Jet Quenching in Heavy Ion Collisions at RHIC and LHC

Urs Achim WiedemannCERN PH-TH

Erice, 2008

From elementary interactions to collective phenomena

How do collective phenomena and macroscopic properties of matter emerge from fundamental interactions ?

1973: asymptotic freedom

QCD = quark model +gauge invariance

Today: mature theory with a precision frontier

• background in search for new physics• TH laboratory for non-abelian gauge theories

QCD much richer than QED:

• non-abelian theory• degrees of freedom change with

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Q2

Question:Why do we need collider energies

to test properties of dense QCD matterwhich arise on typical scales €

sNN = 200GeV [RHIC]

€

sNN = 5500GeV [LHC]

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T ≈150 MeV , Qs ≈1− 2 GeV ?

Answer 1: Large quantitative gains

Increasing the center of mass energy implies

Denser initial system, higher initial temperature

Longer lifetime

Bigger spatial extension

Stronger collective phenomena

A large body of experimental data from RHIC supports this argument.

Answer 2: Qualitatively novel access to properties of dense matter

To test properties of QCD matter, large- processes provide well-controlled tools (example: DIS).

Heavy Ion Collisions produce auto-generated probes at high

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sNN

Q: How sensitive are such ‘hard probes’?

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Q >> T ≈150 MeV€

Q2

Bjorken’s original estimate and its correction

Bjorken 1982: consider jet in p+p collision, hard parton interacts with underlying event collisional energy loss

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ΔE rad ≈ α sˆ q L2

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dE coll dL ≈10GeV fm

Bjorken conjectured monojet phenomenon in proton-proton

Today we know (th): radiative energy loss dominates

Baier Dokshitzer Mueller Peigne Schiff 1995

• p+p:

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L ≈ 0.5 fm, ΔE rad ≈100 MeV

• A+A:

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L ≈ 5 fm, ΔE rad ≈10 GeV

Negligible !

Monojet phenomenon!

observed at RHIC

(Bjorken realized later that this estimate was numerically erroneous.)

Parton energy loss - a simple estimate

Medium characterized bytransport coefficient:

● How much energy is lost ?

Number of coherent scatterings: , where

Gluon energy distribution:

Average energy loss

€

ˆ q ≡μ 2

λ∝ ndensity

Phase accumulated in medium:Characteristic gluon energy

€

kT2 Δz

2ω ≈ˆ q L2

2ω=

ωc

ω

€

Ncoh ≈tcoh

λ

€

tcoh ≈2ω

kT2

≈ ω ˆ q

€

ω dImed

dω dz≈

1

Ncoh

ωdI1

dω dz≈ α s

ˆ q

ω€

kT2 ≈ ˆ q tcoh

€

ΔE = dz0

L

∫ dω0

ωc∫ ωdImed

dω dz~ α sωc ~ α s

ˆ q L2

High pT Hadron Spectra

€

RAA ( pT ,η ) =dN AA dpT dη

ncoll dN NN dpT dη

Centrality dependence:0-5% 70-90%

L large L small

Discovery at RHIC: Au+Au vs. d+Au● Final state suppression ● Initial state enhancement

partonic energy

loss

The fragility of leading hadrons

?

• The quenching is anomalously large (I.e. exceeds the perturbative estimate by ~ 5)

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ˆ q • Why is RAA = 0.2 natural ? Surface emission limits sensitivity to

Eskola, Honkanen, Salgado, WiedemannNPA747 (2005) 511, hep-ph/0406319

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ˆ q (τ =1 fm /c) ≥ 5GeV 2

fm≈ 5 ˆ q QCD

pert

Coherent effort of community to arrive at quantitative statements

‘True jet’ quenching

Where does thisassociated radiationgo to?

How does this partonthermalize?

What is the dependence on parton identity?

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ΔEgluon > ΔEquark, m= 0 > ΔEquark, m>0

Characterize Recoil: What is kicked in the medium?

Jet multiparticle final states provide qualitatively novel characterizations of the medium.

Multi-particle final states above background

Borghini,Wiedemann, hep-ph/0506218 €

Nmedh pT > pT

cut( ) /Nvac

h pT > pTcut

( )

€

pTcut in GeV( )

above

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pTcut

~1

~4

~7

€

ETjet

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Nch Exp. measurement

~10 GeV ~2 GeV

~100 GeV ~4 GeV

~7 GeV~200 GeV

RAA only

RAA + multi-particle final states

Novel challenge for LHC, high luminosity RHIC & TH

Jet modifications in dense QCD matter

Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 93 (2004) 242301

• Jets ‘blown with the wind’ Hard partons are not produced in the rest frame comoving with the medium

• ‘Longitudinal Jet heating’: The entire longitudinal jet multiplicity distribution softens due to medium effects. €

dN h dξ

Borghini,Wiedemann, hep-ph/0506218

€

ξ =ln ETjet pT

h[ ]

Going beyond RAA sharpens our understanding of jet quenching

Standard open questions, e.g.

are addressed by being sensitive to

orE’

E

E’

E1.) ?

2.) What is ?

E’

E

E’

E

This instead of only this

Going beyond RAA – implications for theory Benchmarking: Ideally, models account for p-p baseline, before being extended to medium modifications.

a) Single inclusives:

b) Jet energy flows:

a) Multi-hadron (multi-parton) final states:

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dσ ∝ PDF( )⊗ Hard( )⊗ Fragm( )

TH task: multi-dim integral, MC event generator optional

MLLA: resums ln[1/x]-contributions for intrajet single- (and multi-) parton distributions

Alternative: MC event generator

IR safe jet shapes, such as thrust, are perturbatively calculable.Preferred TH tool: MC event generators

Motivation for developing MC event generator models of jet quenching

JEWEL- Jet Evolution With Energy Loss K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0804.3568 [hep-ph]

Baseline: stand-alone Q2-ordered final state parton shower without keeping track of color flow (since this would complicate medium interaction) Hadronization models: string fragmentation (associating strings between nearest neighbors in momentum space)

Medium effects: Q2-ordering used to embed parton shower in nuclear geometry. Lifetime of virtual state:

determines probability of no scattering

with probability 1-S, parton undergoes elastic scattering

radiative e-loss modeled by fmed-enhanced splitting functions (so far).

€

τ = E Q f2

( ) − E Qi2

( )

€

Sno scatt τ( ) = exp −σ elasnτ[ ]

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dσ elas

d t=

π α s2

s2CR

s2 + u2

t 2

JEWEL gets the vacuum baseline

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T ≡ maxnT

pi ⋅nTi∑

pii∑

€

Tmaj ≡ maxnT ⋅n= 0

pi ⋅ni∑

pii∑

€

Tmin ≡pixi

∑pii

∑

For these jet shape observables, results are insensitive to details of hadronization.

JEWEL vacuum baseline for n-jet fractionK. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0804.3568 [hep-ph]

€

y ij ≡ 2min E i2, E i

2( ) 1− cosθ ij( ) / E cm

2

Durham cluster algorithm: define distance between particles

particle belong to same jet if

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y ij < ycut

JEWEL baseline for jet multiplicity distribution

A sensitive test of hadronization model

JEWEL - modeling “collisional” e-lossK. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0804.3568 [hep-ph]

JEWEL reproduces standard features of collisional e-loss models.

JEWEL can follow the recoil dynamicallyK. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0804.3568 [hep-ph]

At which angle and which pt does recoil parton emerge? Recoil partons can be further showered, scattered and hadronized thermal mass dependence of recoil can be studied.

Caveat: prior to hadronization

JEWEL: disentangling elas / inelas processesK. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0804.3568 [hep-ph]

JEWEL: disentangling elas / inelas processesK. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0804.3568 [hep-ph]

JEWEL: e-loss with minor pt-broadeningK. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U.A. Wiedemann, arXiv:0805.4759 [hep-ph]

Almost no broadening despite extreme choices: E=100 GeV, T=500 MeV, L=5fm, fmed = 3

Back to MLLA + LPHD: (controlling the p+p baseline)

argued to give support to particular picture of hadronization, namely

LPHD = local parton hadron duality which assumes that at scale Q0=Mhadron, there is a one-to-one correspondence between partons and hadrons

But how accurate is the MLLA approximation to the coherent branching formalism?

S.Sapeta and U.A. Wiedemann, Eur. Phys. J. C in press

Coherent branching formalism

€

∂Y Dq x,Y( ) = dzα s kt

2( )

2πz−

z+

∫

× Pqq z( ) Dq

x

z,Y + ln z

⎛

⎝ ⎜

⎞

⎠ ⎟+ Dq

x

1− z,Y + ln 1− z( )

⎛

⎝ ⎜

⎞

⎠ ⎟− Dq x,Y( )

⎡

⎣ ⎢

⎤

⎦ ⎥

⎧ ⎨ ⎩

⎫ ⎬ ⎭

Pgq z( ) Dg

x

z,Y + ln z

⎛

⎝ ⎜

⎞

⎠ ⎟+ Dq

x

1− z,Y + ln 1− z( )

⎛

⎝ ⎜

⎞

⎠ ⎟− Dg x,Y( )

⎡

⎣ ⎢

⎤

⎦ ⎥

Defines evolution equations for inclusive single-parton distributions inside quarks and gluon jets (single log accuracy, angular ordering)

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∂Y Dg x,Y( ) = dzα s kt

2( )

2πz−

z+

∫

× Pgg z( ) Dg

x

z,Y + ln z

⎛

⎝ ⎜

⎞

⎠ ⎟+ Dg

x

1− z,Y + ln 1− z( )

⎛

⎝ ⎜

⎞

⎠ ⎟− Dg x,Y( )

⎡

⎣ ⎢

⎤

⎦ ⎥

⎧ ⎨ ⎩

⎫ ⎬ ⎭

+ 2n f Pqg z( ) Dq

x

z,Y + ln z

⎛

⎝ ⎜

⎞

⎠ ⎟+ Dq

x

1− z,Y + ln 1− z( )

⎛

⎝ ⎜

⎞

⎠ ⎟− Dg x,Y( )

⎡

⎣ ⎢

⎤

⎦ ⎥

€

Y = lnE θ

Q0

⎡

⎣ ⎢

⎤

⎦ ⎥, l = ln

E

phadron

⎡

⎣ ⎢

⎤

⎦ ⎥= ln 1/ x[ ],

λ = lnQ0

ΛQCD

⎡

⎣ ⎢

⎤

⎦ ⎥

Variables:

Coherent branching formalism – initial conditionsdenotes hadronic scale

€

Dq x,Y0( ) = δ 1− x( ), Dg x,Y0( ) = 0.€

Y0

1. For the initial conditions

1. To count only gluons in a quark or gluon jet, start from

2. To count all partons in a quark or gluon jet, start from

the function counts all quarks in a quark jet

the function counts all quarks in a gluon jet

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Dq x,Y0( ) = 0, Dg x,Y0( ) = δ 1− x( ).

€

Dq x,Y0( ) = δ 1− x( ), Dg x,Y0( ) = δ 1− x( )

€

Dg x,Y( )

€

Dq x,Y( )

S.Sapeta and U.A. Wiedemann, in preparation

Coherent branching formalism vs. MLLA

For small Y,significant deviationsfrom MLLA-> agreement of MLLA with data should not be taken as strong support for LPHD

S.Sapeta and U.A. Wiedemann, in preparation

Coherent branching formalism vs. MLLA

For very large Y, MLLA shape is approached(but Y=10 is for jets well above TeV scale!)

S.Sapeta and U.A. Wiedemann, in preparation

Coherent branching formalism

Disentangling quark and gluon contributions to single-parton distributions points to origin of small-Y deviations: (kinematics of parent parton biases distribution)

S.Sapeta and U.A. Wiedemann, in preparation

Instead of a Conclusion:

Et

• The physics: ‘True’ jet rates are abundant at LHC. ‘True’ jets not (yet) in kinematical reach of RHIC.

• The challenge: characterize medium-modifications of jets in high multiplicity background.

- jet-like particle correlations

• The jet as a thermometer: jets as a far out-of-equilibrium probe participating in equilibration processes.

• Sensitive jet features:

- jet shapes (i.e. calorimetry)

- jet multiplicity distributions (in trans. and long. momentum)

- jet composition (i.e. hadrochemistry)

Prerequisite: determine ET-distribution of final state hadrons.

I walked you through recent work on how to use jets in HICs@ LHC

And where this will be tested