”.

Standard back-to-back

di-jet topology.

Au+Au periph

Au+Au central

P.Stankus, RIKEN-Correlations Workshop '05

“Double peak” = Mach wave cone ?

Double peak structure at at  1 rad reminiscent of ...

Mach wave conical shock (“sonic boom”) Þ speed of sound accessible

Caveats:

M

Trigger

Calculations based on collisional energy loss alone (does rad. energy

loss fill the cone ?)

(2) The direction of the Mach cone should change w/ the expansion.

Stoecker, Dumitru, nucl-th/0406018.
Casalderrey, Shuryak, Teaney, hep-ph/0411315.

Mach cone:

 = arccos(cavs) = 1.2 rad = 71º

c2s ¹ constant:

QGP (1/ Ö3) ® phase transition (0.) ® HRG (Ö0.2)

(exp ~ 1. rad ~ 57º)

“Fragmentation functions”: Central AuAu

syst. error

Near side:

overall

enhancement

from pp to AA

larger initial

parton energy

(and modest

energy loss)?

Away side:

softening of

spectra from pp

to central Au+Au

energy from

initial parton

converted to

low pT particles.

energy loss in

medium!

Associated (pT assoc = 0.15 – 4 GeV/c) near- and away- side hadron pT spectra:

Associated near-side jet yields overall enhanced (enhanced underlying evt.)

Associated away-side jet yields “shifted down” in pT: spectra closer to pure “soft” inclusive hadron production (“thermalized”)

Near-side

Away-side

pT trigg = 4 – 6 GeV/c

pT assoc = 0.15 – 4 GeV/c

STAR, submitted to PRL

nucl-ex/0501016

d+Au, 40-100%

Au+Au, 0-5%

STAR preliminary

3 < pT(trig) < 6 GeV
2 < pT(assoc) < pT(trig)

[D. Magestro, HP'04]

Dihadron AuAu  correlations

Significant broadening of pseudo-rapidity

correlations in AuAu compared to pp,dAu.

(“stretching” of jet cone along η).

STAR preliminary

p+p

Au+Au, 0-5%

near-side width

d+Au, 40-100%

Au+Au, 0-5%

STAR preliminary

3 < pT(trig) < 6 GeV
2 < pT(assoc) < pT(trig)

Armesto, Salgado, Wiedemann

PRL 93, 242301 (2004)

Dihadron AuAu  correlations

Significant broadening of pseudo-rapidity

correlations in AuAu compared to pp,dAu.

(“stretching” of jet cone along η).

Coupling of g radiation w/ longitudal

expanding medium?

“Cartoon summary”: Jet-quenching at RHIC

Jet profile in pp (dAu) collisions:

Jet profile in AuAu central collisions:





°





°

Near-side width: <jT> ~ 600 MeV/c

Dijet acoplanarity: <kT> ~ 1.8 GeV/c

Factor ~5 suppression of leading hadron.

Unmodified near-side  width: <jT>AuAu ~<jT>pp

Increased dijet acoplanarity: <kT> ~ 3 GeV/c

“Dip” (& double peak) structure in away-side dN/d

“Thermalized” associated low pT yields

Dijet broadening in η.

Strong medium effects at work !

Outlook

15 times more data available (ongoing DST production) !

.. and exciting jet-physics expected ahead at LHC: g-, Z-, jet-jet corrs., ...

All data shown so far

from here

backup slides ...

“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)

“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]

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<Eg, Eq

Zhang & XNW

“Jet quenching”: modified (di)jet structure

Strongly modified away-side dNpair/d jcorrelations in central AuAu:

F. Wang

STAR, nucl-ex/0501016.

Enhanced and broadened distribution at low pT.

Away side suppression at high pT.

Dihadron azimuthal correlations: AuAu “mono-jets”

Low pT:

Near-side peak: IAA~ 1

Away-side peak: IAA~ 0

Centrality dependence of near- and away- side correlations “strengths”:

[A.Majumder, nucl-th/041261]

STAR, PRL90, 082302 (2003)

Dihadron azimuthal correlations: AuAu “mono-jets”

Centrality dependence of near- and away- side correlations “strengths”:

F.Wang. RIKEN Corrs. Workshop '05

IAA

Centrality dN/d

Old

New

Line = new with old v2.

IAA

Centrality dNch/deta

“Fragmentation functions”: Central AuAu (200 GeV)

Baryon-meson dependence of associated near- and away- side hadron pT spectra:

Associated yields similar for meson & baryon triggers (perhaps weak reduction for baryons in very central collisions).

Slight increase of associated near-side jet yields in mid-central AuAu.

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

PHENIX

nucl-ex/0408007

Near-side

Away-side

pT trigg= 2.5 – 4 GeV/c

pTassoc = 1.7 – 2.5 GeV/c

“Fragmentation functions”: xE distributions pp,dAu

 Dq(z)~e-6z

independent of pTt

<ztrig>=0.85 measured*

Away-side associated hadron pT spectra:

xE ~ z/<ztrig> represents away jet fragmentation z

xE variable: Two-particle equivalent of fragmentation variable z

At high pT, i.e. when di-jets are nearly back-to-back.

“Fragmentation functions”: xE distributions pp,dAu

PHENIX preliminary

J. Jia

independent of pTt

Away-side associated hadron pT spectra:

Preliminary

“Fragmentation functions”: xE distributions pp

PHENIX preliminary

independent of pTt

Away-side associated hadron pT spectra:

CCOR (ISR) s = 63 GeV

see A.L.S. Angelis, Nucl Phys B209 (1982)

Correct +-

1/xE  -5.3

1/xE  -5.8 to –7.8

David d'Enterria

2004-01-14T06:23:12