parton energy loss

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ALICE - Catania, 11.01.2005 Andrea Dainese 1 Parton Energy Loss Parton Energy Loss Andrea Dainese Andrea Dainese Padova – Universit Padova – Universit à à e INFN e INFN I I ° ° Convegno Italiano sulla Fisica di ALICE Convegno Italiano sulla Fisica di ALICE

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I ° Convegno Italiano sulla Fisica di ALICE. Parton Energy Loss. Andrea Dainese Padova – Universit à e INFN. Layout. The discovery of jet quenching in Au-Au coll. at RHIC High- p t particle production in nucleus-nucleus collisions, according to perturbative QCD. PARTON ENERGY LOSS. - PowerPoint PPT Presentation

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Page 1: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 1

Parton Energy LossParton Energy Loss

Andrea DaineseAndrea Dainese

Padova – UniversitPadova – Universitàà e INFN e INFN

II°° Convegno Italiano sulla Fisica di ALICE Convegno Italiano sulla Fisica di ALICE

Page 2: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 2

LayoutThe discovery of jet quenching in Au-Au coll. at RHIC

High-pt particle production in nucleus-nucleus collisions, according to perturbative QCD

Calculating parton energy loss (BDMPS framework)

Application: the Parton Quenching Model vs. RHIC data

Energy loss for heavy quarks

Limitations of the leading-particle appraoch

Summary

PARTON ENERGY LOSS

Page 3: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 3

“Jet Quenching” – J.D.Bjorken 1982

Page 4: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 4

Discoveries at RHIC:high-pt suppression

Nuclear modification factor of pt distributions:

PHENIX 0 (0-10%)

dd/d

dd/d1)(

2

2

tpp

tCAA

CcolltAA pN

pN

NpR

factor 5suppression!

Page 5: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 5

What about jets?

Jets via di-hadron correlations:

trigger: highest pt track, pt > 4 GeV

distribution: 2 GeV < pt < pttrigger

normalize to number of triggers

p+p di-jet

STAR Coll., Phys.Rev.Lett. 90 (2003) 082302

near side away side

Page 6: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 6

Discoveries at RHIC:no away-side jet in central Au-Au

Quantified via ratio of integrals:

pedestal and flow subtracted

STAR Coll., Phys.Rev.Lett. 90 (2003) 082302

awayaway

AA ppAAI ~1 in peripheral AA

~0 in central AA

Page 7: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 7

Is it final-state energy loss?

… or saturation of the parton densities in the initial nuclei?

Control experiment, without medium: d-Au

no high-pt suppression the away-side jet is there

Final-state effect

Page 8: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 8

High-pt particle prod. in AA collisionsA schematic view:

Ingredients:pp baseline (pQCD)initial-state effects:PDF (anti)shadowingkT broadening (Cronin)

final-state effects:energy loss in-medium hadronization ( V.Greco)

medium formed in the collision

Au

Au q

qh

hq

p

p q

q h

hpp h+X

x

fgPb / fg

p

LHC RHIC SPSRAA <1 ~1 >1

Q2 = 5 GeV2

pT

RA

B

1

~2-4 GeV/c

kL kT

Cronin enhancement

Page 9: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 9

Parton Energy LossPartons travel ~4 fm in the high colour-density medium

Bjorken (`82): energy loss due to elastic scattering

Successive calculations (`92 ): a QCD mechanism dominates, medium-inducedmedium-induced gluon radiationgluon radiation

Coherent wave-function gluon acummulates kT due to multiple inelastic scatterings in the medium; it decoheres and is radiated

hardparton

path length L

Bjorken, Gyulassy, Pluemer, Wang, Baier, Dokshitzer, Mueller, Pegne, Schiff, Levai, Vitev, Zhakarov, Wang, Salgado, Wiedemann,…

hardparton

path length L

Page 10: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 10

Calculating Parton Energy Losspath length L

2 ˆ d

d d LqC

IE Rs

Casimir coupling factor:4/3 for quarks3 for gluons

Medium transport coefficient gluon density and momenta

R.Baier, Yu.L.Dokshitzer, A.H.Mueller, S.Peigne' and D.Schiff, (BDMPS), Nucl. Phys. B483 (1997) 291.C.A.Salgado and U.A.Wiedemann, Phys. Rev. D68 (2003) 014008 [hep-ph/0302184].

(BDMPS)

Probe the medium

To analyse RHIC data, need more than mean Quenching weights: energy loss probability distributions

Main limitation: calculated in approx. uncertainties

),ˆ,;( LqCEP R

path length Lpath length L

Page 11: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 11

Application: Parton Quenching Model

A.D., C.Loizides and G.Paic, Eur.Phys.J. C in press, hep-ph/0406201.

Quenching weights + Glauber-model-based medium geometry + PYTHIA for parton generation and fragmentation

The procedure in short:1) generate parton (q or g) with PYTHIA (or back-to-back pair)

2) calculate its L and average along the path

3) use quenching weights to get energy loss

4) quench parton and then hadronize it (independent fragm. (KKP))

qL ˆ ,

),ˆ,;( LqCEP RPYTHIA

tR pC ,

Hadroniz a tion

pt

pt – pt

Page 12: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 12

Model vs RHIC data (1)Density ( ) “tuned” to match RAA in central Au-Au at 200 GeV

Extrapolationsin centrality: according to Glauber-model collision geometry

in s: assuming Ngluons/volume (s)0.6 (saturation model)q̂

Caveat:

No initial-state effects and in-medium hadronization: results given for pt > 5 GeV

Band represents theoretical uncertainty

Saturation model: K.J.Eskola, K.Kajantie, P.V.Ruuskanen and K.Tuominen, Nucl. Phys. B570 (2000) 379 [hep-ph/9909456].

/fmGeV 15ˆ 2q

naturally matches pt-indepence of suppression at high pt

Page 13: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 13

Model vs RHIC data (2)centrality dependence of RAA

pt > 4.5 GeV

Page 14: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 14

Model vs RHIC data (3)disappearence of the away-side jet

awayaway

AA ppAAI

Page 15: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 15

Model vs RHIC data (4)intermediate RHIC energy s = 62 GeV

/fmGeV 72/ˆˆ 2GeV 200GeV 62 qq

energy extrapolation works reasonably well

Page 16: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 16

Model prediction for LHCExtrapolation to LHC according to saturation model gives:

Most partons are absorbed

Only those from the surface can escape the medium

/fmGeV 100ˆ 7ˆ 2GeV 200TeV 5.5 qq

h

GeV/c 5hadrontp

… surface emission dominates

Page 17: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 17

Lower E loss for ?In vacuum, gluon radiation suppressed at < mQ/EQ

“dead cone” effect

Dead cone implies lower energy loss (Dokshitzer-Kharzeev, 2001):Energy distribution dI/d of radiated gluons suppressed by angle-dependent factor

suppress high-energy tail

Q

Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett. B519 (2001) 199 [hep-ph/0106202].

1

1d

d

d

d2

2

2

Q

Q

LIGHTHEAVY E

mII

Yu.Dokshitzer

Page 18: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 18

Charm Energy Loss at RHICDetailed calculation confirms this qualitative feature, although effect is small and uncertainties significant

Very small mass effect for charm at RHIC

Data not yet conclusive (better stats expected within 2005)

N.Armesto, C.A.Salgado and U.A.Wiedemann, Phys. Rev. D69 (2004) 114003 [hep-ph/0312106].N.Armesto, A.D., C.A.Salgado and U.A.Wiedemann, in preparation.

c ePHENIX

RHIC

Page 19: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 19

Heavy Quark E Loss at LHCExploit abundant production of charm and beauty quarks at LHC & study the mass dependence of E loss by measuring RAA for D and B and for their decay leptons

LHCLHC

Page 20: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 20

Open points: the opacity problem

Can we really probe the medium?

Need to relate extracted to an energy density QCD estimate for ideal QGP:

A recent analysis* of RHIC data, similar to that presented, extracts energy density 5 larger than that estimated from produced transverse energy dET/dy (Bjorken estimate)

Opacity problem: the interaction of the hard parton with the medium is much stronger than expected

)(2)(ˆ 4/3 q (Baier)

R.Baier, Nucl. Phys. A715 (2003) 209.* K.J.Eskola, H.Honkanen, C.A.Salgado and U.A.Wiedemann, Nucl.Phys.A in press, hep-ph/0406319.

Page 21: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 21

Open points: limited sensitivity of RAA

Strong suppression requires very large density

Surface emission scenario

RAA determined by geometry rather than by density itself

Limited sensitivity to

Need more differential observables:

massive partons

RAA vs reaction plane

study of jet shapes

?

large RAA indep. of q̂ q̂

Page 22: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 22

Summary

One of the most exciting discoveries at RHIC !

We are dealing with energy loss in an extremely opaque medium, but …

theoretically, we are just starting to learn how to probe the medium; still large uncertainties, but improving

experimentally, need more differential studies (vs reaction plane, vs parton species and quark mass)

need to go beyond pt spectra studies, towards ‘real’ jets

The LHC will be a `hard probes machine’ and quenching studies, including heavy quarks and jets, will play a central role

Page 23: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 23

EXTRA SLIDES

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Page 25: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 25

Jet quenching at the SPS (s 17 GeV)?

SPS: only 0 RAA using parameterized pp reference

Need more systematics: other experiments, more particles

WA97/NA57: can provide RAA (Pb-Pb/p-Be) for K0, and charged hadrons

First step: RCP

Re-analysis: D.d’Enterria, nucl-ex/0403055

RAA ~ 1: Cronin enhancementcompensated by E loss ?

path length L

Page 26: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 26

RCP at the SPS from NA57RCP = central/peripheral normalized to 1 NN collision

1 ~ RCP(K0) > RCP(0) but…no Cronin enhancement!

K0 and similar trend at SPSas at RHIChint for recombination?

NA57 preliminary

Page 27: Parton Energy Loss

ALICE - Catania, 11.01.2005 Andrea Dainese 27

RCP at the SPS from NA57

NA57 preliminary

NA57 K0 data compared topQCD calculation by X.N.Wang:

enhancement, expected if no energy loss, not observed

data vs theory suggest significant quenching at the SPS