- 145 Views
- Uploaded on

Download Presentation
## Parton Energy Loss

**An Image/Link below is provided (as is) to download presentation**

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript

### Parton Energy Loss

I° Convegno Italiano sulla Fisica di ALICE

Andrea Dainese

Padova – Università e INFN

ALICE - Catania, 11.01.2005 Andrea Dainese

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

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?

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

- 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?

- … 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

RAB

Au

1

medium formed in

the collision

q

q

Au

q

h

q

~2-4 GeV/c

pT

kLkT

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

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

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

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

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 RAA

pt > 4.5 GeV

ALICE - Catania, 11.01.2005 Andrea Dainese

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

energy extrapolation works reasonably well

ALICE - Catania, 11.01.2005 Andrea Dainese

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

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

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

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

- 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 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

- 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

ALICE - Catania, 11.01.2005 Andrea Dainese

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

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

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

Download Presentation

Connecting to Server..