jet chemistry and contributions to em signals rainer fries texas am university riken bnl...

Jet Chemistry and Contributions to EM

Signals

Rainer FriesTexas A&M University & RIKEN BNL

Quantifying Properties of Hot QCD Matter, INT, SeattleJuly 14, 2010

INT 2010 2 Rainer Fries

Overview Photons and the case for photons from jets

“Flavor” Conversion of Jets

Elliptic Flow and Correlations with Photons

Fluctuations, Tomography and Higher Harmonics with Hard Probes (optional)

[With W. Liu,Phys.Rev.C77:054902,2008Phys.Rev.C78:037902,2008]

[2002-2004]

[2006-2010]

[With R. Rodriguez, E. Ramirez,arXiv:1005.3567 [nucl-th]]


Photons from Jets


Classifying Photon Sources Identify all important

sources 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 cooling and diluting system)

More sophisticated strategies: Elliptic Flow Correlations of photons with

hadrons and jets

E

Hadron Gas Thermal Tf

QGP Thermal Ti

“Pre-Equilibrium”?

Jet Re-interaction √(Tix√s)Hard prompt


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

nuclei.

Calculable in factorized QCD perturbation theory

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

Compton AnnihilationNb

ba

baNa

NN fdfd /,

/

PDFParton cross section PDF

Parton processes at leading order:


Fragmentation Photons Photons can also fragment off jets created in initial collisions

(Bremsstrahlung) Described by photon fragmentation function Factorization:

At NLO, prompt hard and fragmentation photons can be treated consistently.

Possible problem in nuclear matter: Final state suppression for fragmenting photons but not for prompt photons? Induces uncertainty in direct photon baseline.

Parton process:

//

,,/ cNb

cba

cbaNa

NN Dfdfd

PDFParton cross section PDF FF


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)]


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 (u = 4d !) 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 is created.


Thermal Photons Annihilation, Compton and bremsstrahlung processes also

occur between thermalized partons in a QGP.

Hope to measure the temperature T (or its time-average), confirm existence of deconfined quark-gluon phase

Resummation program (hard thermal loop) + collinear radiation (AMY)

A hot hadron gas shines as well. Annihilation, creation and Compton-like

processes with pions + vector mesons, baryons …

[Arnold, Moore & Yaffe, JHEP (2001, 2002)]

[Kapusta, Lichard & Seibert (1991)] [Baier et al. (1996)]

[Aurenche et al. (1996, 1998)]


Summary So Far Thermal + hard photons

Sufficient to give a decent description of RHIC data. [Turbide, Rapp & Gale, PRC (2004)] [d’Enterria & Peressounko (2006)]


There Must Be More! Any process that radiates gluons should be able to radiate real

and virtual photons. 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 and a medium parton.


There Must Be More! Elastic conversion cross sections peak forward and

backward.

Yield from these jet-to-photon conversions:

Induced photon bremsstrahlung

ut

tu

dtd

~ts

st

dtd ~

jet

jet

pp

pp

jet

jet

C

mTE

Tpfpfxdpd

dNE qq

s2

2423

4ln)()(

32

8

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

[Zakharov, JETP Lett. (2004)]

x

vacvac

INT 2010

13

Rainer Fries

Jet-Medium Photons Features:

Spedtrum sensitive to leading jet particle distrubtions at intermediate times.

Strongly dependent on temperature. An independent thermometer?

How bright is this new source? Can be as important as initial hard photons

at intermediate pT !

FMS PRL 90 (2003)

[Zakharov, JETP Lett. (2004)]


Jet-Medium Photons Pitching a wider tent:

Classify particles as either thermal or belonging to a (mini)jet: Photons from these particles in kinetic theory:

Jets will lose only partially energy before conversions Conversion photons provide additional constraints for jet quenching models.

Most comprehensive scheme on the market: expanded AMY Induced gluon + photon radiation Rate equations for jets Elastic conversions included

pfpfpf jetth

jetjetthjetthth fffffff ~

thermal photons

conversion photons

Did we forget these? No, irrelevant atpresent collider energies

[Jeon & Moore]


Adding Jet-Medium Photons Complete phenomenological analysis including simultaneous fit

of pion quenching Extended AMY (+ hadronic gas); hydro fireball; initial state effects

Good description of RHIC single inclusive direct photon spectra. But: little sensitivity to individual sources. How strong are

conversion photons?

[Turbide, Gale, Frodermann & Heinz (2007)][Qin, Ruppert, Gale, Jeon & Moore (2009)]


Adding Jet-Medium Photons More Sensitivity: Nuclear Modification RAA

Jet-medium photons roughly make up for the loss through jet quenching, except for very large PT.

[Qin, Ruppert, Gale, Jeon & Moore (2009)]


Jet-Medium Dileptons Jets can convert into virtual photons

Dileptons w/o hadronic sources:

Possible signals at high transverse momentum. [Turbide, Gale, Srivastava & RJF, PRC 74 (2006)]

[Srivastava, Gale & RJF, PRC 67 (2003)]


“Flavor” Conversions

Simplest possible application: opacity of the medium Drag force on QCD jets or hadrons = jet quenching Most models: energy loss of the leading parton.

Sensitive to transport coefficient

= momentum transfer squared per mean free path. Several calculations on the market using different sets of

assumptions, e.g.


2

ˆ q

I F

AMYBDMPSASW GLV

DGLV

Higher TwistAMY

Perturbative plasma inthe high temperature limit

Extrapolated from DISoff large nuclei (e+A h+X)

Hard Probes Revisited


Hard Probes Revisited How else can we use hard probes? Track changes in flavor

and chemistry in the medium! Identity of a parton can change when interacting with a

medium. Here: 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

I F

Example: Schäfer, Wang, Zhang; HT formalism


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 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 leading parton signals

Changing multiplicities in jets in medium might also change hadron chemistry: changed hadronization

[Sapeta, Wiedemann]


What Can Chemistry Tell Us? Measure equilibrium or rate of approach to equilibrium.

Low PT:

Intermediate PT: recombination, ridge vs jet etc.

inclusive Au+A

u: M. Lam

ont (S

TAR

) SQ

M06 C

u+Cu: C

. Nattrass

(STA

R), Q

M2008

Au+A

u: J.B. (S

TAR

), WW

ND

07


Why Could It Be Exciting? 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 to : Many interactions with small momentum transfer? Few scatterings with large momentum transfer?

Measurements will be challenging Need particle identification beyond 6-8 GeV/c at RHIC, outside of the

recombination region.

q̂


Quark-Gluon Conversions 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, Liu, Zhang; Schäfer, Zhang, Wang; …]

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!


Two Examples for Rare Probes 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.

L

NNN

dtdN

jet

excess rare,jet

rare

1

gssg e.g.%50

for RHIC GeV 10 @ %5

mediumce

jetjet

dusw

dusw

jet photon

g s


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

Need to compare to 2 3 processes. Non-perturbative mechanisms?

),(),( cT

c

acaT

b

baa

NTpNTpdtdN

QQggqgqqggqgQQggqqggqq

Photons and dileptons; inverse reaction negligible

Heavy quarks production?Quark / gluon conversions

234124321

)4(423412

424

34

3

33

33

23

23

1

)2(

)](1)[(2222222

1

MppppM

pfpfEpd

Epd

Epd

E

2g


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 to cut uncertainties from fragmentation functions.

AA

pAA

pp

AAp R

R)(p/)(p/

/

[Ko, Liu, Zhang] [Liu, RJF]

04

KK

Recombination region

[Liu, RJF, PRC (2008)]


Results: 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?

Recombinationregion


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

[Liu, RJF, PRC (2008)]


Recent Results from STAR STAR at QM 2009

Kaon enhancement seen between 6 and 10 GeV/c.

A proper signal of conversions?

Caution: p enhancement too big.

Blast from the past: strangeness enhancement!


Elliptic Flow at High PT


Elliptic Flow v2

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

y z

Initial anisotropy

Final anisotropyElliptic flow v2

Bulk pressure gradient

collective flow v2 > 0

saturated hard probe

path length quenching v2 > 0

rare hard PT probe

path length additional production

v2 < 0

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


Photon Elliptic Flow Have to add other photon sources

with vanishing or positive v2. Almost perfect cancellation, |v2| small

Status: Large negative v2 excluded by experiment. Large uncertainties from fireball model?

[Liu & RJF, PRC (2006)]

[Turbide, Gale & RJF (2006)]

[Chatterjee, Frodermann, Heinz, Srivastava; …]


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.

[Liu & RJF (2008)]

w/ conversions w/o conversions

Recombination taken into account


Correlations at High PT


Correlations with Photons Photon-hadron and photon-jet correlations

can provide a handle on the initial energy of a jet before quenching.

“Gold Plated Measurement” for energy loss.

Caution: additional photon sources + radiative corrections complicate the picture.

[Wang, Huang & Sarcevic (1996)]


Correlations with Photons Dilution of kinematic correlation through different photon

sources!

NLO effects important.

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

[Arleo et al. (2004)]


Spatial Fluctuations and Tomography with Hard Probes


Spatial Structures and Hard Probes Fluctuations in the initial state are important for bulk

observables. Do we expect an impact of spatial fluctuations on hard

probes? They are sensitive to early times! Can hard probes tell us about the spatial structure of the

fireball, i.e. can we do something akin to tomography? Seemingly hopeless: we sum over many events and only see an

average fireball.

b=3.2 fmb=11 fm


Quenching with Fluctuations Density integral along the path of a parton created at point r.

The relevant quantity for energy loss is the emission probability weighted integral.

With fluctuating emission and background densities:

Relevant information contained in the correlation function between emission and background

densities. R

|r2-r1|


Quenching in a Fluctuating Background Simple 2-component model for R :

Fluctuation signal on energy loss: Shows potential cancellation between stronger quenching in regions of

stronger emission and less quenching around those regions. Sign depends on details of R.

Elliptic flow signal in a fireball with short and long axes X and Y resp.

Expect less v2 in this simple model.


Numerical Study: RAA

Numerical study using event-by-event jet quenching. Events from GLISSANDO Glauber model using collision

densities

Two quenching models (simple ~L2 deterministic energy loss [sLPM], Armesto-Salgado-Wiedemann [ASW]).

Both models give less quenching at all centralities and momenta.

[Broniowski, Rybczynski & Bozek, CPC (2009)]

b = 3.2 fm


Numerical Study: RAA

RAA can be refitted across all centralities and momenta after adjusting the quenching strength.

Additional uncertainty to extraction of from geometry.

smooth Event-by-event

csLPM 0.055 0.085cASW 1.6 2.8

q̂


Residual Signatures Elliptic Flow reduced After refitting: small residual suppression.

Di-hadron pair suppression reduced. After refitting: potentially larger suppression.

Spatial structures do leave a finger print in hard probe observables.

Enough so to be useful? Have not studied time-evolution.

b = 11 fm


Higher Harmonics Bulk physics: initial state fluctuations lead to non-vanishing

v3, possibly a larger v4 etc. Same should be true for hard probes.

If observable in experiment, tests for energy loss models. More information about the initial state.

Here: interesting case of v1.

v1

v1

v2

v2

v3

v3

v4

v4

Smooth event Asymmetric event


Higher Harmonics Clear v1 signal in engineered events. Survives on the

percent level in more realistic event sample from GLISSANDO.

Must be compensated by recoil at low PT.

Look for it in bulk events with large momentum triggers?


Summary and Outlook 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.

Photons and dileptons from jets Compatible with data but still not unambiguously confirmed by experiment. New approaches using elliptic flow and photon-jet correlations.

Fluctuations in the fireball are important for hard probes physics. Just another uncertainty or a chance to measure the inhomogeneity of the

fireball? Other harmonics besides v2 are there!

jet chemistry and contributions to em signals rainer fries texas am university riken bnl...

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