High-p T probes of heavy-ion collisions at RHIC and LHC Marco van Leeuwen, LBNL.

High-pT probes of heavy-ion collisions at RHIC and LHC

Marco van Leeuwen, LBNL

Marco van Leeuwen, High-pT probes at RHIC and LHC 2

Introduction

Motivation:Initial production well-calibratedHard processes (high Q2) are only sensitive to short distances and times

Final state particles (partons and/or hadrons) probe the medium through interactions


Introduction

Can we check our understanding of hard processes?

For hard process, expect to scale as number of binary collision Ncoll for A+A

Yes, by showing that p+p results can be explained by pQCD

c

chbbaa

abcdba

T

hpp

z

Dcdab

td

dQxfQxfdxdxK

pdyd

d

0

/222

)(ˆ

),(),( Parton density function Matrix element Fragmentation

function

Measured in DIS e+e-pQCD


Hard processes at RHIC

• High-ptt light hadron production– Most abundant process, lots of data

– Inclusive production, di-hadron correlations, elliptic flow

– Baryon production sensitive to quark vs gluon jets

• Direct production– Non-interaction probe, test Ncoll scaling

• Heavy quark production– Main results so far from semi-leptonic decays

– Test pQCD theory for production and suppression (dead-cone effect)

Goal:- Understand production rates and suppression in A+A- Determine medium properties (density, dynamics) in heavy-ion collisions


STARSTAR

RHIC accelerator and experiments

PHENIX

Focus: rare probes , e±, Partial coverage High-granularity calorimetry and trackingForward muon detectors

STAR

Focus:global observables

Large volume TPC (2)+EM calorimetry (coarse)

Maximum energy: sNN=200 GeV for Au+Aus=500 GeV for p+p (default 200 GeV)

Recent runs2004: large statistics Au+Au (~80M events), most results in this presentation2005: large statistics Cu+Cu, analysis in progress2006: dedicated polarised p+p run, data-taking in progress


p+p jet spectrum @ s=200 GeV

4.0 , 200 RGeVs

First direct measurement of jet spectrum at RHIC

Statistics out to pT=50 GeV… more being collected

Measured spectrum agrees with NLO pQCD

Dominant uncertainty: jet energy scale

Prefer particle spectra, di-hadron correlationsfor Au+Au baseline

(backgrounds too large for jet reconstruction in Au+Au)


Light hadron production in p+p

NLO calculations: W. Vogelsang

Star, PRL 91, 172302Brahms, nucl-ex/0403005

Light hadron production at RHIC in good agreement with NLO pQCD

Caveat: gluon fragmentation not so well constrained from e+e-

PRL 91, 241803


Baryon production in p+pAlbino, Kniehl, Kramer, Nucl Phys B725, 181 hep-ph/0510173

{

Proton spectra used to be problematic (KKP FF)

New parameterisation of FF (AKK) from full flavour separated datasets (OPAL),(no SU(3) flavour symmetry assumption) shows much better agreement

also well described

FF parametrisation is an ongoing activity

Baryon production at RHIC also described by pQCD


Direct photons

Direct in p+p agree with pQCD

q + g q + PHENIX, PRL 94, 232301

ppTcoll

AuAuTAA dpdNN

dpdNR

/

/

Direct in A+A scales with Ncoll

Centrality

RAA=1 (Ncoll scaling) for incoherent superposition

of p+p collisions

Production through

q + q g +


Light hadron production in A+A

Photons and hadron production measured to well in the (expected) perturbative regime

: RAA = 1

0, h±: RAA ≈ 0.2

Light hadron production suppressed by factor 4-5 in central Au+Au

Au+Au 200 GeV, 0-5% central


Radiative energy loss in QCDCalculational frameworks:• Multiple soft scattering (BDMPS, Wiedemann, Salgado,…)

• Few hard scatterings,opacity expansion (Gyulassy, Vitev, Levai, Wang,…) • Twist expansion (Wang, Wang,…)

Plus details:Longitudinal expansion reduces E~L2 to E~LFinite energy effects may lead to E-dependent energy loss

CS

coherent

LPM Nq

dzd

dI

ldzd

dI ˆHeitlerBethe

2

ˆ q

2ˆ~ˆ~ LqLqdzd

dIddzE SCS

LPML

med

C

cf Lt

Medium properties can be characterized by a single constant

e.g. transport coefficient

‘average kT -kick per mean-free-path’

E does not depend on parton energyE L2 due to interference effects (for a static medium)

321 ˆLqR

Salgado and Wiedemann, Phys. Rev. D68, 014008

dI

/d

~1 GeV at RHIC C

Soft radiation suppressed by phase space requirement kT <

Radiative energy loss is due to moderate number (~3) of finite energy gluons (~0.1-1 GeV)

43

~~ˆ glueq


Non-perturbative dynamics at intermediate pT

Intermezzo

Enhancement depends on:- Particle type (different for , p)- Centrality

d+Au, s=200 GeV

Hadron production in d+Au enhanced compared to Ncoll scaling

‘Cronin effect’ known from fixed target at Fermilab, but mechanism unclear

Effect small compared to effects in Au+Au

p


Baryon production in Au+Au

Intermediate pT (2-4 GeV) p/ much larger in Au+Au than p+p (vacuum fragmentation)

At pt=6 GeV: p/ similar in p+p, d+Au and central Au+Au

Non-perturbative effects large at intermediate pTNote: p/ ratio sensitive to gluon/quark ratio. Probes differences in coupling to medium

This presentation: focus at highest pT

Au+Au, 0-5% central, sNN=200 GeV


Hadron suppression: sNN=200 GeV Au+Au

Different calculations lead to similar medium densitiesdNg/dy=1100, , approx. 30 times nuclear densityfmGeVq 2155ˆ

Reasonable agreement between data and calculations for pT up to 20 GeV

High statistics year-4 data


Centrality dependence

Dainese, Loizides and Paic, Eur.Phys.J. C38, 461 (2005)

pT>4.5 GeV

Data agree with calculated suppression patterns

Path length, density dependence leads to centrality dependence of suppression

More differential tests (e.g. from v2) are under way

On theory side: need to quantify constraints on L-dependence


Surface emission (geometric bias)

? Inclusive measurements insensitive to opacity of bulk Need coincidence measurements to probe deeper

RAA~0.2-0.3 for broad range of q̂

Large energy loss opaque core

Eskola et al., hep-ph/0406319

fmGeVq 20ˆ

fmGeVq 21ˆ

fmGeVq 2155ˆ


Azimuthal di-hadron correlations

Phys Rev Lett 91, 072304

4 < pT,trig < 6 GeVpT,assoc > 2 GeV

p+p

trigger

associated

Au+Au

Need to subtract background in Au+Au

2002 resultNo modification of near sideStrong suppression of away side

No measurable away-side yield; cannot quantify suppression


Jet-like di-hadron correlations

Larger pT allows quantitative analysis of jet energy loss

New results, year-4

Background negligible

at higher pT,assoc

8 < pT,trig < 15 GeVLarger data sample extends pT-range

Emergence of the away side peak

d+Au Au+Au 20-40% 0-5%


Di-hadron correlations: centrality dependence

Fit scaledby x2

8 < pT,trig < 15 GeV/c

Near side yields essentially unmodified

Away-side: Increasing suppression with centrality

Again ‘surface bias’


Di-hadron fragmentation

~0.54

~0.25

8 < pT,trig < 15 GeV/c

Scalingfactors

Near side fragmentation unmodified

Away-side: strong suppression,but shape similar above zT≈0.4


A closer look at azimuthal peak shapes8 < pT(trig) < 15 GeV/c

pT(assoc)>6 GeV

Large energy loss without observable modification of longitudinal and azimuthal distributions

Observations constrain energy loss fluctuations and geometrical bias

No away-side broadening


Discussion of di-hadron resultsStrong suppression (factor 4-5, similar to inclusive hadron suppression) without modification of longitudinal and azimuthal fragmentation shapes

jetTcs p

N

dz

dE 2

8

In contrast to several model expectations

Broadening due to fragments of induced radiation

Induced acoplanarity (BDMPS):

= STAR preliminary

Near-side enhancementdue to trigger bias

Majumder, Wang, Wang, nucl-th/0412061

Observation:

Vitev, hep-ph/0501225


Confronting IAA and RAA

Dainese, Loizides and Paic, QM posterEskola et al., hep-ph/0406319

IAA ≈ RAA ≈ 0.20-0.25

First look: from IAA and RAA in quantitative agreementq̂

≈ 5-7 GeV2/fm in central Au+Au @ RHICq̂

Need to further assess theory uncertainties


Heavy quark suppression (non-photonic electrons)

Suppression of non-photonic electrons larger than expected

Compatible with charm-dominance up to pT ≈ 10 GeV

Comparison of light and heavy quark suppression elucidates energy loss mechanism

Wicks, et al, nucl-th/0512076

Collisional energy loss revisited


Au+Au 0-5%

STAR Preliminary

d+Au 100-40%

Intermezzo II: Jet structure at intermediate pT

3 < pT,trig < 6 GeV2 < pT,assoc < pT,trig

pt,assoc > 2 GeV

absolute ridge yield

New feature in Au+Au: long range correlation

Persist to high pT,trigger likely jet-related

STAR Preliminary

Scenarios: Parton radiates energy before fragmenting

and couples to the longitudinal flow Armesto et al, nucl-ex/0405301

Heating of the medium Chiu & Hwa Phys. Rev. C72:034903,2005

– Radial flow + jet-quenching Voloshin nucl-th/0312065


RHIC Summary

• pQCD applicable for p+p at RHIC• Strong suppression effects seen for light and heavy

flavours• Testing radiative energy loss:

– Path length dependence confirmed– Heavy flavour suppression stronger than expected– No modifications of away-side shapes in di-hadron correlations

• Additional dynamics at intermediate pT

medium response

Newest results at RHIC start to provide quantitative tests of in-medium energy loss

Detailed evaluation ongoing

Jets in nuclear collisions at the LHC

ATLAS

CMS

ALICE

2007: p+p collisions @ 14 TeV2008: Pb+Pb collisions @ 5.5 TeV

ALICE is the dedicated Heavy-Ion experiment (high-density tracking and PID)CMS and ATLAS are likely to participate in HI runs as well

Complementary capabilities in high-Q2 probes


ETjet>100 GeV ~ 106/year

Hard process rates at the LHCAnnual yields for Pb+Pb at LHC

Jet rates and kinematic reach

at LHC are huge compared to RHIC

High statistics measurements over large kinematic range for precision test of theory


Inclusive hadron suppression at LHC

Initial gluon density at LHC ~ 5-10 x RHIC:

/fmGeV10~ˆ 2RHICq

Surface bias leads to relatively small change in RAA: Use full jet structure for more differential measurements

I. Vitev and M. Gyulassy, PRL 89, 252301(2002)A. Dianese et al., Eur.Phys.J. C38, 461(2005)

/fmGeV70~ˆ 2LHCq

{

First test of jet quenching theory at LHC:Different formalisms give different expectations


Jet reconstruction at LHC

Jet yields at high energies (>50 GeV) are large enough for full jet reconstruction

En

erg

y (G

eV)

Full jet reco removes fragmentation bias Study jet quenching (modified

fragmentation) in more detail

Jets accessible over large energy range (50-200 GeV from full jet reco) Validate jet quenching mechanism And more:

– Heavy quark jets– -jet correlations (calibrate kinematics)– Suprises?

ELHC ≈ 40 GeV need ET,Jet~200 GeV for E>>E

100 GeV jet in central Pb+Pb


ALICE+EMCal

Pb+Pb, 5.5 TeV

Rjet=0.3

Jet reconstruction in heavy ion events

CDF, Phys Rev D65, 092002 (2002)

Full jet reconstruction removes fragmentation and geometric biases

PYTHIAHERWIG

pTcharged>5 GeV

pTcharged>30 GeV

80%

80%TeV1.8p,p

Jet cone:22 R

• CDF: ~80% of jet energy containedin R<0.2

• Background from 5.5 TeV Pb+Pb: ~ 75 GeV

Use small cone radius ~ 0.3 to suppress backgrounds:

Further optimisation of jet-finding parameters awaits data

pT-cut for charged hadrons: pT > 2 GeV{

With cuts, only modest influence of background fluctuations


ALICE EMCalLead-scintillator sampling calorimeter||<0.7, =110o

Shashlik geometry, APD photosensor~13k towers (x~0.014x0.014)

ALICE-EMCal upgrade project in full swing:-First module by 2008-Full detector by 2009(Depending on funding)

US contribution to ALICE


MLLA: parton splitting+coherence angle-ordered parton cascadeGood agreement with fragmentation function data

=ln(EJet/phadron)

pThadron~2 GeV for

Ejet=100 GeV

Fragmentation strongly modified at pThadron~1-5 GeV even for the

highest energy jets

Measuring jet quenching

Borghini and Wiedemann

Introduce medium effects in parton splitting

Use large kinematic reach of LHC to test theory

z


More jet quenching at LHC

charm/light

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

Charm and beauty energy loss to distentangle colour charge and mass

(dead-cone) effects

Z,-jet to calibrate recoil energyand change geometric bias


Conclusion

pQCD and jet quenching at RHIC reaches quantitative era:– Jet measurements in p+p– Differential measurements of di-hadron fragmentation

and suppression– Heavy quark energy loss– Baryon suppression to probe colour charge effects

But kinematic reach (‘dynamic range’) is limited

Qualitative improvements expected at LHC:- Large kinematic range- Full jet reconstruction


RHIC outlookDi-hadron correlations

in Cu+Cu-jet correlations

Reducing L with a more penetrating probe

Inclusive -hadron correlations

ET,trig > 10 GeVpT,assoc > 4 GeV

T. Dietel, QM talk

Reducing the couplingto the medium

First results available, need differential studies, model comparisons

Methods need further development and large data samples


Extra slides


Hot and dense QCD matter

Phase diagram of nuclear matter

Baryon density

tem

per

atu

re

Hadronic matter

(Quasi-)free quarks and gluons

Nuclear matter

Neutron stars

Elementary collisions(accelerator physics) High-density

phases?

Thermodynamic approach Microscopic picture

Binding force between quarks

in protons and neutrons

Confinement: isolated quarks cannot exist in vacuum

The strong interaction (QCD)

Nuclear matter Quark Gluon Plasma

High density: large overlap between hadrons quarks are ‘quasi-free’

Goal: understand dense bulk matter of the Standard Model

Ea

rly u

niv

ers

e RNC research

Fundamental phase transition of the Standard Model


Surface and other bias effectsPQM: Dainese, Loizides and Paic

X-N Wang, PLB 595, 165 (2004)

= STAR preliminary

Note also: possible low-z enhancement from fragmentation of induced gluons. Outside measured range, awaits confirmation

‘Surface bias’:- Trigger, associated selection favours short path lengths

Surface bias is not the only possibility:- Energy-loss fluctuations (at fixed path length) potentially large- Fragmentation bias Wicks, Horowitz, Djordjevic, Gyulassy

nucl-th/0512076

Are we selecting pairs, events with small energy-loss?

Alternative:Shape of di-hadron fragmentation changes little if underlying partonic spectrum shape unmodified

This calculation underpredicts suppression

Partonic spectrumEjet

Nuclear geometry

L

Energy lossE(Ejet)

FragmentationD(Ejet,E)

General form:

Need full calculations, a la PQM Different observables probe different parts of convolution

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High-p T probes of heavy-ion collisions at RHIC and LHC Marco van Leeuwen, LBNL.

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