HARD PROBES'04

Ericeira, Portugal - Nov 7th, 2004

David d’Enterria

Nevis Labs, Columbia University, NY

High pT hadron suppression in A+A collisions: from SPS to RHIC

Overview

Motivation: High pT in A+A collisions as a probe of the properties	of QCD media (QGP, CGC) via p+p ↔ p,d+A ↔ A+A comparison

High pT A+A hadroproduction from √sNN = 20 to 200 GeV:

SPS √sNN»20 GeV results revisited:

p+p reference, indications of suppression, comparison to Eloss models

RHIC results at √sNN = 62.4 GeV:

p+p references, preliminary RAA, comparison to Eloss models

A few new RHIC results at √sNN = 200 GeV:

very high pT π, η suppression, data vs. Eloss models

Excitation function of high pT suppression.

Summary. 3 lessons learnt.

Hard scattering in A+A collisions

[ Experimental handle: p+p ]

Hard scattering in A+A collisions

Leading-twist shadowing

or

Gluon saturation (CGC)

pT broadening

(Cronin enhancement)

[Experimental handle: p,d+A]

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

Initial-state effects

Hard scattering in A+A collisions

pT broadening

(Cronin enhancement)

medium-induced

parton energy loss:

“jet quenching” in QGP

possible hadronic

rescattering

[Experimental handle: p,d+A]

[Experimental handle: A+A]

Final-state effects

Leading-twist shadowing

or

Gluon saturation (CGC)

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

Initial-state effects

Hard scattering in A+A collisions

pT broadening

(Cronin enhancement)

medium-induced

parton energy loss:

“jet quenching” in QGP

possible hadronic

rescattering

[Experimental handle: p,d+A]

[Experimental handle: A+A]

Final-state effects

Leading-twist shadowing

or

Gluon saturation (CGC)

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

Initial-state effects

Approach: Study modifications (dN/dpT, particle composition, dNpair/dφ) of high pT in A+A with respect to p+p, p+A to learn about the properties of QCD media:

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

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

Expected hard scattering yields in A+A

Production yields calculable theoretically 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”

RHIC A+A “breakthroughs”(*)
in the high pT sector
(*) as given by PRL covers

(1) High pT suppression in central Au+Au @ 200 GeV

x4-5 suppression

RAA << 1: well below pQCD

(collinear factoriz.) expectations

for hard cross-sections

Consistent with “jet quenching”

expectations for leading hadrons

Neutral pions

Charged

hadrons

(2) High pT enhancement in d+Au @ 200 GeV

d+Au @ RHIC shows “Cronin” pT broadening as seen at lower Ös p+A

Suppression in central Au+Au due to final-state effects

Charged Hadrons

Neutral

pions

much of the excitement lies in the Ös dependence ...

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

WA98, EPJ C 23, 225 (2002)

PHENIX, PRL 88 022301 (2002)

PHENIX, PRL 91, 072303 (2003)

CERN-SPS: Pb+Pb central (√sNN = 17.3 GeV): strong Cronin enhancement

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

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

RAA ~ 0.4

RAA (p0) in central A+A collisions (as of ~1 year ago):

RAA ~ 2.0

RAA~ 1.5

RAA~0.2

D.d'E

nucl-ex/0401001

A+A at SPS, ISR ≅ fixed-target p+A at Fermilab ...

Std. argument: “Initial-state effects dominate hard hadro-production in A+A at SPS energies. Final-state effects do not play a significant role.”

How can it be, however, that: εBjSPS » 3 GeV/fm3 (⇒ dNq+g/dy»2·ε3/4 ·t0·AT» 600) and yet there is no high pT suppression at SPS !? (whereas there is a factor x5 suppression at RHIC w/ εBjRHIC » 5 GeV/fm3)

A+ A @ √sNN = 17.3, 31 GeV	p+A @ √sNN = 20 - 40 GeV

(as of ~1 year ago)

High pT in A+A at SPS (√sNN ≈ 20 GeV):
Cronin or suppression ?

High pT A+A spectra @ CERN-SPS

WA98 Collab.

EPJ C23 (02)225

Pb+Pb → π0 + X

(√s = 17.3 GeV)

CERES Collab

P.Wurm, J. Bielcikova

nucl-ex/0407019

Pb+Au → π± + X

(√s = 17.3 GeV)

WA80 Collab.

EPJ C5 (98)225

S+Au → π0 + X

(√s = 19.4 GeV)

Spectra go up to pT~3.5-4 GeV/c (large stat. uncertainties in higher pT bins).

Evidence of hard scattering processes: power-law deviation from “soft”

(exponential) behaviour observed above pT ~2 GeV/c.

High pT baseline p+p spectra @ CERN-SPS ?

NO p+p  π +X reference measurement at SPS Pb+Pb energy (√s = 17.3 GeV)

Closest existing π

p+p data for:

√s = 16.9 GeV

√s = 18.2 GeV

√s = 19.4 GeV

(not completely

consistent among

each other at high pT)

Enhanced high pT production @ CERN-SPS ?

RAA for central Pb+Pb constructed using 2 different p+p parametrizations:

WA98 Collab.

EPJ C23 (02)225

Huge Cronin

enhancement

above pT~2 GeV/c

Wang&Wang

PRC 64 (01) 034901

Moderate Cronin

enhancement

above pT~1 GeV/c

p+p ref. constructed from

higher energy ISR π0 data

+ xT scaling

p+p ref. constructed from

ISR π± data + pQCD-

based √s scaling

Enhanced high pT production @ CERN-SPS ?

RAA for central Pb+Pb constructed using 2 different p+p parametrizations:

WA98 Collab.

EPJ C23 (02)225

Huge Cronin

enhancement

above pT~2 GeV/c

Wang&Wang

PRC 64 (01) 034901

Moderate Cronin

enhancement

above pT~1 GeV/c

But suppressed Rcp at

the same time (?!)

p+p → π +X @ √s ≈ 20 GeV : data vs. references

WA98, Wang&Wang p+p parametrizations confronted to π data (√s = 16 – 20 GeV):

D.d'E. nucl-ex/0403055

PLB 596 (2004) 32

WA98 parametrization

undershoots the data

by up to a factor of ~3

Wang&Wang parametriz.

undershoots the data

by up to a factor of ~2

New p+p → π +X reference @ √s ≈ 20 GeV

D.d'E. nucl-ex/0403055

PLB 596 (2004) 32

New parametrization [Blattnig et al. PRD62 (2000) 094030] versus p+p data √s = 16 – 20 GeV:

Better agreement in shape & magnitude

(within large exp. uncertainties).

sqrt(s) dependence (√s = 16 – 20 GeV) of yields correctly reproduced.

p+p → π0+/- + X

(√s = 19.4 GeV)

“New” nuclear modification factors at SPS

High pT π0 production in ~0 ─10% central A+A at SPS and ISR energies:

... not enhanced but consistent with “collision scaling” (RAA ~ 1) above pT~2 GeV/c

D.d'E. nucl-ex/0403055

PLB 596 (2004) 32

Indications of high pT suppression @ SPS

Centrality evolution of high pT π0 production (WA98 Pb+Pb √s = 17.3 GeV):

RAA GLV prediction at SPS:

w/o energy loss (just Cronin)

[See A. Accardi talk also]

“Cronin” enhancement

in peripheral Pb+Pb

Suppression in 1% (!?)

most central Pb+Pb

Peripheral

1% top central

0-8% central

Indications of high pT suppression @ SPS

Centrality evolution of high pT π0 production (WA98 Pb+Pb √s = 17.3 GeV):

Peripheral

1% top central

0-8% central

WA98 Collab.

EPJ C23 (02)225

New RAA is now consistent with originally

published WA98 suppressed Rcp

High pT @ SPS: data vs. theory

New RAA at SPS agree better with SPS parton energy loss predictions:

Vitev, Gyulassy

PRL 89 (2002) 252301

High pT @ SPS: data vs. theory

New RAA at SPS agree with parton energy loss calculations [I.Vitev nucl-th/0404052] in moderately dense system: dNg/dy ~ 400-600 (more consistent with estimated εBj ~ 3 GeV/fm3 ).

D.d'E. nucl-ex/0403055

PLB 596 (2004) 32

Measuring a high pT p+p reference @ √s ≈ 20 GeV ?

Option (1):

SPS-NA49 ?

SPS- NA57 / WA97 ?

Option (2): RHIC Au+Au, p+p run at √s ≈ 20 GeV (feasible, though would need more support from RHIC community, long runs required to collect stat !)

Au+Au @ RHIC:
High pT suppression at √sNN = 62.4 GeV

High pT Au+Au spectra @ RHIC 62.4 GeV

preliminary

nucl-ex/0405003

π0

preliminary

High pT baseline p+p spectra @ 62.4 GeV ?



No concurrent p+p @ 62.4 GeV measured at RHIC in Run-4 ...

p+p @ 62–63 GeV measured at ISR: π0 (8), π± (4), charged hadrons (2)

D.d'E.

HotQuarks'04

Proceeds.

p+p → π +X spectra @ 62.4 GeV are not consistent



Discrepancies as large as a

factor of ~3 between data sets !

D.d'E.

HotQuarks'04

Proceeds.

Final corrected p+p→ π +X spectra @ 62.4 GeV



Correction for: unsubtracted η

& direct photon “contaminations”

hi-pT yields @ 62  62.4  63 GeV

(+ elimination of 2 “outlier” data sets)

Final corrected data consistent

among each other within ±25%

D.d'E.

HotQuarks'04

Proceeds.

Parametrized p+p → π +X reference @ √s = 62.4 GeV



Calculations by

Werner Vogelsang

+/-25% uncertainty

after “corrections”

D.d'E.

HotQuarks'04

Proceeds.

Parametrization:	Good agreement with NLO:

(purely empirical)

Parametrized p+p → h± +X reference @ √s = 62.4 GeV



Reasonable agreement (within ~30% norm. uncertainties) of

PHOBOS/STAR/PHENIX h+/- parametrizations and existing data

High pT suppression in central Au+Au @ 62.4 GeV

STAR preliminary

RAA ~ 1 at pT~2-3 GeV/c (h±)

RAA ~ 0.8 at pT~2-3 GeV/c (π0,± )

RAA ~ 0.3 for pT > 6 GeV/c (h±,π0,±)

Charged hadrons

Neutral & charged pions

preliminary

nucl-ex/0406201

Zhangbu Xu (STAR), nucl-ex/0411001

RAA	RAA

h± less suppressed than π0,± at intermediate

pT's (“baryon enhancement”) [See talks by

B.Jacak, R.Hwa, J.Velkovska]

pT (GeV/c)

High pT Au+Au @ 62.4 GeV : data vs theory

I. Vitev, nucl-th/0404052

X.N. Wang, nucl-th/0405029

Adil & Gyulassy, nucl-th/0405036

A.Dainese et al., hep-ph/0406201

High pT Au+Au @ 62.4 GeV : data vs theory

I. Vitev, nucl-th/0404052

X.N. Wang, nucl-th/0405029

Adil & Gyulassy, nucl-th/0405036

A.Dainese et al., hep-ph/0406201

RAA~0.3 for

dNg/dy~800

Au+Au @ RHIC:
High pT suppression at √sNN = 200 GeV
some more recent stuff(*) ...

[ (*) More details in Henner Buesching's talk tomorrow ]

Latest high pT Au+Au spectra @ RHIC 200 GeV

π0

preliminary

Au+Au  p0 +X	Au+Au  η+X

Measured spectra well in the perturbative domain: pTmax ~ 15 (10) GeV/c

Latest high pT baseline p+p spectra @ 200 GeV

Truly perturbative spectra (pTmax = 17 GeV/c) well described by NLO pQCD

(PDF: CTEQ6M)

p+p  p0 +X	p+p  η+X

preliminary

preliminary

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

preliminary

preliminary

Latest RAA in Au+Au @ 200 GeV

Au+Au central: Strong suppression (RAA~0.2)

Au+Au semi-central: Suppression (RAA~0.4)

Au+Au peripheral: consistent w/ Ncoll scaling (RAA~0.9)

preliminary

Au+Au η+ X @ √sNN = 200 GeV

Latest RAA @ 200 GeV

Coincident suppression pattern for π0 and η: magnitude, pT dependence

Agreement with parton energy loss (GLV) predictions in dense medium

(flat behaviour up to the highest pT values measured so far)

preliminary

Preliminary

GLV RAA π0

dNg/dy = 1100

Excitation function of high pT suppression
from SPS to RHIC

Excitation function of high pT suppression



X.N. Wang, PLB579 (2004)299

Predicted Ös dependence of suppression in

std. parton Eloss scenario (propto initial

parton density) + 1-D Bjorken expansion

Excitation function of high pT suppression



X.N. Wang, PLB579 (2004)299

h± less suppressed than π0 still at

pT = 4 GeV/c (non perturbative

baryonic contribution)

Excitation function of high pT suppression



X.N. Wang, PLB579 (2004)299

Excitation function of high pT suppression



Excitation function of high pT suppression



{

Excitation function of high pT suppression



62.4 GeV RAA

Excitation function of high pT suppression



Parton Eloss predictions reproduce well

the observed Ös dependence of suppression

Summary

RAA for high pT hadroproduction at CERN-SPS energies revisited using a new p+p reference:

(1) RAA(cent) » 1. No Cronin. RAA(cent) < RAA(periph) consistent w/ factor ~1.6 suppress.

(2) (new) RAA consistent w/ (old) Rcp

(3) RAA(cent) in agreement w/ parton Eloss calculations in dense system:	dNg/dy » 500 (consistent with estimated εBj »3 GeV/fm3).

(4) (Re)measurement of A+A, p+p at √sNN ≈ 20 GeV desirable (look for onset of suppr.)

RAA at high pT in Au+Au √s = 62.4 GeV

(1) Current ISR-averaged p+p refs. have uncertainties of order ~30%.

(2) RAA(cent) in agreement w/ parton Eloss in dense system: dNg/dy » 800

(3) More quantitative study of high pT suppression requires actual measurement of	p+p at at √s = 62.4 GeV (RHIC Run-5 ?).

Latest RAA at high pT in Au+Au √s = 200 GeV

(1) Universal suppression for all hadrons (π0, η, h±) above pT~5 GeV/c.

(2) Very high pT suppression (pT>10 GeV/c) in agreement w/ parton Eloss predictions

Excitation function of suppression described by parton Eloss models

Corollary

3 lessons learnt:

(1) At CERN SPS energies one is likely creating the same (less

dense) strongly interacting matter as at RHIC. Indications of

high pT suppression (need actual exp. confirmation !) are now

more consistent w/ previous observations: J/ψ suppression, εBj, ...

(2) If we want to constraint / challenge parton energy loss models

(and we want to, in order to learn more about the properties of	the dense QCD medium produced), we need more differential	observables than (ratios of) spectra [RAA vs. azimuthal angle, ...].

(3) If we want to characterize quantitatively the properties of the	produced media in A+A collisions (QGP,CGC), we need a

concurrent measurement of the p+p baseline at the same sqrt(s) ! [This applies for RHIC 62.4 GeV, but also for 5.5 TeV at CERN-

LHC where we need to convince the “Higgs – beyond SM –

SUSY...” community to run at ~1/3 of the nominal (maximal)

p+p collision energy].

backup slides ...

Unsubstracted π 0 “contaminations” at ISR (1)

All but one measurement at ISR didn't substract the η and direct-

“World average” η/π0 ~0.45 ratio at high pT in hadronic colls.

18% η contribution needs to be substracted from “unresolved” π0 spectra.

BRη →  • Rη/π0 = 0.39 • 0.45 ≈ 0.18

Unsubstracted π 0 “contaminations” at ISR (2)

All but one measurement at ISR didn't substract the η and direct-

/π0 ratio at high pT in p+p at 62 GeV (data compared to NLO pQCD):

Prompt  are a significant source of e.m. clusters above pT ~6 GeV/c

that needs to be substracted too

High pT suppression - 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)

_ _

David d'Enterria

2004-07-18T02:48:03