High pT at RHIC:
Highlights.

AGS&RHIC Users Meeting

BNL, NY - May 12, 2004

David d’Enterria

Nevis Labs, Columbia University, NY

QGP & CGC search @ RHIC in the high pT sector

 “QCD vacuum” reference results – p+p @ 200 GeV

1. Inclusive hadron spectra: Ed3s/d3p

2. Jet properties (dNpair/dφ): <jT>, <kT>

 “Hot QCD medium” highlights – central Au+Au @ 200, 62 GeV

1. Suppressed hadron spectra: dN/dpT

s, pT, centrality dependence

2. Suppressed di-jet correlations (dNpair/dφ)

reaction-plane dependence, low pT assoc. correlations ...

3. Baryon “anomalies” @ intermediate pT

enhanced dN/dpT and v2, jet-like dNpair/dφ

 “Cold QCD medium” highlights – d+Au @ 200 GeV

1. Inclusive hadron dN/dpT enhancement at y ≤ 0

2. Inclusive hadron dN/dpT suppression at y ≥ 1

Comparison to

pQCD (NLO) expectations

Comparison to

QGP expectations

(“jet quenching” & “q recomb.”)

Comparison to

“Color-Glass-Condensate”

expectations

NEW

Hard scattering in A+A collisions

Hard scattering in A+A collisions

Leading-twist shadowing

or

Gluon saturation (CGC)

pT broadening

(Cronin enhancement)

[Experimental handle: p,d+A]

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

Initial-state effects

Hard scattering in A+A collisions

Leading-twist shadowing

or

Gluon saturation (CGC)

pT broadening

(Cronin enhancement)

medium-induced

parton energy loss:

“jet quenching” in QGP

possible hadronic

rescattering

[Experimental handle: p,d+A]

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

[Experimental handle: A+A]

Initial-state effects

Final-state effects

Hard scattering in A+A collisions

Leading-twist shadowing

or

Gluon saturation (CGC)

pT broadening

(Cronin enhancement)

medium-induced

parton energy loss:

“jet quenching” in QGP

possible hadronic

rescattering

Approach: Study modifications (dN/dpT, particle composition, dNpair/dφ, ...) of high pT in A+A with respect to p+p, d+A to learn about QCD many-body dynamics:

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

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

[Experimental handle: p,d+A]

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

[Experimental handle: A+A]

Initial-state effects

Final-state effects

Part I:
p+p @ 200 GeV
“Calibrating” the QCD vacuum

p+p @ 200 GeV: “baseline” spectra

Good theoretical (NLO pQCD) description ...

KKP FF

Kretzer FF

(PDF: CTEQ6M)

PHENIX Collab.

PRL91, 241803

hep-ex/0304038

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

High quality data: sensitive to different

parametrizations of gluon FF

p+p @ 200 GeV: “baseline” spectra

Good theoretical (NLO pQCD) description ...

... at variance with lower sqrt(s) results (factors of ~2-4 discrepancy):

non-perturbative effects (intrinsic kT), partly cured by NLL soft g resummation

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

p+p @ 200 GeV: “baseline” spectra

Good theoretical (NLO pQCD) description:

KKP FF

Kretzer FF

(PDF: CTEQ6M)

PHENIX Collab.

PRL91, 241803

hep-ex/0304038

Well calibrated (experimentally &

theoretically) p+p references at hand !

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

p+p @ 200 GeV: jet properties (I)

Intrinsic kT (di-jet acoplanarity):

Jet 2

Jet 1



R=0.35

=0.22±0.02±0.06

=0.31±0.05±0.06

d+Au

<kT> ~ 1.8 GeV/c

= +

p+p @ 200 GeV: jet properties (II)

Jet “width” jT :

Jt

pythia-GEANT

< jT> ~350 MeV/c

preliminary

< jT> ~ 500 MeV/c

p+p

p+p

p+p

Part II:
Au+Au @ 200 GeV
Probing the hot & dense
QCD medium

Au+Au @ 200 GeV (central): suppression !

Discovery of

high pT suppression

(one of most significant

results @ RHIC so far)

x4-5 suppression

RAA << 1: well below pQCD

(collinear factorization) expectations

for hard scattering cross-section

p0

h+/-

d+Au @ 200 GeV: enhancement !

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

1/Ntrigger dN/d()

 (radians)

Au+Au @ 200 GeV (central): mono-jet correlations !

p+p, peripheral Au+Au: 2 jets

Central Au+Au: no away-side jet

d+Au control: 2 jets

Correlation of

trigger particles 4<pT<6.5 GeV with
associated particles 2<pT<pT,trig

Associated particles

Near side jet

Trigger particle

Away side jet



Low pT:

Associated low-pT yield (“jet remnants”):

near-side: jet-like

away-side: broad cos()-like from

momentum conservation

Au+Au @ 200 GeV (central): jet properties

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

Near-side Away-side

Associated hadron pT spectra:

Jets widths:

Central Au+Au: Associated away-side yields are “moved down” in pT

Au+Au @ 200 GeV (central):
reaction-plane dependence of monojet topologies

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

y

x

in-plane

Out-of-plane

Effect of path length on suppression is experimentally accessible

-/4

/4

3/4

-3/4

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

Au+Au @ 200 GeV: “anomalous” hadron composition

p+p collisions: hadron/meson ~ 1.6

Particle composition inconsistent with known fragmentation functions.

RHIC

Au+Au (central): Baryon/meson ~ 2.5

within pT = 1 - 4 GeV/c

ISR

Au+Au @ 200 GeV (central): baryons > mesons !

Baryon/meson ratios: ~1 !

Rcp baryons vs. mesons: very heavy mesons ( ,k*) =! baryons

Additional production mechanism for baryons in the intermediate pT range

Theory vs. data (I): final-state very dense medium

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]

Such large opacities imply:

(i) large rescattering: thermalization, (ii) energy densities εcrit QCD >> 1 GeV/fm3

Theory vs. data (II): partonic or hadronic medium ?

Energy loss in Dual-Parton-Model based approaches ...

(Elena Ferreiro)

... Mostly partonic

Theory vs. data (III): final-state “quark” medium

Hwa & Yang

PRC 67, 034902

Quark recombination (coalescence) mechanisms provide an additional mechanism for baryon production at intermediate pT's (2-5 GeV/c):

Via quark momenta addition:

pT(baryons)> pT(mesons)> pT(quarks)

Fragmentation dominates for pT > 5 GeV/c:

pT(hadrons)= z pT(partons) , with z<1

However ... thermal + shower parton recombination

needed to explain jet-like baryon near-side azimuthal

correlations

Hwa & Yang

nucl-th/0404066

Large quark densities in thermal medium required

Part IIb:
Au+Au @ 62.4 GeV
Probing the (sligthly less)
hot & dense QCD “medium”

Nuclear modification factor (p0): √sNN dependence

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

WA98, EPJ C 23, 225 (2002)

(*) Reanalysis: D.d'E. nucl-ex/0403055

PHENIX, PRL 88 022301 (2002)

PHENIX, PRL 91, 072303 (2003)

CERN-SPS: Pb+Pb central (√sNN = 17.3 GeV): small suppression (?) (*)

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

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

RHIC Au+Au (√sNN = 62.4 GeV): (i) probe suppression excitation function, (ii) confirm/constraint models of (partonic) energy loss

RAA ~ 0.4

RAA ~ 1.0

RAA~ 1.5

RAA~0.2

(*) RHIC should run Au+Au

and p+p @ √sNN ~ 20-30 GeV

Au+Au @ 62.4 GeV: pT spectra

π0

preliminary

0-10% p0

10-30% p0

0-85% p0

nucl-ex/0405003

p+p @ 62.4 GeV: reference pT spectra

π0

Reasonable agreement (within

~30% norm. uncertainties)

among h+/- parametrizations

No concurrent p+p measured at RHIC. Have to rely on ISR measurements ...

+/-25% uncertainty

Au+Au @ 62.4 GeV (RAA central): h± suppression !

STAR preliminary

pT~2-3 GeV/c:

RAA ~ 1

pT >6 GeV/c:

RAA ~ 0.3

x3 suppr.

Central	Peripheral

Au+Au @ 62.4 GeV (RAA central): π0 suppression !

Pions more suppressed than h+/- at intermediate pT (also found at 200 GeV):

RAA ~ 0.6 at pT~2 GeV/c

“Universal” (PID) and constant suppr. at high pT: RAA ~ 0.3 for pT > 6 GeV/c

Neutral pions

Charged hadrons vs. pions

p+p  h± X ref. in denominator

p+p  πo X ref. in denominator

Au+Au @ 62.4 GeV (RAA central): baryons again !

Central p/π+ ratio similar to 200 GeV Au+Au result at high pT.

Central pbar/π- ratio at 2-3 GeV is smaller than 200 GeV results.

Smaller pbar/p ratio due to more baryon transport to mid-rapidity and less pbar production.

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

Au+Au @ 62.4 GeV (central): data vs. theory

I. Vitev nucl-th/0404052 Comparison by Saskia Mioduszewski

Reasonably good agreement (esp. high pT) within uncertainties

Caveat: uncertainty in the pT shape (esp. low pT) of the p+p reference ...

Part III:
d+Au @ 200 GeV at y~3
Probing the QCD cold “medium”

The quest for gluon saturation effects @ RHIC ...

RHIC kinematical regime:

High pT @ midrapidity:

y = 0, Q2 = 1-100 GeV2/c2

- pQCD collinear factorization

- DGLAP evolution (g splitting)

- small nuclear effects in PDFs

(LT shadowing).

Moderate pT, rapidities:

y ≈ 1-3, Q2 ≈ 10 GeV2/c2

- kT factorization

- linear BFKL evolution (g splitt.)

- “moderate” nuclear effects (LT

shadowing).

Low pT @ large rapidities:

y > 3, Q2 < Qs2 ≈ 5 GeV2/c2

- pQCD factorization breakdown

- non-linear evolution (g fusion)

- strong nuclear effects in the

initial-state

xT = pT /√s (e-y+ey)

(2 → 2)

x small: Look forward in rapidity !

d+Au @ 200 GeV (η = 3.2): forward suppression !

First time a large “shadowing” is seen at small-x and high pT in nuclear syst.

So far unexplored perturbative region of nuclear (x,Q2) plane.

Perturbative

non-perturbative

Factor ~2 suppression

pT = 1-3 GeV/c hadron

production at η = 3.2

(x2 ~ 10-4 in Au).

Is this “standard” nuclear shadowing ?

Maximum gluon shadowing at x~10-4 (indirectly) constrained by leading-twist approaches fitted to available DIS data on nuclear targets is ~0.8

D. de Florian & R.Sassot

hep-ph/0311227

Q2=10 GeV2

RG(x)

NLO DGLAP global analysis

of nuclear PDFs

Is this “Color Glass Condensate” ?

CGC + quantum evolution seem to reproduce data:

Possible hint of extra higher-twist effects at small-x (breakdown of QCD factorization). But, soft physics effects can still be playing a role here ...

(K. Tuchin)

Summary

 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 d+Au vs p+p at midrapidity at RHIC:

(4) Spectra enhanced by a factor ~1.3

“Explanation” (1,2 via 4): pQCD hard scattering + final-state parton	energy loss + quark recombination in dense thermal QCD medium.

QGP ? : thermal γ ?, J/Ψ suppression ? (Run-4 AuAU @ 200 GeV)

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

(5) Spectra suppressed by a factor ~2.

“Explanation” : possible evidence of higher twist effects at small-x.

backup slides ...

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).

d+Au @ 200 GeV : rapitidy dependence of suppression

Rapidity

Low xAu

High xAu

Suppression at low xAu (shadowing, saturation,…)

Enhancement at high xAu (Cronin, anti-shadowing,…)

J/ behaves like other mesons

AB

QCD factorization in A+B collisions

pQCD (factorization theorem) expectation for inclusive A+B hard cross-sections:

Independent scattering of “free” partons:

dAB → hard = A·B·dpp → hard

dNAB → hard (b) = TAB(b)·dpp → hard

Nuclear

Modification

Factor:

RAA = 1

A+A = “simple superposition of p+p

collisions” at high-pT where hard scattering dominates

AB

At imp. param. b:

geom. nuclear overlap at b

production is “shadowed”

TAB ~ # NN colls. (“Ncoll scaling”)

Confirmation... unsuppressed hard colorless production in Au+Au central

“Control” observable: direct photons (clean, penetrating = directly coupled to partonic vertex, no fragmentation) non-hadronic hard probes.

Photons (insensitive to final-state effects) show collision scaling at high pT:

pQCD parton scattering holds for hard processes in central Au+Au !

g NLO pQCD x Ncoll Vogelsang/CTEQ6 (M = 0.5 - 2.0 pT)

preliminary AuAu 200 GeV 0-10%

[ J.Frantz QM'04 ]

“ g (direct+decay) / g decay ”

David d'Enterria

2004-01-14T06:23:12