Hadron production at high pT:
experimental survey

“QUARK MATTER 2004”

17th International Conference on

Ultra-Relativistic Nucleus-Nucleus Collisions

Oakland, CA, Jan. 14, 2003

David d’Enterria

Nevis Labs, Columbia University, NY

Overview

1. Introduction:

The goal: study Quantum Chromo many-body Dynamics: QGP, CGC.

The means: compare hard scattering production in diff. colliding systems.

2. “QCD vacuum” production – high pT spectra in p+p:

Baseline reference data of hard scattering in free space.

3. “Hot QCD medium” production – high pT spectra in central A+A:

Light-flavour (u,d,s): Suppressed (compared to free space).

- s, pT, centrality, y, particle species dependence

 Heavy-flavour (c): Unsuppressed (?)  Non-hadronic (g): Unsuppressed.

4. “Cold QCD medium” production – high pT spectra in d+Au:

 Light-flavour (u,d,s) production: Enhanced at y ≤ 0 (mid&forward)

Suppressed at y ≥ 1 (backwards)

- ntrality, and particle species dependence.

5. What have we learnt ? Data vs. theory.

6. Summary

Light-flavor (u,d,s): Suppressed (compared to free space).

s, pT, y, centrality, and particle species dependence.

 Heavy-flavor (c): Unsuppressed (?).

 Colorless particle (g): Unsuppressed.

 Light-flavor (u,d,s) :	Enhanced at y ≤ 0 (mid-rapid. & high x2 in Au)

pT, centrality, and particle species dependence.

Suppressed at y ≥ 1 (small x2 in Au)

CGC ?

QGP ?

}

}

Hard QCD probes. Motivation (I)

Hard probes: High-pT, jets, direct , heavy-quarks (D, B), ...

[1] Early production ( ~ 1/pT< 0.1 fm/c) in parton-parton scatterings with large Q2:

Closest experimental probes to underlying QCD (q,g) degrees of freedom.

[2] Direct probes of partonic phase(s)  Sensitive to QCD medium properties:

[3] Incoherent processes: Direct comparison A+A to p+p yields via “ Ncoll scaling” :

dAB → hard (b) = TAB (b)·dpp → hard

Nuclear overlap:

TAB (b)  Ncoll(b) : number of binary inelastic NN colls.

q: fast color triplet

g: fast color octet

Q: slow color triplet

QQbar: slow color

singlet/octet

Virtual photon: colorless

Real photon: colorless

Unknown QCD Medium

Induced

gluon

radiation ?

Energy

Loss ?

Dissociation ?

Controls

QCD probe in

QCD medium

(possible quark-gluon plasma?)

Modification?

QCD probe out

Hard QCD probes. Motivation (I)

[4] Production yields theoretically calculable via:

perturbative-QCD or ...	classical-field QCD:

A

B

A

B

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 modifs. (incl. spectra, partic. composition) of high pT production in A+A with respect to p+p, p+A to learn about QCD many-body dynamics:

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

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

Initial-state effects:	Final-state effects

[Experimental handle: p,d+A]

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

[Experimental handle: A+A]

Hard scatt. in A+A (large soft backgd) studied via high pT (leading) hadron = jet tag.

Initial-state effects

Final-state effects

High pT spectra @ RHIC

π0 spectra as of QM'01

2 centralities

4 orders of magnitude

pTmax = 4 GeV/c

High pT spectra @ RHIC

π0 spectra as of QM'02

10 centralities

7 orders of magnitude

pTmax = 12 GeV/c

High pT spectra @ RHIC

π0 spectra as of QM'04

10 centralities

8 orders of magnitude

pTmax = 15 GeV/c !

High pT spectra @ RHIC

Impressive amount of

high quality high pT data !

π0 spectra as of QM'04

10 centralities

8 orders of magnitude

pTmax = 15 GeV/c !

High pT spectra in Au+Au @ 200 GeV

π0

Λ,Λbar

e± p,pbar

High pT p+p @ 200 GeV: “baseline” data

p+p  p0 X	p+p  h± X (Non Singly Diffractive)

Good theoretical (NLO pQCD) description

STAR Collab.

PRL91, 172302

nucl-ex/0305015

KKP FF

Kretzer FF

(PDF: CTEQ6M)

PHENIX Collab.

PRL91, 241803

hep-ex/0304038

Well calibrated (experimentally &

theoretically) p+p references at hand !

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 collision scaling central Au+Au collisions

D.d'E. nucl-ex/0309015

Nuclear modification factor (p0)

Discovery of

high pT suppression

(one of most significant

results @ RHIC so far)

PHENIX Collab.

PRL 91, 072303 (2003)

nucl-ex/0304022

Ncoll scaling

(“hard” production)

x4-5 suppression

Npart scaling

(“soft” production)

Nuclear modification factor: √sNN 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: Pb+Pb (√sNN = 17.3 GeV), a+a (√sNN = 31 GeV): Cronin enhancement.

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

Ncoll scaling

(“hard” production)

Npart scaling

(“soft” production)

RAA ~ 0.4

RAA (p0) compilation in nucleus-nucleus collisions:

RAA ~ 2.0

RAA ~ 1.5

RAA~0.2

High pT @ CERN-SPS: “Cronin” or “quenching” ?

Small suppression

Both figures in the same paper:

WA98, EPJ C 23, 225 (2002)

However ... how does one reconcile Rcp ~ 0.8 with RAA >> 1. for π0 at SPS ?!

huge enhancement !

Pb+Pb (0-10%) @ √sNN = 17.3 GeV

High pT @ CERN-SPS: “Cronin” or “quenching” ?

Small suppression

Huge enhancement !

Both figures in the same paper:

WA98, EPJ C 23, 225 (2002)

However ... how does one reconcile Rcp ~ 0.8 with RAA >> 1. for π0 at SPS ?!

Better p+p → π0 ref. @ √sNN = 17.3 GeV

[details in D.d'E. poster S26].

No “Cronin” effect ! (but Ncoll scaling)

Look for onset of suppression at RHIC Au+Au,p+p @ √sNN≈ 20 GeV ?

New RAA

Pb+Pb (0-10%) @ √sNN = 17.3 GeV

High pT suppression: centrality dependence (I)

peripheral	central

Smooth evolution of suppression w.r.t. Ncoll scaling

(in agreement with pQCD+parton energy loss expectations):

pQCD+partonic ΔEloss

(1+1D expansion)

PHENIX Collab.

to appear in PRC

nucl-ex/0308006

X.N.Wang,

nucl-th/0307036

back to RHIC energies ....

pQCD expectation

(w/o energy loss)

CGC prediction

RAANpart

Npart

High pT suppression: centrality dependence (II)

Approx. Npart scaling (in accord with CGC predictions too):

D.Kharzeev, E.Levin, L.McLerran

PLB 561, 93 (2003)

PHENIX Collab.

to appear in PRC

nucl-ex/0308006

PHOBOS Collab.

PLB 578, 297 (2003)

nucl-ex/0302015

Npart

High pT production per

participant pair ~constant

in wide range of centralities

pQCD expectation

(w/o energy loss)

CGC prediction

RAANpart

Npart

High pT suppression: centrality dependence (II)

Approx. Npart scaling (in accord with CGC predictions too):

D.Kharzeev, E.Levin, L.McLerran

PLB 561, 93 (2003)

PHENIX Collab.

to appear in PRC

nucl-ex/0308006

PHOBOS Collab.

PLB 578, 297 (2003)

nucl-ex/0302015

Npart

High pT production per

participant pair ~constant

in wide range of centralities

Rcp RAA(0-10%)

pT (GeV/c)	pT(GeV/c)

BRAHMS

PRL91: 072305(2003)

High pT suppression: (pseudo)rapidity dependence

Similar high pT suppression at η = 0 and η = 2.2

"The quenching medium extends also in the longitudinal direction.”

Additional initial-state depletion at work too in this kinematic range	(given the new d+Au results at forward rapidities).

High pT suppression: particle dependence (I)

Inclusive charged hadrons suppressed a factor ~ 4 – 5 at pT= 5 GeV/c

~15% overall normalization

errors not included

High pT suppression: particle dependence (I)

Inclusive charged hadrons suppressed a factor ~ 4 – 5 at pT= 5 GeV/c

... but less than π0 in the range pT = 2 – 5 GeV/c.

Universal (PID-wise) suppression above pT = 5 GeV/c

~15% overall normalization

errors not included

High pT suppression: particle dependence (II)

Rc p (ratio central/peripheral) at intermediate pT = 2 – 4 GeV/c:

1. Baryons: p, p, Λ, Λ NOT suppressed in central Au+Au.

2. Mesons:	p0, k0s, η equally suppressed.

Particle composition inconsistent with known fragmentation functions.

Different production mechanism for baryons and mesons in the intermediate pT range (e.g. fragmentation vs. q recombination in dense partonic medium).

STAR Collab.

subm. to PRL,

nucl-ex/0306007

PHENIX Collab.

PRL91:172301(2003)

_ _

High pT suppression: particle dependence (III)

Central colls.: p/π ~ 0.8 (at pT = 2 - 4 GeV/c) at variance with perturbative production mechanisms (favour lightest meson).

Periph. colls.: p/π ~ 0.2 as in p+p,p (ISR,FNAL) & in e+e- jet fragmentation

Charged hadron and π0 equally suppressed above pT ~ 5 GeV/c:

h / π ~ 1.6 as in p+p (perturbative ratio).

Since h± = π± + p(pbar) + K±  baryon enhancement limited to pT < 4.5 GeV/c

PHENIX Collab.

PRL91:172301(2003)

nucl-ex/0305036

Unsuppressed (?) hard heavy-quark production

Indirect measurement via semileptonic open-charm decays: D e± X .

Within uncertainties, single electron Au+Au central spectra and x-section(*) consistent with Ncoll scaled p+p charm production:

Strong(*) medium effects on heavy flavor production precluded so far.

(*) at least as strong as for light-quark mesons.

(*) Charm production is intrinsically hard:

Ncoll scaling expected down to low pT

Possible reduction (1σ) at high pT ?

factor ~2 less suppression expected

for D than for p (RAA=0.2) in models of

medium-induced energy loss

Wait for results from hi-stat. Run-4.

[ Sean Kelly QM'04 ]

Unsuppressed hard colorless production

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

Probes insensitive to colored final-state do show collision scaling at high pT:

pQCD incoherent 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 ”

High pT in d+Au (“control” experiment)

hot & dense medium	cold medium

(initial+final-state effects)	(initial- state effects only)

A+Au collision

p,d+Au collision

d+Au nuclear modification factor (η=0)

RdAu > 1 in min. bias, central d+Au and p+Au (neutron-tagged) colls.

High pT d+Au unquenched: reminiscent of p+A “Cronin enhancement” (initial-state soft & semihard scattering).

No Au gluon saturation effects in kinematic region probed (η = 0).

d+Au nuclear modification factor (η=0)

Cronin effect disappears

(Rcp≈1) above pT≈ 8 GeV/c

RdAu > 1 in min. bias, central d+Au and p+Au (neutron-tagged) colls.

High pT d+Au unquenched: reminiscent of p+A “Cronin enhancement” (initial-state soft & semihard scattering).

No Au gluon saturation effects in kinematic region probed (η = 0).

RAA vs. RdA (η = 0) : centrality dependence

PERIPHERAL Au+Au & d+Au

preliminary

RAA vs. RdA (η = 0) : centrality dependence

MID-PERIPHERAL Au+Au & d+Au

preliminary

RAA vs. RdA (η = 0) : centrality dependence

MID-CENTRAL Au+Au & d+Au

preliminary

RAA vs. RdA (η = 0) : centrality dependence

CENTRAL Au+Au & d+Au

preliminary

RAA vs. RdA (η = 0) : centrality dependence

Opposite centrality dependence of d+Au nuclear enhancement compared to Au+Au nuclear suppression.

(Model-independent) conclusion: Au+Au suppression at y = 0 not due to a “cold” (initial-state) nuclear matter effect: gluon saturation effects not relevant, final-state (QGP) interpretation favoured.

preliminary

Nuclear attenuation factor in e+A

Cronin-like pT broadening observed in nuclear DIS too.

Nuclear attenuation ratio is free from modified nPDF effects:

HERMES Collab.

PLB577 (2003)37

plab=27.6 GeV e-+A

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 (~20%) 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.

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)

d+Au nuclear modification factor (η = 3.2)

Significant suppression (factor ~2-3) of moderately high pT hadro- production at η = 3.2 (small x2 in Au).

Qualitative agreement with gluon saturation / strong shadowing effects.

BRAHMS preliminary

η = 0. (x ≈ 10-2)

η = 3.2 (x ≈ 5∙10-4)

xT = pT /√s (e-y+ey)

for pT=2 GeV/c:

d+Au nuclear modification factor (other results @ η≠0)

Increasing suppression with rapidity: RdAu(η=1)≈1, RdAu(η≈2)≈0.7, ...

preliminary

preliminary

<η> = -1.7

<η> = 1.8

why so much excitement ...

Take the predictions of a standard “leading twist” approach ...

NLO DGLAP global analysis of nuclear PDFs (fit to ~450 experimental points from e+A, p+A Drell-Yann data):

Maximum gluon shadowing at x~10-4 (indirectly) constrained by all available DIS data on nuclear targets is ~0.8

IF indeed RG(x=10-4) ≈ 0.2-0.4 (as suggested by BRAHMS), this could be an evidence of breakdown of QCD factorization at high pT (due to high twist effects at small-x).

D. de Florian & R.Sassot

hep-ph/0311227

Q2=10 GeV2

RG(x)

but (1) ... soft production is also suppressed in d+Au !

Particle multiplicities (low pT) in d+Au well below expectations from Npart tot scaling compared to p+p at forward rapidities (d fragmentation) !

Well known from p+A at lower sqrt(s). How this affect high pT production?

Bottom line: Be careful with blind application of “usual” scaling laws for particle production at forward rapidities in asymmetric systems !

p+p

d+Au

but (2) ... valence q (not g) dominate BRAHMS data

h+ > h- : deuteron valence quarks (high x1) dominate over “wee” gluons from Au (small x2).

(Personal) Conclusion: It's premature to claim RGluon(x=10-4) ≈ 0.2-0.4

It's premature to claim CGC at RHIC.

BRAHMS preliminary

What hard scattering data at RHIC
tell us(*) about the properties of the
underlying QCD matter ...

Summary of possible physical scenarios:

1. Dense final-state partonic medium: Parton energy loss +

quark recombination.

2. Dense initial-state partonic medium: Gluon saturation.

3. Dense final-state hadronic medium: hadronic energy loss.

(*) via confronting data to theory

Final-state “QGP” effects vs. data (I)

Dense 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, 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 εcrit QCD in thermalized syst.

Final-state “QGP” effects vs. data (II)

Hwa & Yang

PRC 67, 034902

Greco, Ko, Levai

PRL 90, 202302

Fries, Mueller, Nonaka, Bass

PRL 90, 202303

Quark recombination (coalescence) mechanisms provide a simple explanation of anomalous chemistry at intermediate pT's (2-5 GeV/c):

By quark momenta addition, recombination

dominates for pT ~ 1- 4 GeV/c:

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

Fragmentation dominates for pT > 5 GeV/c:

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

High parton densities in a thermal medium

are required.

However... is recomb. consistent with

(p+p-like) Au+Au dN/dφ near-side widths ?

Final-state effects in a dense hadronic medium ?

Assumption: fast parton hadronization time implies rescattering of “pre-hadrons” (colorless but not fully formed) inside expanding (hadronic) fireball.

Nuclear modification factor: [expanding system with εinit≈1 GeV/fm3 (*) ]

State-of-the-art hadronic models (HSD, UrQMD) produce suppression but ...

Cassing, Gallmeister, Bratkovskaya,

Greiner, Stoecker, nucl-th/0312049

(*) NB: Such a dense hadronic medium

should have gone first through an

(even) denser partonic phase

of course ...

Summary

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

 Observation 1: Light-flavor (u,d,s) spectra suppressed by a factor 4-5.

(possible suppression already at √s ≈ 20 GeV).

 Observation 2: Intermediate pT light-flavor composition inconsistent	with known fragmentation functions in free space.

 Observation 3: Heavy-flavor (c) spectra unsuppressed (?).

 Observation 4: Direct photon spectra unsuppressed.

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

 Observation 5: Spectra enhanced by a factor ~1.3

 “Explanation” (1,2 via 4,5): pQCD hard scattering + final-state parton	energy loss + parton recombination:

 Dense thermal QCD medium.

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

 Observation 6: Spectra suppressed by a factor ~2-3.

 “Explanation” (6): possible evidence of high twist effects at small-x.

backup slides ...

High pT @ CERN-SPS: “Cronin” or “quenching” ?

High pT @ SPS: “Cronin” or “quenching” ?

David d'Enterria

2004-01-14T06:23:12