Jet quenching in high-energy
nuclear collisions

QCD @ Work 2005

Conversano, June 19th, 2005

David d’Enterria

Nevis Labs, Columbia University, NY

Overview

0. Physics motivation: Jet production in QCD medium (AA) vs QCD

vacuum (pp) as a signature of QGP formation at RHIC.

1. Empirical observation I: High pT (leading) hadron suppression.

- Magnitude of suppression (x5 in central AuAu @ RHIC-200 GeV) provides

direct info on transport (<q0>) & thermodynam. (dNg/dy) properties of medium

- Properties of suppression (pT-, Ös-, ... dependence) in agreement w/

non-Abelian gluon radiation off hard scattered partons.

2. Empirical observation II: Modified high pT di-hadron φ-,η-correlations.

- Disappearance of back-to-back dNpair/dφ peak (“monojets”)

- "Double peak" structure in away-side dNpair/dφ (“Mach boom” in medium ?)

- Di-jet pseudo-rapidity dNpair/dη broadening (coupling of g rad. w/ long. expansion ?)

3. Summary

Disclaimer: This is a limited selection of a vast number of exp. nucleus-nucleus data (no mention to high pT	baryon or heavy-Q spectra, no space for discussion on detailed jet properties <jT>,<kT>, ...)

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” = 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:

DEloss ~ gluon (gluon density)

DEloss ~ DL2 (medium length)

Energy is carried away by gluons emittted

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

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

(1) High pT leading hadron pT spectra
in high-energy pp, dA, AA 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 !

Hard spectra: AA = incoherent sum of pp

Hard yields 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”

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)

“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 direct photon production in AuAu

Color-less hard probes (direct g) are unsuppressed.

AuAu collision = incoherent sum of pp collisions (expected “Ncoll scaling” for perturbative probes).

D.d'E, HP'04

EPJ C to appear

(nucl-ex/0504001)

PHENIX.

PRL 94, 232301

Unquenched high pT hadroproduction 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)

Dense medium properties

From data vs. model (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]

Such large opacities imply fast thermalization.

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

[BDMPS, F.Arleo ]

[Salgado-Wiedemann]

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

Ös- 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

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 g density:

Medium transport coeff.:

SPS

RHIC

RHIC

High pT suppression: Excitation function

Ös-dependence (and 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 (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

}

(2) High pT di-hadron φ,η correlations
in high-energy pp, dAu and AuAu collisions

Jets in AA collisions at RHIC

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

p+p ®jet+jet [Ös = 200 GeV]

STAR @ RHIC (2003)

Au+Au ® X [ÖsNN = 200 GeV]

STAR @ RHIC (2003)

3-5 GeV/c

d+Au

Near-side

away-side

Jets via high pT di-hadron φ 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:

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

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

Di-hadron AuAu: Δφ correlations (I)

Near-side: Jet-like Gaussian. Unmodified (AuAu ~ dAu ~ pp)

STAR, PRL 90, 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

Away-side:

peak disappearance.

“monojet”-like topologies

in central AuAu.

Di-hadron AuAu: Δφ correlations (II)

W.Holtzman, RIKEN-Corrs. Workshop '05

Strongly modified away-side D correlations in central AuAu:

F.Wang, RIKEN-Corrs. Workshop '05

“peak”

“dip”

Standard

away-side

Gaussian

Preliminary

Away-side () “dip” and excess

of energy (“double peak”) at 1

Broader double peak at large pTassoc

(more pronounced for lower pTtrig)

Au+Au

periph

Au+Au

central

PHENIX Collab. PRL to be submitted

PHENIX Preliminary

Di-hadron AuAu: Δφ correlations (III)

Same dNpair/dD result in polar coords. now:

Strongly non-Gaussian

away-side “peak”.

Standard back-to-back

di-jet topology.

Au+Au peripheral

Au+Au central

PHENIX Collab. PRL to be submitted

“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, Satarov, Mishutin, hep-ph/0505245.
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)

d+Au, 40-100%

Au+Au, 0-5%

STAR preliminary

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

[D. Magestro, HP'04]

Di-hadron 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)

Di-hadron 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/ longitudinal

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

unmodified in pp,dAu

Away-side width and acoplanarity

unmodified in pp and dAu

Factor ~5 suppression of leading hadron

(very large initial parton densities: dNg/dy~1000)

Disappearance of back-to-back peak (“monojets”)

“Double peak” structure at lower pT in away-side

(“sonic boom” in medium ?)

Dijet broadening in η

(coupling of g radiation w/ expanding medium ?)

Strong QCD medium effects at work !

“QCD vacuum” & “cold QCD medium”

“hot & dense QCD Medium”

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

from here

backup slides ...

High pT suppression: Reaction-plane 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 not reproduced by “canonical” L2

(or L) path-length dependence.

 = 0°

 = 90°

PRELIMINARY

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

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

RAA(φ) versus parton energy loss model:

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]

Jets via high pT di-hadron φ correlations: AuAu

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

(1) Increased “underlying event” background

(2) Collective elliptic flow (harmonic) contribution

Ajitanand, ICPAQGP'04

and nucl-ex/0501025

Delicate subtraction procedure (esp. in finite acceptances).

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).

“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

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| :

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).

“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)

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

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

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) ?

Preliminary

Suppressed charm

production above

pT~2 GeV/c ?

“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

Jet production in AA : (a few) theoretical expectations

Mono-jets:

Jet broadening in eta:

Wang&Gyulassy PRL 68, 1480 (1992)

Armesto et al

hep-ph/0405301

Leading hadron suppression:

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

Medium-modified FFs:

Mach cone:

X.N.Wang;

Salgado&Wiedem.

Arleo, ...

Hagedorn, 1982

David d'Enterria

2004-01-14T06:23:12