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Hard Probes in Heavy Ion Collisions: Jet Chemistry and Tomography . Rainer Fries Texas A&M University & RIKEN BNL. Seminar, Tokyo University,Komaba November 11, 2009. Overview. Introduction: heavy ion collisions and hard probes Hadron chemistry and flavor conversions Tomography Summary.

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Hard probes in heavy ion collisions jet chemistry and tomography

Hard Probes in Heavy Ion Collisions: Jet Chemistry and Tomography

Rainer Fries

Texas A&M University & RIKEN BNL

Seminar, Tokyo University,Komaba

November 11, 2009

Overview Tomography

  • Introduction: heavy ion collisions and hard probes

  • Hadron chemistry and flavor conversions

  • Tomography

  • Summary

[With W. Liu,



[In progress, with R. Rodriguez ]

Tokyo 2009

Introduction Tomography

Tokyo 2009

Matter in extreme conditions
Matter in Extreme Conditions Tomography

  • Evolution of the universe:

    • Microseconds after the Big Bang we start with a hot soup of matter

    • Quarks, gluons (and leptons)

    • Phase transition to hadrons (and leptons)

    • Temperature T ~ 1012 K

Hadrons melt:

quark gluon plasma (QGP)




Too hot for nucleons to be

bound inside nuclei













Molecules, atoms dissolve:

EM plasma

Today’s universe: 2.7 K

Tokyo 2009

Qcd phase diagram
QCD Phase Diagram Tomography

Static quark potential (Karsch et al.)

  • Asymptotic freedom for T 

  • Lattice QCD predicts deconfinementphase transition around Tc ~ 170-195 MeV for B = 0 with c ~ 1 GeV/fm3.

  • Possible cross over at B =0 with critical point around Bc ~ 100-400 MeV.

Energy density (T)

Tokyo 2009

How to produce 10 12 k matter
How to Tomography Produce 1012 K Matter?

  • In nature: not observed in the last 13 billion years; maybe in extreme energy cosmic rays impacting on planetary objects.

  • In the lab: high energy nuclear collisions!


Au+Au @ s = 200 GeV

Running since 2000

T  170 MeV at freeze-out;

thermalizedpartonic degrees

of freedom


Pb+Pb @ s = 5.5 TeV

First run 2010 ?


Tokyo 2009

Sketch of a heavy ion collision
Sketch of a Heavy Ion Collision Tomography

  • Soft particles (PT< 1-2 GeV): Bulk fireball

    • Equilibration, thermalization Quark Gluon Plasma

    • Formation & evolution: likely non-perturbative

  • Hard particles (PT> 1 -2GeV): Rare probes

    • Can be used as probes of the bulk fireball (QGP)

    • Creation: perturbative, evolution: maybe perturbative

  • Stages of a heavy ion collision

Soft Hard

99% 1%


< 1fm/c

~ 10 fm/c




Tokyo 2009

Hard probes

q Tomography


Hard Probes


  • Use hard processes as plasma probes

    • hard processes  jet production, photons etc.

    • Calibration under control

      • perturbative calculations

      • p+p baseline experiment

  • Strong final state interactions for jets

    • Energy loss of fast quarks/gluons traveling through the plasma  jet quenching

    • Jet broadening, deflection, Mach cones, …

  • Photons / leptons can escape the fireball

    • Penetrating probes

Tokyo 2009

Energy loss
Energy Loss Tomography

  • Simplest possible probe: measure opacity of the medium

    • Drag force on QCD jets or hadrons = jet quenching

    • Energy loss of the leading parton.

    • Related to broadening in transverse direction.

  • Several models on the market.

    • Calculating energy loss through induced gluon radiation with different sets of assumptions

    • AMY (full thermal QCD HTL calculation)

    • Medium modified higher twist (from deep inelastic scattering)

    • GLV, BDMPS in many varieties .

  • Energy loss determined by the momentum transfer in collisions

    • Sensitive to transport coefficient

      = momentum transfer squared per mean free path/collision.



Tokyo 2009

Jet quenching
Jet Quenching Tomography

  • RHIC: dramatic suppression effect for single hadrons


Nuclear modification


Factor 4 …5 suppression

of the particle yield for

PT > 2 GeV

Tokyo 2009

Jets correlations
Jets: Correlations Tomography

  • Away-side jets vanish

    • Trigger on a high PThadron and look for associated hadron as a function of relative azimuthal angle and rapidity

Away side




Tokyo 2009


Broadening/Ridge on the near side

Towards precision measurements
Towards Precision Measurements (?) Tomography

  • Fits of to data have begun.

  • Different energy loss models give vastly different results

  • Within a fixed model different observables give incompatible results.

  • Lots of details matter!

    • Calibration (shadowing, Cronin effect)

    • Treatment of the initial fireball

J. Nagle, HP 2008


c2 (total) – c2 (min)

e0 [GeV/fm]

Tokyo 2009

Armesto et al., arxiv:0907.0667

Chemistry of hadrons at high p t
Chemistry of Hadrons at High P Tomography T

First: a Look Back at Low PT

Tokyo 2009

Hadro chemistry in the bulk
Hadro-Chemistry in the Bulk Tomography

  • Hadron ratios given by statistical model.

  • Important to establish thermalization/chemical equilibration

    • Chemical freeze-out temperature: lower bound on T.

    • Low net baryon number


Chemistry at intermediate p t recombination
Chemistry at Intermediate Tomography PT : Recombination


Two great discoveries
Two Great Discoveries Tomography

  • Enhanced baryon yield

    • p/ ~ 1 in Au+Au (for PT ~ 2 …4 GeV/c)

    • p/ ~ 0.3 in p+p,

    • p/ ~ 0.1….0.2 in e++e-



Two great discoveries1
Two Great Discoveries Tomography

  • v2 scaling for mesons and baryons


Instantaneous coalescence
Instantaneous Coalescence Tomography

  • Simple realization of a recombination model

    • Recombine valence quarks of hadrons

    • Dressed quarks, no gluons

  • Instantaneous projection of quark states on hadron states:

  • Shortcoming: projection conserves only 3 components of 4-momentum.

  • Caution: recombination itself does not make absolute predictions. If successful it predicts the difference between hadron species.

Meson Wigner function

Production hypersurface

Product of quark distributions


Baryon meson ratios
Baryon / Meson Ratios Tomography

  • Not all particles at intermediate PT are equal.

  • Example for non-equilibrium effect @ intermediate PT : jet-like correlations.

    • Correlations induced by Soft/Hard Reco (pick up reactions)

    • Hadron correlations arise from correlations between soft partons.

  • Baryon/meson ratios are a measure for equilibration for certain processes / parts of phase space.

    • Quantifiable?

  • Examples:

    • Away-side and near-side of trigger-associated hadrons.

    • Jet vs ridge on the near-side

Hot spots: fully or partially thermalized jets  correlated soft partons


Part Tomography icle composition in “jet” and ridge

pTtrig > 4.0 GeV/c

2.0 < pTassoc< pTtrig

inclusive Au+Au: M. Lamont (STAR) SQM06 Cu+Cu: C. Nattrass (STAR), QM2008

Au+Au: J.B. (STAR), WWND07

C. Suarez (STAR), poster, QM2008

Au+Au: 2<pTtrig<3 GeV/c,Cu+Cu:3<pTtrig<6 GeV/c

  • Baryon/meson ratios:

  • “jet”: smaller than inclusive

  • and similar to p+p

  • ridge: similar to inclusive

PHENIX, arXiv:0712.3033


Jana Bielcikova, Hard Probes 2008

Jana Bielcikova

Hadron correlations
Hadron Correlations Tomography

  • Understandable with Recombination?

  • Basic features can be reproduced in simple recombination + fragmentation model with 2-body correlations.

[RJF, Bass & Müller]

Near side

Away side

Away-side: Preliminary RJF + Nonaka

frag-frag + reco-reco only



In progress …


Chemistry at high p t
Chemistry Tomography at High PT ??


Hard probes revisited
Hard Probes Revisited Tomography

  • How else can we use hard probes? Measure the flavor!

  • Obviously: flavor of a parton can change when interacting with the medium.

  • Here: very general definition of flavor:

    • Gluons g

    • Light quarks q = u,d

    • Strange quarks s

    • Heavy quarks Q = c,b

    • Real photons, virtual photons (dileptons) 

  • Measure flavor conversions  jet chemistry



Example: Schäfer, Wang, Zhang; HT formalism


Jet chemistry
Jet Chemistry Tomography

  • Flavor of a jet here = identity of the leading parton.

    • Flavor of a jet is NOT a conserved quantity in a medium.

    • Only well-defined locally!

  • The picture here:

    • Parton propagation through the medium with elastic or inelastic collisions

    • After any collision: final state parton with the highest momentum is the new leading parton (“the jet”)

  • Hadronization: parton chemistry hadron chemistry

    • Hadronization washes out signals; need robust flavor signals on the parton side.

  • Other mechanisms might also change hadron chemistry in jets:

    • E.g. changed multiplicities

[Sapeta, Wiedemann]


Applications photons
Applications: Photons Tomography

  • Photon and dilepton conversions:

  • First estimate:

    • Competitive photon sources for PT~ 4-6 GeV at RHIC energies, even more important at LHC.

  • Experimental situation: not resolved

    • From a recent calculation:

FMS PRL 90 (2003)

[RJF, Müller, Srivastava] [Srivastava, Gale, RJF]

[Zakharov], …..

[Zhang, Vitev]

[Turbide, Gale, Frodermann, Heinz]


Applications gluons and protons
Applications: Gluons and Protons Tomography

[Ko, Liu, Zhang; Schäfer, Zhang, Wang; …]

  • Gluon  (light) quark conversions

  • Available in some jet quenching schemes (HT, AMY, …)

  • Relative quenching of gluons and quarks: color factor 9/4

    • Not explicitly observed in data

    • Shouldn’t be there in a system with short mean free path!

  • Ko et al: elastic g  q conversions

    • Lose 30% of quark jets at RHIC

    • enhance p/ ratio; need elastic cross sections  4 to get p+p values

    • Dependence on fragmentation functions!


Why could it be exciting
Why Could It Be Exciting? Tomography

  • For chemistry, momentum transfer is not important (unless there are threshold effects)

  • Rather: flavor conversions are sensitive to the mean free paths  of partons in the medium.

  • Complementary information, could help settle interesting questions

    • Many interactions with small momentum transfer?

    • Few scatterings with large momentum transfer?

  • But: measurements will be challenging

    • Need particle identification beyond 6-8 GeV/c at RHIC, outside of the recombination region.


Two examples for rare probes
Two Examples for Rare Probes Tomography

  • Example 1: excess production of particles which are rare in the medium and rare in the probe sample

    • Example: photons

    • Need enough yield to outshine other sources of Nrare.

  • Example 2: chemical equilibration of a rare probe particle

    • Example: strangeness at RHIC

    • Coupling of jets (not equilibrated) to the equilibrated medium should drive jets towards chemical equilibrium.





Tokyo 2009

Conversion rates
Conversion Rates Tomography

  • Coupled rate equations for numbers of jet particles (flavors a, b, c, …) in a fireball simulation.

  • Here: reaction rates from elastic 2  2 collisions

  • Need to compare to 2  3 processes.

  • Non-perturbative mechanisms?

Quark / gluon conversions

Heavy quarks production?

Photons and dileptons;

inverse reaction negligible

Tokyo 2009

Results protons
Results Tomography : Protons

  • Use the model by Ko, Liu and Zhang:

    • Rate equations plus energy loss.

    • Elastic channels; cross sections with K-factor

    • Longitudinally and transversely expanding fireball

      • RHIC: Ti = 350 MeV @ 0.6 fm/c

      • LHC: Ti = 700 MeV @ 0.2 fm/c

  • Use double ratios to cut uncertainties from fragmentation functions.

Recombination region

[Liu, RJF]

[Ko, Liu, Zhang]


Results strangeness
Results Tomography : Strangeness

  • Kaons: see expected enhancement at RHIC

    • Measure above the recombination region!

  • No enhancement at LHC

    • Too much initial strangeness!

    • Maybe it works with charm at LHC?



Tokyo 2009

Numerical results heavy quarks
Numerical Results: Heavy Quarks Tomography

  • Have to take into account threshold effect

  • At RHIC: additional heavy quark production marginal

  • LHC: not at all like strangeness at RHIC; additional yield small

    • Reason: charm not chemically equilibrated at LHC

    • Results in small chemical gradient between jet and medium charm

    • Also: threshold effect



Tokyo 2009

Elliptic flow v 2

z Tomography



Elliptic Flow v2

  • Azimuthal anisotropy for finite impact parameter.

  • Three different mechanisms:

Tokyo 2009

[Turbide, Gale & RJF, PRL 96 (2006)]

Photon elliptic flow
Photon Elliptic Flow Tomography

  • Have to add other photon sources with vanishing or positive v2.

  • Status:

    • Large negative v2 excluded by experiment.

    • Large uncertainties from fireball model?

[Turbide, Gale, RJF;

Chatterjee, Frodermann, Heinz, Srivastava; …]

[Liu, RJF]

[Turbide, Gale, RJF]

Tokyo 2009

Strangeness elliptic flow
Strangeness Elliptic Flow Tomography

  • Strangeness as non-equilibrated probe at RHIC: additional strange quarks have negative v2.

  • Expect suppression of kaon v2 outside of the recombination region.

Recombination taken into account

w/o conversions

w/ conversions

[Liu, RJF]

Tokyo 2009

New results from star
New Results from STAR Tomography

[Liu, RJF, PRC (2008)]

  • STAR at QM 2009

    • Kaon enhancement seen between 6 and 10 GeV/c.

    • A first signal for conversions?

    • Caution: p enhancement too big.

  • Blast from the past: remember strangeness enhancement from the 1980s?

Tokyo 2009

New simulation of hard probes
New Simulation of Hard Probes Tomography

Tokyo 2009

Plans for the near future
Plans for the Near Future Tomography

  • We develop a standardized test bed to simulate N jets/hard particles in a fireball.

    • Part of a NSF project with R. Rodriguez, R.J. Fries, E. Ramirez

  • Input:

    • initial phase space distributions

    • background (aka fireball)

    • specifics of dynamics (energy loss, fragmentation)

  • What it should do:

    • Evolution of particle distributions;

    • (modified) fragmentation and hadronization

    • analysis of results in terms of experimentally relevant observables

Tokyo 2009

Propagating particles in a medium ppm
Propagating Particles in a Medium (PPM) Tomography

  • Some results from the testing process

    • Using vacuum fragmentation and GLV average energy loss

  • Neutral pion RAAvs PHENIX data

    • Estimate

  • Triggered away side fragmentation function for charged hadrons.

STAR preliminary

Direct photon trigger

Direct prompt photon trigger

Ch. pion with 0 trigger

8-16 GeV trigger PT

Tokyo 2009

Propagating particles in a medium ppm1
Propagating Particles in a Medium (PPM) Tomography

  • Map functions

    • Example: emissivity for 8-10 GeV up-quarks going to the right, b=7.4 fm collision of Au ions.

  • Goals:

    • Build a flexible test bed for hard and electromagnetic probes.

    • Comprehensive, quantitative studies of observables.

    • photon/Z – jet/hadron correlations at NLO accuracy.

  • Understanding photon/Z – jet/hadron correlations  understanding electromagnetic sources and conversion processes.

  • Eventually code can be made public and/or be made part of a larger effort (Techqm, JET)

Tokyo 2009

Probing the spatial structure
Probing the Spatial Structure Tomography

  • What can hard probes tell us about the spatial structure of the fireball?

    • Or: can we do true tomography?

    • Seemingly hopeless: we sum over many events and only see an average fireball.

  • What is the effect of an inhomogeneous fireball on hard probes?

    • Realistic initial conditions with fluctuations have been studied for hydrodynamics.

    • Applicability of hydro and elliptic flow seem to set some boundaries for times > 1 fm/c.

    • Hard probes from the surface layer and in peripheral collisions might be affected by fluctuations in the background.

b=3.2 fm

b=11 fm

RIKEN 2009

Probing the spatial structure1
Probing the Spatial Structure Tomography

  • Use events with fluctuating transverse profiles of the energy density.

    • Have tried simple transverse profiles with Gaussian fluctuations.

    • Here: initial state Glauber estimates of the number of participants (i.e. no energy flow build-up taken into account that would wash out inhomogeneities);

    • Assume .

    • Will run some existing initial state simulations (e.g. GLISSANDO)

    • For every observable take average over events.

  • Check single pion spectra:

    • As expected: fluctuations lead to slightly less suppression.

    • This can be absorbed in the definition of your favorite energy loss parameter,

    • Fit data well with

    • Nothing won, effect no longer observable.

    • But the extracted strength of energy loss changes by 25%. Add to the list of uncertainties.

RIKEN 2009

Probing the spatial structure2
Probing the Spatial Structure Tomography

  • Centrality dependence and PT dependence:

  • With inhomogeneities dependence stays the same after rescaling of .

    • Results fit PHENIX pion data.

  • No signature.

RIKEN 2009

Probing the spatial structure3

z Tomography



Probing the Spatial Structure

  • Elliptic Flow v2:

  • 10-20% reduction in v2 even after redefinition of .

RIKEN 2009

Probing the spatial structure4
Probing the Spatial Structure Tomography

  • Dipion spectrum:

    • Visible deviation with inhomogeneities

  • Relative deviation on linear scale:

  • 10-20% reduction in correlation strength after redefinition of .

RIKEN 2009

Summary outlook
Summary & Outlook Tomography

  • Hadro-chemistry for hard probes

    • Flavor changing processes are present in jet-medium interactions.

    • Jet chemistry contains information complementary to jet quenching measurements.

    • Predict strangeness enhancement at high PT.

  • Work progressing on a new framework for studies of hard and electromagnetic probes.

  • Study of partons in inhomogeneous backgrounds:

    • Found 25% change in the extracted quenching strength to explain the data.

    • Even after adjusting the quenching strength, up to 20% secondary effect visible in elliptic flow and di-hadron correlations.

Tokyo 2009

Backup Tomography

Tokyo 2009

Bulk evolution elliptic flow

z Tomography



Bulk Evolution: Elliptic Flow

  • Finite impact parameter b > 0:

    • Spatial anisotropy in the initial state

    • Momentum anisotropy in the final state

  • Fourier analysis

    • v2 = second harmonic (“elliptic flow”)

  • Described very well by hydrodynamics

Tokyo 2009

Conversion rates1
Conversion Rates Tomography

  • Coupled rate equations for particle numbers (flavors a, b, c, …)

    • Reaction rates (here for elastic 2  2 collisions)

  • Elastic channels:

Quark / gluon conversions

Photons and dileptons;

inverse reaction negligible

Heavy quarks; status ?

Tokyo 2009

Numerical results photons
Numerical Results: Photons Tomography

[Liu, RJF]

  • Real photons (RHIC):

  • Real photons (LHC):

Tokyo 2009

Numerical results heavy quarks1
Numerical Results: Heavy Quarks Tomography

  • Have to take into account threshold effect

  • At RHIC: additional heavy quark production marginal

  • LHC: not at all like strangeness at RHIC; additional yield small

    • Reason: small gradient between jet and medium charm

    • Threshold effect


[Liu, RJF]


Tokyo 2009

Jet photon correlations
Jet-Photon Correlations Tomography

  • Additional photons are background for photon tagged jets.

  • Turn this into an advantage: measure photon spectrum opposite to a jet of known energy to disentangle different sources.

  • Photons opposite 10 and 20 GeV jets:


Tokyo 2009