parton energy loss
DESCRIPTION
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 PresentationTRANSCRIPT
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
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
ALICE - Catania, 11.01.2005 Andrea Dainese 3
“Jet Quenching” – J.D.Bjorken 1982
…
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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!
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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
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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
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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
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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
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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
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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
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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))
q̂
qL ˆ ,
),ˆ,;( LqCEP RPYTHIA
tR pC ,
Hadroniz a tion
pt
pt – pt
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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
q̂
naturally matches pt-indepence of suppression at high pt
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Model vs RHIC data (2)centrality dependence of RAA
pt > 4.5 GeV
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Model vs RHIC data (3)disappearence of the away-side jet
awayaway
AA ppAAI
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Model vs RHIC data (4)intermediate RHIC energy s = 62 GeV
/fmGeV 72/ˆˆ 2GeV 200GeV 62 qq
energy extrapolation works reasonably well
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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
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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
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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
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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
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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
q̂
)(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.
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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
…
q̂
?
large RAA indep. of q̂ q̂
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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
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EXTRA SLIDES
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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
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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
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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