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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.

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parton energy loss

I° Convegno Italiano sulla Fisica di ALICE

Parton Energy Loss

Andrea Dainese

Padova – Università e INFN

ALICE - Catania, 11.01.2005 Andrea Dainese

layout
Layout
  • The discovery of jet quenching in Au-Au coll. at RHIC
  • High-pt particle production in nucleus-nucleus collisions, according to perturbative QCD

PARTON ENERGY LOSS

  • 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

ALICE - Catania, 11.01.2005 Andrea Dainese

jet quenching j d bj o rken 1982

“Jet Quenching” – J.D.Bjorken 1982

ALICE - Catania, 11.01.2005 Andrea Dainese

discoveries at rhic high p t suppression

PHENIX p0 (0-10%)

Discoveries at RHIC:high-pt suppression
  • Nuclear modification factor of pt distributions:

factor 5

suppression!

ALICE - Catania, 11.01.2005 Andrea Dainese

what about jets
What about jets?

Jets via di-hadron correlations:

  • trigger: highest pt track, pt > 4 GeV
  • Df distribution: 2 GeV < pt < pttrigger
  • normalize to number of triggers

p+p  di-jet

near side

away side

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

ALICE - Catania, 11.01.2005 Andrea Dainese

discoveries at rhic no away side jet in central au au
Discoveries at RHIC:no away-side jet in central Au-Au
  • Quantified via ratio of integrals:

pedestal and flow subtracted

~1 in peripheral AA

~0 in central AA

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

ALICE - Catania, 11.01.2005 Andrea Dainese

is it final state energy loss

Final-state effect

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

ALICE - Catania, 11.01.2005 Andrea Dainese

high p t particle prod in aa collisions

h

RAB

Au

1

medium formed in

the collision

q

q

Au

q

h

q

~2-4 GeV/c

pT

kLkT

fgPb / fgp

Q2 = 5 GeV2

Cronin

enhancement

h

p

pp  h+X

LHCRHICSPS

RAA<1~1>1

q

p

h

x

High-pt particle prod. in AA collisions
  • A schematic view:
  • Ingredients:
    • pp baseline (pQCD)
    • initial-state effects:
      • PDF (anti)shadowing
      • kT broadening (Cronin)
    • final-state effects:
      • energy loss
      • in-medium hadronization

( V.Greco)

ALICE - Catania, 11.01.2005 Andrea Dainese

slide9

path length L

hard

parton

path length L

hard

parton

Parton Energy Loss

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

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

ALICE - Catania, 11.01.2005 Andrea Dainese

slide10

path length L

path length L

path length L

Calculating Parton Energy Loss

  • To analyse RHIC data, need more than mean DE
  • Quenching weights: energy loss probability distributions
  • Main limitation: calculated in E approx. uncertainties

(BDMPS)

Medium transport coefficient

gluon density and momenta

Casimir coupling factor:

4/3 for quarks

3 for gluons

Probe the medium

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].

ALICE - Catania, 11.01.2005 Andrea Dainese

application parton quenching model

pt

PYTHIA

Hadronization

pt – DE

pt

Application: Parton Quenching Model
  • Quenching weights + Glauber-model-based medium geometry + PYTHIA for parton generation and fragmentation
  • The procedure in short:
    • generate parton (q or g) with PYTHIA (or back-to-back pair)
    • calculate its L and average along the path
    • use quenching weights to get energy loss
    • quench parton and then hadronize it (independent fragm. (KKP))

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

ALICE - Catania, 11.01.2005 Andrea Dainese

model vs rhic data 1

naturally matches pt-indepence of suppression at high pt

Model vs RHIC data (1)
  • Density ( ) “tuned” to match RAA in central Au-Au at 200 GeV
  • Extrapolations
    • in centrality: according to Glauber-model collision geometry
    • in s: assuming  Ngluons/volume (s)0.6 (saturation model)

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].

ALICE - Catania, 11.01.2005 Andrea Dainese

model vs rhic data 2 centrality dependence of r aa
Model vs RHIC data (2)centrality dependence of RAA

pt > 4.5 GeV

ALICE - Catania, 11.01.2005 Andrea Dainese

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

ALICE - Catania, 11.01.2005 Andrea Dainese

model vs rhic data 4 intermediate rhic energy s 62 gev
Model vs RHIC data (4)intermediate RHIC energy s = 62 GeV

energy extrapolation works reasonably well

ALICE - Catania, 11.01.2005 Andrea Dainese

model prediction for lhc
Model prediction for LHC
  • Extrapolation to LHC according to saturation model gives:
  • Most partons are absorbed
  • Only those from the surface can escape the medium

… surface emission dominates

ALICE - Catania, 11.01.2005 Andrea Dainese

slide17

Yu.Dokshitzer

Q

Heavy Quarks

Lower E loss for ?

  • In vacuum, gluon radiation suppressed at q < mQ/EQ

 “dead cone” effect

  • Dead cone implies lower energy loss(Dokshitzer-Kharzeev, 2001):
    • Energy distribution w dI/dw of radiated gluons suppressed by angle-dependent factor
    • suppress high-energy tail

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

ALICE - Catania, 11.01.2005 Andrea Dainese

slide18

c  e

PHENIX

Charm Energy Loss at RHIC

  • Detailed 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)

RHIC

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.

ALICE - Catania, 11.01.2005 Andrea Dainese

heavy quark e loss at lhc

LHC

LHC

Heavy Quark E Loss at LHC
  • Exploit 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

ALICE - Catania, 11.01.2005 Andrea Dainese

open points the opacity problem
Open points: the opacity problem
  • Can we really probe the medium?
  • Need to relate extracted to an energy density e
  • 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

(Baier)

R.Baier, Nucl. Phys. A715 (2003) 209.

* K.J.Eskola, H.Honkanen, C.A.Salgado and U.A.Wiedemann, Nucl.Phys.Ain press, hep-ph/0406319.

ALICE - Catania, 11.01.2005 Andrea Dainese

open points limited sensitivity of r aa

?

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

ALICE - Catania, 11.01.2005 Andrea Dainese

summary
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

ALICE - Catania, 11.01.2005 Andrea Dainese

extra slides
EXTRA SLIDES

ALICE - Catania, 11.01.2005 Andrea Dainese

jet quenching at the sps s 17 gev

path length L

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

Jet quenching at the SPS (s  17 GeV)?
  • SPS: only p0 RAA using parameterized pp reference
  • Need more systematics: other experiments, more particles
  • WA97/NA57: can provide RAA (Pb-Pb/p-Be) for K0, L and charged hadrons
  • First step: RCP

RAA ~ 1:

Cronin enhancement

compensated by E loss ?

ALICE - Catania, 11.01.2005 Andrea Dainese

r cp at the sps from na57

NA57 preliminary

RCP at the SPS from NA57

RCP = central/peripheral normalized to 1 NN collision

K0 and L similar trend at SPS

as at RHIC

hint for recombination?

1 ~ RCP(K0) > RCP(p0) but…

no Cronin enhancement!

ALICE - Catania, 11.01.2005 Andrea Dainese

r cp at the sps from na571

NA57 preliminary

RCP at the SPS from NA57
  • NA57 K0 data compared to
  • pQCD calculation by X.N.Wang:
  • enhancement, expected if no energy loss, not observed
  • data vs theory suggest significant quenching at the SPS

ALICE - Catania, 11.01.2005 Andrea Dainese