Hirschegg 2005

Kleinwalsertal, Austria, Jan. 19, 2005

David d’Enterria

Nevis Labs, Columbia University, NY

Jets and jet quenching in
high energy nuclear collisions

Overview

Introduction:

- Jet production in pp, pA and AA collisions.

- What can be learnt about high-energy-density QCD via jet physics in AA collisions ?

Results I: Leading hadron production in pp, dAu, AuAu @ RHIC:

- High pT hadro-production in baseline pp collisions (data vs. pQCD)

- High pT suppression in central AuAu: pT-, sqrt(s)-, reaction-plane dependence.

- Physics [Final state (FS) effects]: Parton energy loss ® QCD medium properties.

Results II: Jet production in QCD vacuum (pp) & cold QCD medium (dAu):

- Full jet reconstruction (pp).

- Dijets via dihadron Δφ correlations (pp, dAu).

- Extraction of jet properties: jT, kT

- Physics [Initial state (IS) effects]: multiple scattering in cold nuclear matter

Results III: Jet production in a hot & dense QCD medium (AA):

- Dijets: away-side disappearance: sqrt(s)-, reaction-plane dependence.

- Extraction of jet properties (jT, kT, “FFs”).

- Dihadron Δη correlations.

- Physics [Final state (FS) effects]: Jet quenching ® QCD medium properties.

Jet production in pp collisions

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

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

(ii) Parton fragmentation transverse momentum (jet “width”): jT

N.B.(1) -- High pT (leading) hadron: pp, ppbar

pT< 2 GeV/c: Expo. (Ed3s/d3p ~ e-6pT) w/ constant inv. slope: ~ 160 MeV ~ Tcrit

Thermally-shaped Soft Production: e-6pT s-indep.

N.B.(1) -- High pT (leading) hadron: pp, ppbar

pT< 2 GeV/c: Expo. (Ed3s/d3p ~ e-6pT) w/ constant inv. slope: ~ 160 MeV ~ Tcrit

pT> 2 GeV/c: Power-law (Ed3s/d3p ~ 1/pTn, n~ 6 -10) with strong Ös-dependence.

Hard Scattering -- varies with s

Thermally-shaped Soft Production: e-6pT s-indep.

Jet production in pp collisions

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

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

(ii) Parton fragmentation transverse momentum (jet “width”): jT

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

N.B.(2) -- Jets in pp, ppbar collisions

p+pbar ® jet+jet [Ös = 560 GeV]

UA2 @ CERN-SppS (1983)

nucleon

nucleon

parton

jet

p+pbar ® jet+jet [Ös = 1.96 TeV] – D0 @ Tevatron (2001)

p+p ® jet+jet [Ös = 200 GeV] – STAR @ RHIC (2003)

p+p ® jet+jet [Ös = 63 GeV]

AFS @ CERN-ISR (1982)

Jet production in pp collisions

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

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

(ii) Average jet fragmentation transverse momentum (jet “width”): <jT>

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

<jT>

Jet production in pp collisions

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

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

(ii) Average jet fragmentation transverse momentum (jet “width”): <jT>

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

<kT>

<jT>

<kT>

Jet production in pA collisions

kT broadening

(Cronin enhancement)

Initial-State effects

Extra nuclear kT broadening due to multiple scattering in the nuclear medium (Cronin enhancement of leading hadron).

:<kT> = <kTintr> + <kTnuc>

Jet production in pA collisions

Leading-twist Shadowing

or

Gluon saturation (CGC)

kT broadening

(Cronin enhancement)

Initial-State effects

Extra nuclear kT broadening due to multiple scattering in the nuclear medium (Cronin enhancement of leading hadron, dijet broadening).

At (very) small-x values (forward rapidities) there is a reduced effective number of parton scattering centers due to gluon-gluon fusion processes.

Jet production in AA collisions

Leading-twist Shadowing

or

Gluon saturation (CGC)

Initial-State effects

kT broadening

(Cronin enhancement)

medium-induced

parton energy loss:

“jet quenching” in QGP

Final-State effects

Parton energy loss due to final-state gluon-strahlung in dense medium:

(i) Reduces energy of the leading hadron: high pT hadron suppression,

(ii) Modifies (di)jet shape properties: kT, jT, dN/dxE, ...

Jet production in AA collisions

Leading-twist Shadowing

or

Gluon saturation (CGC)

Initial-State effects

kT broadening

(Cronin enhancement)

Medium-induced

parton energy loss:

“jet quenching” in QGP

Final-State effects

Approach: Compare jet production in p+p,d+A & A+A :

I. Inclusive leading hadron: dN/dpT

II. Di-hadron correlations: dNpair/dφ, dNpair/dη

III. Obtain (di)jet properties: kT, jT, dN/dxE (~FF)

... to learn about transport & thermodyn. properties of dense QCD medium.

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

Leading hadron production
at RHIC: pp, dAu, AuAu

Results I

_________________

Leading hadron spectra: pp @ 200 GeV

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

KKP FF

Kretzer FF

(PDF: CTEQ6M)

PHENIX Collab.

PRL91, 241803

hep-ex/0304038

High quality data: sensitive to different

parametrizations of gluon FF

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

Leading hadron spectra: pp @ 200 GeV

High pT p0 well described by collinear NLO pQCD: No apparent need of large intrinsic kT (though this is maybe FF-dependent) at variance w/ lower sqrt(s) results:

Higher-order theoretical corrections (soft-g rad., threshold resumm.) cure the problem.

p+p  p0 X

Bourrely & Soffer

EPJC 36, 371 (2004)

hep-ph/0311110

factors of ~2 - 4 discrepancy

below pT~5 GeV/c

[Vogelsang & deFlorian, to be submitted]

Leading hadron spectra: 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.

PRL91, 241803

hep-ex/0304038

Well calibrated (experimentally &

theoretically) p+p baseline spectra at hand !

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

Leading hadron spectra: AuAu, dAu @ 200 GeV

π0

Au+Au:

d+Au:

High pT p0, h±,h, ...spectra (well) above pT ~ 10 GeV/c.

Comparing AA, pA and pp hard inclusive spectra

Production yields computable in 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 “shadowed”

TAB ~ # NN collisions (“Ncoll scaling”)

“Factorization theorem”:

Independent scattering of “free” partons:

A+B = “simple superposition of p+p collisions”

( dpA → hard = A·dpp → hard )

High pT p0: AuAu vs pp @ 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

AuAu @ 200 GeV (central): high pT suppression !

Discovery of

high pT suppression

(one of most significant

results @ RHIC so far)

x4-5 suppression

RAA << 1: well below pQCD

(factorization) expectations

for hard scattering cross-sections

p0

h+/-

dAu @ 200 GeV: high pT enhancement !

Conclusion: Suppression in central Au+Au is due to final-state effects.

p0

h+/-

High pT suppression: QCD medium properties

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 ecrit QCD in thermalized syst.

High pT suppression: Excitation function

sqrt(s)-dependence of RAA consistent w/ parton Eloss models (ΔEloss~ dN/dy) + Bjorken expansion:

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

Qu Wang & X.N.Wang (nucl-th/0410049)

[D.d'E , Hard Probes'04]

High pT suppression: Reaction-plane dependence

Observe:

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

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

Ongoing study to constraint path-length dependence (L, L2 ?) of energy loss.

 = 0°

 = 90°

PRELIMINARY

[B. Cole, S. Mioduszewski

HP'04]

Jet production:
QCD vacuum (pp) versus
cold QCD medium (dAu)

Results II

__________________

Full jet reconstruction in pp

First attempt to fully reconstruct jets @ RHIC (p+p @ Ös = 200 GeV)

Uses STAR EMC (neutral)

+ TPC (charged).

Two different triggers.

“Cone algorithm” (R=0.7)

Obtained jT spectrum (see later)

Raw Per Event Yield

p+p MinBias

p+p High Tower

Uncorrected Et in cone (GeV)

Trigger bias

[T.Henry, QM'04]

Pythia-GEANT

< jT> ~ 500 MeV/c

Preliminary

3-5 GeV/c

d+Au

Near-side

away-side

Dijets via 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 (“unseen” in inclusive high pT hadron spectra <kT>pair¹ <kT>incl)

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]

Di-jet acoplanarity (“intrinsic” kT): Excitation function (pp)

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).

Jet production in hot & dense
QCD matter (AA collisions)

Results III

___________________

Jets in high-energy AA collisions

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

p+p ®jet+jet [Ös = 200 GeV]

STAR @ RHIC (2003)

Au+Au ® X [ÖsNN = 200 GeV]

STAR @ RHIC (2003)

Dijets via dihadron azimuthal correlations: AuAu (200 GeV)

[Same analysis as in pp, dAu plus elliptic flow subtraction]

Discovery of “monojet”-like topologies in central AuAu: (disappearance of away-side jet correlations):

STAR, PRL90, 082302 (2003)

STAR, PRL90, 082302 (2003)

Associated low pT hadrons

(“jet remnants”):

1/Ntrigger dN/d()

 (radians)

pT trigg= 4 – 6 GeV/c

pTassoc = 0.15 – 4 GeV/c

[F.Wang, QM'04]

Dijets via dihadron azimuthal correlations: AuAu (200 GeV)

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

Low pT:

Away-side peak: IAA~ 0

STAR, PRL90, 082302 (2003)

Near-side peak: IAA~ 1

STAR, PRL90, 082302 (2003)

Reaction-plane dependence of away-side dissapeareance

pTtrigger=4-6 GeV/c, 2<pTassociated<pTtrigger, ||<1

y

x

in-plane

-/4

/4

3/4

-3/4

[K.Filimonov, QM'04]

Reaction-plane dependence of away-side dissapeareance

pTtrigger=4-6 GeV/c, 2<pTassociated<pTtrigger, ||<1

y

x

Out-of-plane

-/4

/4

3/4

-3/4

Back-to-back suppression out-of-plane stronger than in-plane

(consistent with increasing energy loss in larger path-length)

[K.Filimonov, QM'04]

Reaction-plane dependence of away-side dissapeareance

pTtrigger=4-6 GeV/c, 2<pTassociated<pTtrigger, ||<1

y

x

in-plane

Out-of-plane

-/4

/4

3/4

-3/4

Constraints on path-length dependence of partonic “absorption”

Back-to-back suppression out-of-plane stronger than in-plane

(consistent with increasing energy loss in larger path-length)

[K.Filimonov, QM'04]

Dihadron azimuthal correlations: AuAu (62 GeV)

Low pT:

Away-side disappearance also at 62 GeV (statistics limited):

STAR preliminary

[near side] – [away side] diff. indicates similar suppression pattern @ 200 and 62 GeV

STAR preliminary

[D. Magestro, HP'04]

Dihadron azimuthal correlations: PbPb (17 GeV)

Low pT:

Large broadening of away-side correlations seen in di-pion azimuthal correlations at CERN-SPS:

Gaussian peaks have diff. widths: sN = 0.27  0.05 rad, sF = 0.55  0.17 rad.

Centrality dependence: sN is constant, sF increases w/ centrality

Significant kT broadening: <kT> = 2.8 0.6 GeV/c

[J. Bielcikova, HP'04]

Elliptic flow

pT > 1.2 GeV/c

PbPb: 21-26% central

CERES

Jet properties (jT, kT): AuAu (200 GeV)

(3.0pTtrigg5.0)(1.5pTassoc3.0)

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

Centrality dependence of |jTy| and z |kTy| in Au+Au:

<jT>AuAu~ <jT>pp: near-side fragmentation unaffected by QCD medium.

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

[J.Rak, QM'04]

“Fragmentation functions”: Central AuAu (200 GeV)

Near-side Away-side

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

Associated near-side jet yields unmodified (pp ~ AuAu)

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

Azimuthal distribution of “jet remnants”:

near-side: jet-like

away-side: broad cos()-like from

momentum conservation

[F.Wang, QM'04]

“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

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  correlations: AuAu (200 GeV)

Significant broadening of pseudo-rapidity

correlations in AuAu compared to pp,dAu.

(“stretching” of jet cone along η).

Coupling of g radiation w/ longitud. expanding medium?

STAR preliminary

p+p

Au+Au, 0-5%

near-side width

Summary

Summary: “Jet quenching” at RHIC

Results I: Leading hadron production in pp, dAu, AuAu @ RHIC:

- Strong (factor ~5) high pT suppression in central AuAu: pT-, sqrt(s)- dependence consistent with parton energy loss in dense QCD medium (dNg/dy~1100).

- Reaction-plane dependence of the suppression provides additional constraints to the path-length dependence of the energy loss.

Results II: Jet production in QCD vacuum (pp) & cold QCD medium (dAu):

- Small differences on the extracted properties (jT, kT) of the jets emitted in pp & dAu

- Relatively small initial state effects (multiple scattering) in cold nuclear matter.

Results III: Jet production in a hot & dense QCD medium (AA):

- Away-side disappearance consistent with:

(i) “Mono-jet” predictions due to high-energy parton “absorption” in dense QCD matter. (enhanced suppression following line of longest path).

(ii) Large broadening of di-jet acoplanarity (kT~3 GeV/c) due to multiple scattering of

away-side parton in the medium. (Can we extract a transport coeffic. consistent w/

the observed dNg/dy ? ).

- Unmodified near-side azimuth. jet properties (jT): Vacuum fragm. of unquenched trigger had.

- No strong flavor dependence at intermediate pT observed

- Dihadron eta correlations consistent w/ coupling of g rad. w/ longitud. expand. medium

“Cartoon summary”: Jet-quenching at RHIC

Jet profile in pp (dAu) collisions:

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

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.

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

“Thermalized” associated low pT yields

Dijet broadening in eta.

(Unmodified) near-side width: <jT> ~ 600 MeV/c

Backup slides

“Fragmentation functions”: xE distributions pp,dAu

PHENIX preliminary

J. Jia

 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

Preliminary

Dijets via dihadron azimuthal correlations: AuAu

2.5 < pTtrig < 4 GeV/c, 1.0 < pTassoc < 2.5 GeV/c

Additional associated yield in same jet in Au+Au

But same angular width observed !!!

PHENIX preliminary

PHENIX preliminary

Summary

Jet production and fragmentation:

Good agreement of the jet properties in pp collisions with other lower s experiments

dAu jT and kT consistent with pp

In AuAu significant broadening of “effective” kT - with centrality

Back-to-back  correlations

Suppression of away-side jet → jet quenching picture

Suppression dependent on reaction plane orientation

No strong flavor dependence at intermediate pT observed

Near-side  correlations

Evidence for near-side jet broadening in central Au+Au

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

(1) Inclusive spectra suppressed by a factor of 4-5 at 200 GeV	and by a factor of ~3 at 62.4 GeV

(2): Intermediate pT hadron composition inconsistent	with known fragmentation functions in free space.

(3) Disappearance of away-side jet correlations. Enhanced	mono-jet pattern following line of longest path.

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

(5) Spectra suppressed by a factor ~2.

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)

Dijets via dihadron azimuthal correlations: AuAu

.. 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: jets in dAu, pp

..

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 !

“ 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

Au+Au @ 62.4 GeV (central): suppression predictions

I. Vitev nucl-th/0404052

X.N. Wang nucl-th/0405029

Adil & Gyulassy nucl-th/0405036

RAA (π0) ~ 0.5 - 0.3

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+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).

David d'Enterria

2004-01-14T06:23:12