Suppression of light- n’ heavy-flavored hadrons from RHIC to LHC

CERN Heavy Ion Forum, April 13th, 2005 Andrea Dainese 1

Suppression of Suppression of light- n’ heavy-flavored hadrons light- n’ heavy-flavored hadrons

from RHIC to LHCfrom RHIC to LHC

Andrea DaineseAndrea Dainese

Padova – University and INFNPadova – University and INFN

based on work in collaboration with:

N.Armesto, C.Loizides, G.Paic, C.Salgado, U.Wiedemann

CERN Heavy Ion ForumCERN Heavy Ion Forum


LayoutThe discovery of “jet quenching” at RHIC

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

Calculating parton energy loss (BDMPS-Z framework)

Light-flv hadron suppression in the Parton Quenching Model

versus RHIC data

predictions for LHC

Energy loss for heavy quarks

PARTON ENERGY LOSS


Discovery 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!

compilation by D.d’Enterria


What about jets?

Jets via di-hadron correlations:

trigger: highest-highest-ppTT track track, (4--6 GeV)

distribution: 2 GeV < pT < pTtrigger

pp 2 back-to-back jets

STAR Coll., PRL 90 (2003) 082302.slide courtesy of P.Jacobs

near side away side


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

Quantified via ratio of integrals:

awayaway

AA ppAAI ~1 in peripheral AA

~0 in central AA

STAR Coll., PRL 90 (2003) 082302.


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 effectPHENIX Coll., PRL 91 (2003) 072303.STAR Coll., PRL 91 (2003) 072304.


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 (coalescence)

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


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

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

…

Bjorken, FERMILAB-Pub-82/59-THY (1982).


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

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

Gyulassy, Pluemer, Wang, Baier, Dokshitzer, Mueller, Peigne’, Schiff, Levai, Vitev, Zhakarov, Salgado, Wiedemann, …

hardparton

path length L

kT


Calculating Parton Energy Loss

cc

ccRsC

I

for )/(

for /

d

d2

Baier, Dokshitzer, Mueller, Peigne‘, Schiff, NPB 483 (1997) 291.Zakharov, JTEPL 63 (1996) 952.Salgado, Wiedemann, PRD 68(2003) 014008.

BDMPS-Z formalismpath length L

kT

Radiated-gluon energy distrib.:

2

ˆTk

q transport coefficient

2/ˆ 2Lqc LR c

sets the scale of the radiated energyrelated to constraint kT < , controls shape at << c

Casimir coupling factor: 4/3 for q, 3 for gRC

STATICMEDIUM

(BDMPS case)


Calculating Parton Energy Loss

2

0 ˆ

d

d d LqCC

IE RscRs

c

Probe the medium

qE ˆ gluons volume-density and interaction cross section

Finite parton energy (qualitatively)

LqECECI

E RscRs

E ˆ

d

d d

0

2/1ˆˆ qqLL 2

If E < c (e.g. small pT parton with large L):

dependence on parton energy : smaller sensitivity to density


Medium ExpansionThe density of scattering centers is time-dependent:

Dynamical scaling law: same spectrum obtained for equivalent static trasport coefficient

Calculations for a static medium apply to expanding systems

0

0ˆ)(ˆ qq

)(ˆ)(2

ˆ0

002

qd

Lq

L

Salgado and Wiedemann, PRL 89 (2002) 092303

= 1.5, 1.0, 0.5, 0


Quenching Weights

BDMS, JHEP 0109 (2001) 033.Salgado and Wiedemann, PRD 68 (2003) 014008.

Goal: compute high-pT suppression in BDMPS-Z framework

QW = energy loss probability distributions

Calculated from d/d , in approximation

),ˆ,;( LqCEP R

),,ˆ,;( ELqCEP R + uncertainties

]3/1[ s


Application: Parton Quenching Model

Dainese, Loizides, Paic, EPJC 38 (2005) 461.

QW + Glauber-based medium geometry and density profile + 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.)

q̂

qL ˆ ,

),ˆ,;( LqCEP RPYTHIA

TR pC ,

Fragm

entat ion

pT

pT – pT

dN/dpT

)]()(ˆ[ sTTsq BA


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

Note:

No initial-state effects nor “reco”: results given for pT > 5 GeV

Band represents theoretical uncertainty

/fmGeV 14ˆ 2q

q̂

matches pT-indepence of suppression at high pT

Eskola, Honkanen, Salgado, Wiedemann, NPA 747 (2005) 511


Model vs RHIC dataCentrality dependence of RAA

pT > 4.5 GeV

Centrality evolution according to Glauber-model collision geometry );();(ˆ bsTTkbsq BA

Dainese, Loizides, Paic, EPJC 38 (2005) 461.


Model vs RHIC dataDisappearence of the away-side jet

near side away side

STAR Coll., PRL 90 (2003) 082302.STAR Coll., nucl-ex/0501016.

awayaway

AA ppAAI


Extrapolation in c.m.s. energyIntermediate RHIC energy s = 62 GeV

/fmGeV 72/ˆˆ 2GeV 200GeV 62 qq

energy extrapolation works reasonably well

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

First test:

q̂

EKRT: Eskola, Kajantie, Ruuskanen, Tuominen, NPB 570 (2000) 379.PHENIX Coll., JPG 31 (2005) S473.


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

/fmGeV 100ˆ 7ˆ 2GeV 200TeV 5.5 qq

2

GeV 200TeV 5.5 AA

AA

RR

indep. of pT

h

?!?

Dainese, Loizides, Paic, EPJC 38 (2005) 461.Vitev and Gyulassy, PRL 89 (2002) 252301.


Why RAA is flat Surface effectMost partons from inner part are totally absorbed

Energy loss is “saturated”: many partons radiate all their energy before getting outLong path lengths exploited only by high energy partons effectively, RAA doesn’t increase at high pT

Exercise: RAA increases with pT

Prod. points in (x,y) for partons giving hadrons with pT > 5 GeV:

increasing s

10.5 with , EE

LL

Loizides, PhD Thesis, nucl-ex/0501017


Open points (1): 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

Eskola, Honkanen, Salgado, Wiedemann, NPA 747 (2005) 511.

?


Further handle on L-dependence*

First RAA vs appearing …(see talk by d’Enterria)

= 0

= /2PHENIX 0 prel. vs PQM

Data show stronger dep. than PQM model

PQMPQM

STAR Coll., PRL 93 (2004) 252301

Note: model is not E L2, rather E L * Beware: effect of collective flow on RAA vs !?!


Open points (2): 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)

Baier, NPA 715 (2003) 209.* Eskola, Honkanen, Salgado, Wiedemann, NPA 747 (2005) 511.



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

“dead cone” effect

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

suppress high- tail

Q

Dokshitzer, Khoze, Troyan, JPG 17 (1991) 1602.Dokshitzer and Kharzeev, PLB 519 (2001) 199.

1

1d

d

d

d2

2

2

Q

Q

LIGHTHEAVY E

mII

Dokshitzer


Lower E loss for heavy quarks ?Detailed massive calculation shows that:

Armesto, Salgado, Wiedemann, PRD 69 (2004) 114003.

Medium-induced gluon radiation fills the dead cone

Still, kT-integrated radiation is suppressed, but only for quite large m/E (i.e. large m, low pT)*

R = 105

*Warning: large uncertainties

kT

)0.1/( / 2 cdddI

)( 22Tk

m/E = 0.1

massive

massless

dead cone


Charm Energy Loss at RHICCompute RAA for c, D, e (from D)

Baseline: PYTHIA, with EKS98 shadowing, tuned to match pT-shape of D cross section measured in d-Au by STAR

c-quark E loss as for light-flv hadrons, but using (mc/Ec)-dependent QW

extracted from 0,h RAA (central)

Thermalize charms that lose all energy dN/dmT mT exp(-mT/T), T = 300 MeV

Use a range in to visualize limited sensitivity of RAA to itself

Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027.EKS: Eskola, Kolhinen, Salgado, EPJC 9 (1999) 61.

q̂

q̂q̂

baseline dN/dpT

baseline RAA


Charm RAA at RHIC

effect of the mass

thermalized component

/fmGeV 144ˆ 2q

Small effect of mass for charm (~50% for D, ~30% for e) at low pT [large uncertainties!]Basically no effect in “safe” pT-region

Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027.


Charm Electron RAA from PHENIX

RAA down to 0.3 at pT ~ 4 GeV,where effect of c masspredicted ~20-30%

Reasonable agreementof prediction with data,

however …

RAA

pT (GeV)


Charm+Beauty e RAA from PHENIX

PHENIXFONLL pQCDCacciari, Nason, Vogt, hep-ph/0502203

PHENIX e spectrum may be ~50% charm + ~50% beauty for 3 < pT < 8 GeV

For pT < 3 GeV, computed RAA depends on T used for thermalized charm


The heavy-to-light ratio:What enters the game:

1) (c) quark vs gluon (Casimir factor) RD/h > 1

2) harder charm pT distribution: RD/h up

3) harder charm fragm. RD/h down

4) mass effects RD/h up

)(

)()(/

ThAA

TDAA

ThD pR

pRpR

1) dominates > 12-13 GeV2) and 3) ~ compensate4) dominates below 12 GeV

Warning: significant non-pert. effects (e.g. reco) below 6-7 GeV

RHIC


Heavy Quark Energy Loss at LHCCharm (beauty) production expected to increase from RHIC to LHC by factor 10 (100)

~100 cc pairs and ~5 bb pairs per central Pb-Pb collision

Experiments will measure with good precision RAA for D and

B, and for their decay leptons (see talk by R.Turrisi)

What can we learn from a comparative quenching study of

massive and massless probes at the LHC?


Heavy Flavor RAA at LHCBaseline: PYTHIA, with EKS98 shadowing, tuned to reproduce c and b pT distributions from NLO pQCD (MNR)

E loss with (m/E)-dep. QW and

MNR: Mangano, Nason, Ridolfi, NPB 373 (1992) 295.

/fmGeV 10025ˆ 2q

Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027.


Heavy-to-light ratios at LHC: charm

(Small) mass effect limited to pT < 8-10 GeV, where shadowing and “reco” may be significantFor 10 < pT < 20 GeV, charm behaves like a m=0 quark,light-flv hadrons come mainly from gluonsRD/h enhancement probes color-charge dep. of E loss

gq EE


Heavy-to-light ratios at LHC: beauty

For 10 < pT < 20 GeV, dominant effect due to b mass

RB/h enhancement probes mass dep. of E loss

mass effect


SummaryOne 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 experimentally, already going to more differential observables

vs azimuthal angle i.e. path-length -- (not understood, flow?)

vs parton species and quark mass -- c (b) SUPPRESSED!

The LHC will be a `hard probes machine’ and quenching studies will play a central roleExample: Heavy-to-light ratios as probes of E loss…… color-charge dependence (RD/h)… parton-mass dependence (RB/h)


EXTRA SLIDES


Comparison to massive GLV

Djordjevic, Gyulassy, Wicks,hep-ph/0410372


RAAD at LHC, step by step

Documents

Suppression of light- n’ heavy-flavored hadrons from RHIC to LHC