Thanks to my hosts at Nagoya University
Some successful
collaborations in nuclear physics in the pastand to JSPS for a wonderful andproductive stay in Japan!

Rainer FriesHI Pub 20092

Osaka University

Tokyo University
Nagoya University

Kyoto University

Rainer FriesHI Pub 20093

Overview

Introduction : Electromagnetic probes



Photon Sources

Initial hard photons



Thermal radiation

Jet-medium interactions



Flavor Conversions

Elliptic Flow

Photon correlations

Rainer FriesHI Pub 20094

Introduction

Rainer FriesHI Pub 20095

How to Investigate 1012 K Matter?


Look at the Ashes

Look at projectiles
Soft bulk physics

Some objects are violently
ejected from an explosion.

Hard probes

Look at radiation
Electromagnetic probes

Rainer Fries

T ~ 1300 K

HI Pub 2009

T ~ 270 K

6

Electromagnetic ProbesReal and virtual photons are perfect probes for nuclear matter.


Quarks (and charged hadrons) couple to photons.

Weak coupling αem << αS:Photon mean free path ~ 100 fm in hot nuclear matter >> typical system sizephoton probes usually interact only once with the systemleast possible disruption of the system by the probe

but very low production rates

What we can hope to measure is a current-current correlator.
Photons couple toRainer FriesHI Pub 20097

Learning From Success: DISLongitudinal structure of hadrons and nuclei can be revealed
through deep inelastic scattering:Probe: virtual photon in the initial state

Quarks exist!


Bjorken scaling (∂F1/∂Q2 = 0 = ∂F2/∂Q2)

Callan-Gross relation (F2 = 2xF1)

Rainer Friesx

HI Pub 20098

Learning From Success: DISRigorous procedure to extract parton distributions
for protons and other hadrons.
CTEQ6



Today: toward 3-d picture of hadrons

E.g. DVCS

Rainer FriesHI Pub 20099

High Energy Nuclear Collisions
Can we have a similarly successful program in Heavy Ion Physics?


E.g.: measure the distribution function of quarks in a QGP?

In principle yes … but:

Many sources: system is far from homogenous

Initial prompt photons, pre-equilibrium phase, QGP, jets, hadronic phase

Moving target: system changes radically as a function of time

Photon signals are integrated over system history

Rainer FriesHI Pub 200910

Very Good Data Available

Rainer FriesHI Pub 200911

Photons in Nuclear Collisions

Rainer FriesHI Pub 200912

Classifying Photon Sources

Identify all importantsources and develop a
strategy to measure them
individually.
Transverse momentum spectra of single direct photons


Hierarchy in momentum

Reflects hierarchy in average momentum
transfer (or temperature) in a coolingand diluting system)

Hadron Gas Thermal Tf
QGP Thermal Ti

More sophisticated strategies:

Elliptic Flow

Correlations of photons withhadrons and jets
Rainer Fries“Pre-Equilibrium”?

Jet Re-interaction √(Tix√s)

Hard prompt
HI Pub 2009

13

Eγ	

Initial Hard Photons
Prompt photons from initial hard scattering of partons in the
nuclei.

Parton processes at leading order:Calculable in factorized QCD
perturbation theory

PDF

Partoncross
sectionComptonAnnihilation

PDF

p+p collisions: important baseline to understand prompt photons
in heavy ion collisions despite somewhat different initial state.

Rainer FriesHI Pub 200914

Fragmentation Photons
Photons can also fragment off jets created in initial collisions(Bremsstrahlung)Described by photon fragmentation function



Factorization:

PDF





Parton process:Partoncross
sectionPDF

FF

At NLO, prompt hard and fragmentation photons can be treatedconsistently.
Possible problem in nuclear matter:

Final state suppression for fragmenting photons but not for prompt
photons?

Induces uncertainty in direct photon baseline.

Rainer FriesHI Pub 200915

Initial Hard PhotonsPrompt photon data in p+p well described by NLO calculations.
This seems like a safe
baseline!

Photon world data @ hadron colliders[Aurenche et al., PRD (2006)]

Rainer FriesHI Pub 200916

Initial Hard Photons: Nuclear Effects

Do we have control over initial state effects for prompt photons
in nuclear collisions?
Isospin: correct blend of protons and neutrons in colliding nuclei is important(αu = 4αd !)

Shadowing and EMC effect: usually taken into account by modified
parameterizations for nuclear PDFs (EKS …); source of some uncertainty!

Cronin effect: initial state scattering leading to broadening.

Final state effects for fragmentation photons: most calculations
assume final state parton is quenched until the photon iscreated.
Which often means full quenching until the parton leaves the fireball!

Rainer FriesHI Pub 200917

Initial State Effects


Shadowing:[Eskola et al., (1998, 2008)]

Large uncertainties innuclear gluon distribution at small x!

Estimated combined effect ofisospin and shadowing on initial
hard photons.
[Srivastava @ HP06]



Isolation cuts can in principle
distinguish fragmentation photons.
Rainer FriesHI Pub 200918

Initial State EffectsCronin et al. (1975): enhancement of hadron production
at moderate PT in proton-nucleus interactions.
Initial state multiple scattering leads to additional PT
McGaughey, Moss,
Peng (1999)



Nicely explained for Drell-Yan as a higher twist
effect[RJF (2003)]
After resummation: diffusion equation fortransverse momentum distribution.
[Majumder, Müller (2006)]



d+A at RHIC:


Effect might be small for photons.



Large (non-perturbative ?) effect for protons.
Rainer FriesHI Pub 200919

Thermal Photons
Annihilation, Compton and bremsstrahlung processes also occur
between thermalized partons in a QGP.
Emission Rate
(β = 1/T, Π = polarization tensor)



Hope to measure the temperature T (or its time-average).

Resummation program (+ hard thermal loop)

AMY: complete leading order results[Kapusta, Lichard & Seibert (1991)][Baier et al. (1996)][Aurenche et al. (1996, 1998)][Arnold, Moore & Yaffe, JHEP (2001, 2002)]
Rainer FriesHI Pub 200920

More Thermal Photons
A hot hadron gas shines as well.
Annihilation, creation and Compton-likeprocesses with pions



Vector mesons, baryons …
[Kapusta, Lichard & Seibert (1991); …]

From rates to spectra:

Need time evolution of the temperature over the system volume.



Plug rates into fireball evolution.

State of the art: hydrodynamics

Challenge:

Need reliable rates to test fireball models and extract temperatures



But we would like to experimentally check rates first
Rainer FriesHI Pub 200921

Summary So FarThermal + hard photons[d’Enterria & Peressounko (2006)][Turbide, Rapp & Gale, PRC (2004)]

Sufficient to give a decent description of RHIC data.
Rainer FriesHI Pub 200922

But Wait There’s More!


Final state interactions of jets can give us additional photons.
Compton, annihilation and Bremsstrahlung processes can also
occur between a fast parton in a jet a medium parton.
⇒

⇒

jet

jet

⊗

⊗

Elastic cross sections peak forward and backward.
In ~ 50% of cases the photon ends up with half of the jet momentum or
more.

Yield from these jet-to-photon conversions:

Rainer FriesHI Pub 2009

[RJF, Müller & Srivastava, PRL (2002)][Zakharov (2004)]

23

Jet-Medium PhotonsInteresting features:

Shape proportional to leading jet particle spectra (power law!)

Still strongly dependent on temperature.



An independent thermometer?

How bright is this new source?


FMS PRL 90 (2003)

Our first quick check:

Can be as important as initial hard
photons at intermediate pT !

Rainer FriesHI Pub 200924

Jet-Medium Photons
The bigger picture:Classify particles as either thermal or belonging to a (mini)jet:Photons from these particles in kinetic theory:

thermalphotons

conversion
photons

Did we forget these? No, irrelevant atpresent collider energies

Careful: jets will lose energy before conversion!



Leads to additional uncertainties of photon observablesAdditional constraints for jet quenching models?

Most comprehensive scheme on the market: expanded AMYInduced gluon + photon radiation

Rate equations for jets
Rainer Fries[Jeon & Moore]

HI Pub 200925

Adding Jet-Medium Photons

Recent phenomenological analysis[Turbide, Gale, Frodermann & Heinz (2007)]



AMY + thermal hadron gas + elastic jet-medium conversions

Standard hydro fireball + initial state nuclear effects

Good description of RHIC single inclusive direct photon spectra.
But: little sensitivity to individual sources. How strong areconversion photons?
Rainer FriesHI Pub 200926

Adding Jet-Medium PhotonsMore Sensitivity: Nuclear Modification RAA[Turbide, Gale, Frodermann & Heinz (2007)]



Jet-medium photons roughly make up for the loss through jet
quenchingExcept for very large PT.
Rainer FriesHI Pub 200927

“Flavor” Conversions

Rainer FriesHI Pub 200928

Hard Probes Revisited

Simplest possible hard 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 DIS)GLV, BDMPS in many varieties .

F

I

Energy loss determined by the momentum transfer incollisions


Sensitive to transport coefficient= momentum transfer squared per mean free path/collision.
Rainer FriesHI Pub 200929

Hard Probes Revisited







How else can we use hard probes? Measure the flavor!
Obviously: flavor of a parton can change when interacting withthe medium.
Here: very general definition of flavor:
F

I

Gluons gLight quarks q = u,dStrange quarks sHeavy quarks Q = c,bReal photons, virtual photons (dileptons) γ

Measure flavor conversions → jet chemistry

Example: Schäfer, Wang, Zhang;
HT formalism
Rainer FriesHI Pub 200930

Jet Chemistry

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 withelastic or inelastic collisions



After any collision: final state parton withthe 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.
[Sapeta, Wiedemann]

Other mechanisms might also change hadron chemistry in jets:E.g. changed multiplicities

Rainer FriesHI Pub 200931

Connection with Jet-Medium Photons
Conversions into photons (and dileptons) corresponds to the jetmedium photon source discussed earlier.
[RJF, Müller, Srivastava][Srivastava, Gale, RJF]
[Zakharov], …..
[Zhang, Vitev]



Unambiguous proof of conversion processes?
No, experimental situation not resolvedUnlikely that single inclusive photon
measurements at RHIC will deliver a
clear answer[Turbide, Gale, Frodermann, Heinz]
Rainer FriesHI Pub 200932

Another Application: Gluons and Protons


Gluon ↔ (light) quark conversions

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

[Ko, Liu, Zhang; Schäfer, Zhang, Wang; …]Relative quenching of gluons andquarks: color factor 9/4


Not explicitly observed in dataShouldn’t be there in a systemshort mean free path!

with



Ko et al: elastic g ↔ q conversions
Lose 30% of quark jets at RHIC
 enhance p/π ratio; need elastic crosssections × 4 to get p+p values
 Dependence on fragmentation
HI Pub 200933
functions!


Rainer FriesWhy Could It Be Exciting?








For chemistry, momentum transfer is not important (unlessthere are threshold effects)
Rather: flavor conversions are sensitive to the mean free paths
λ of partons in the medium.
Complementary information, could help settle interestingquestions

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 therecombination region.
Rainer FriesHI Pub 200934

What Can Chemistry Tell Us?


Measure equilibrium or rate of approach to equilibrium.



Low PT:

Intermediate PT:
recombination, ridge vs jet etc.

HI Pub 2009

inclusive Au+Au: M. Lamont(STAR) SQM06 Cu+Cu: C. Nattrass(STAR), QM2008
Au+Au: J.B. (STAR), WWND07

Rainer Fries

35

Two Examples for Rare Probes

Example 1: excess production of particles which are rare in themedium and rare in the probe samplejetphoton

Example: photons



Need enough yield to outshine other sources of Nrare.

Example 2: chemical equilibration of a rare probe particlegs

Example: strangeness at RHIC



Coupling of jets (not equilibrated) to the equilibrated medium should drive
jets towards chemical equilibrium.
Rainer FriesHI Pub 200936

Conversion Rates


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

Here: reaction rates from elastic 2 → 2 collisions

Quark / gluon conversions

Heavy quarks production?
Photons and dileptons;
inverse reaction negligible

Need to compare to 2 → 3 processes.

Non-perturbative mechanisms?
Rainer FriesHI Pub 200937

Results: 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
fragmentation functions.

to cut uncertainties from
Recombination region[Liu, RJF]

[Ko, Liu, Zhang]
Rainer FriesHI Pub 200938

Results: Strangeness

Kaons: see expected enhancement at RHIC


Measure above the recombination region!

Recombination
region

No enhancement at LHCToo much initial strangeness!

Maybe it works with charm at LHC?
Rainer FriesHI Pub 200939

Numerical Results: Heavy Quarks
Additional 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

LHC

LHC

Rainer FriesHI Pub 200940

Elliptic Flow at High PT

Rainer FriesHI Pub 200941

Elliptic Flow v2

Azimuthal anisotropy for finite impact parameter.
Three different mechanisms:zyx

Initialanisotropy

Final anisotropy

Bulkpressure
gradient

collective flowv2 > 0saturated hard

path length

quenchingv2 > 0path length

additional
production

v2 < 0Elliptic flow v2

probe

rare hard

PT probeRainer FriesHI Pub 2009

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

42

Photon Elliptic Flow

Have to add other photon sources
with vanishing or positive v2.
[Turbide, Gale, RJF]

Almost perfect cancellation, |v2| small

Status:

Large negative v2 excluded by experiment.



Large uncertainties from fireball model?

[Liu, RJF]
Rainer FriesHI Pub 2009

[Chatterjee, Frodermann, Heinz, Srivastava; …]

43Strangeness Elliptic Flow

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 accountw/ conversions

w/o conversions[Liu, RJF]
Rainer FriesHI Pub 200944

New Results from STAR
[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 fromthe 1980s?

Rainer FriesHI Pub 200945

Correlations at High PT

Rainer FriesHI Pub 200946

A New Playground: Correlations


Hadron-hadron correlations


Away-side jet extinct

Ridges and cones …

Away side
gone/diffuse

p+p

STAR
Rainer Fries

Broadening/Ridge onHIthenear side
Pub 2009

A+A

47

Importance for Precision Measurements

to data.

Different energy loss models give
results.

RAA or IAAQuantitative fits of[J. Nagle, HP 2008]
different[Formalism:
Zhang, Owens, Wang2]

Within a fixed model different
observables give incompatible results.

ε0 [GeV/fm]

Details matter!


Calibration (shadowing,
Cronin effect)Treatment of the initial
fireball…
Rainer Friesχ2 (total) – χ2 (min)HI Pub 200948

Armesto et al., arxiv:0907.0667

Correlations with Photons
Photon-hadron and photon-jet correlations
can provide a handle on the initial energy ofa jet before quenching.
[Wang, Huang & Sarcevic (1996)]

γ	





“Gold Plated Measurement” for energy loss?
Caution: this is again parton model thinking, not QCD. Additional
photon sources + radiative corrections complicate the picture.
Rainer FriesHI Pub 200949

Correlations with Photons


Dilution of kinematic correlation through different photon
sources!

[Qin, Ruppert, Gale, Jeon, Moore,(2008); (2009)]

Some NLO effects havebeen studied[Arleo et al. (2004)]
Rainer FriesHI Pub 200950

A New Twist on CorrelationsInstead of using photons to measure jet modification: use jets
to measure photon sources.
To disentangle photon sources measure associated photon
spectrum opposite to a jet of known energy ET.
Photons opposite 10 and 20 GeV jets:

Rainer FriesHI Pub 2009

[RJF @ DNP 2004]51

Concluding RemarksFor precision probes we need precision tools: need forconsistent integration of NLO hard processes + fragmentation
with final state interactions.


Need to address factorization issues.

Back to the drawing board: study simple processes like DIS on
nuclei.
E.g.



Could make connections with transport
description (e.g. diffusion equation for transverse momentum)[Majumder, RJF & Müller, PRC (2008)]

Understanding photons and understanding energy loss are very
closely related.
Rainer FriesHI Pub 200952

SummaryElectromagnetic probes are still a very promising tool.

Solid understanding of different sources needed.

Precision in both theory and experiment!

Conversions of high-PT particles in quark gluon plasma: a new ideaStrangeness enhancement at high PT.

v2, correlations

What I haven’t talked about (with apologies):Pre-equilibrium photons, dileptons, SPS, …

Rainer FriesHI Pub 200953

THANKS

Thanks to my collaborators:

T. C. Awes, C. Gale, A. Majumder, B. Müller, D. K. Srivastava, S. Turbide

Some slides borrowed from:

C. Gale, J. Kapusta, G. Y. Qin, D. K. Srivastava

Rainer FriesHI Pub 200954

BackupRainer FriesHI Pub 200955

New Simulation of Hard Probes

Rainer FriesHI Pub 200956

Plans for the Near Future

We develop a standardized test bed to simulate N jets/hardparticles 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

Rainer FriesHI Pub 200957

Propagating Particles in a Medium (PPM)

Some results from the testing

Neutral pion RAA vs PHENIX dataUsing vacuum fragmentation and
GLV average energy loss

Estimate

Triggered away side fragmentation function for charged
hadrons.
STAR preliminary

Direct photon trigger

Ch. pion with π0 trigger
8-16 GeV trigger PT

Direct prompt photon trigger
Rainer FriesHI Pub 200958

Propagating Particles in a Medium (PPM)

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 andelectromagnetic probes.

Comprehensive, quantitative studies ofobservables.

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 alarger effort (Techqm, JET)
Rainer FriesHI Pub 200959