Transcript "Jet Chemistry and Contributions to EM Signals"

Jet Chemistry and
Contributions to EM
Signals
Rainer Fries
Texas A&M University & RIKEN BNL
Quantifying Properties of Hot QCD Matter, INT, Seattle
July 14, 2010
Overview

Photons and the case for photons from jets
[With W. Liu,
Phys.Rev.C77:054902,2008
Phys.Rev.C78:037902,2008]

“Flavor” Conversion of Jets

Elliptic Flow and Correlations with Photons

[2002-2004]
[2006-2010]
Fluctuations, Tomography and Higher Harmonics with Hard
Probes (optional)
[With R. Rodriguez, E. Ramirez,
arXiv:1005.3567 [nucl-th]]
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Photons from Jets
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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)
Hadron Gas Thermal Tf
QGP Thermal Ti
More sophisticated strategies:

Elliptic Flow

Correlations of photons with
hadrons and jets
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“Pre-Equilibrium”?
Jet Re-interaction √(Tix√s)
Hard prompt
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Eg
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
d N  N g   f a / N  d a bg  f b / N
a ,b
PDF

Parton
cross
section
Compton
Annihilation
PDF
p+p collisions: important baseline to understand prompt photons
in heavy ion collisions despite somewhat different initial state.
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Fragmentation Photons

Photons can also fragment off jets created in initial collisions
(Bremsstrahlung)

Described by photon fragmentation function

Factorization:
d N  N g 
f
a/N
Parton process:
 d a bc  f b / N  Dc / g
a ,b ,c
PDF


Parton
cross
section
PDF
FF
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.
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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)]
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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 = 4d !)

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.
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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 …
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[Kapusta, Lichard & Seibert (1991)]
[Baier et al. (1996)]
[Aurenche et al. (1996, 1998)]
[Arnold, Moore & Yaffe, JHEP (2001, 2002)]
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Summary So Far

Thermal + hard photons
[d’Enterria & Peressounko (2006)]
[Turbide, Rapp & Gale, PRC (2004)]

Sufficient to give a decent description of RHIC data.
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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.
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There Must Be More!

Elastic conversion cross sections peak forward and backward.

jet
d u t
~ 
dt
t u


jet
pg  p jet
dNg
d 3 pg

d t s
~ 
dt s t

[RJF, Müller & Srivastava, PRL (2002)]
Yield from these jet-to-photon conversions:
Eg


pg  p jet
4 Eg T

s 4 2
2




d
x
f
(
p
)

f
(
p
)
T
ln

C
q
g
q
g
2 
2

8
3
m


Induced photon bremsstrahlung
vac
vac
[Zakharov, JETP Lett. (2004)]
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x
12
Jet-Medium Photons


Features:
FMS PRL 90 (2003)

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
intermediate pT !
photons at
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[Zakharov, JETP Lett. (2004)]
Jet-Medium Photons

Pitching a wider tent:

Classify particles as either thermal or belonging to a (mini)jet: f  p   f th  p   f jet  p 

Photons from these particles in kinetic theory:
fg ~ f th  f th  f jet  f th  f jet  f jet
thermal
photons

Did we forget these? No, irrelevant at
present collider energies
Jets will lose only partially energy before conversions


conversion
photons
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
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[Jeon & Moore]
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Adding Jet-Medium Photons

Complete phenomenological analysis including simultaneous fit of
pion quenching

Extended AMY (+ hadronic gas); hydro fireball; initial state effects
[Qin, Ruppert, Gale, Jeon & Moore (2009)]


[Turbide, Gale, Frodermann & Heinz (2007)]
Good description of RHIC single inclusive direct photon spectra.
But: little sensitivity to individual sources. How strong are
conversion photons?
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Adding Jet-Medium Photons

More Sensitivity: Nuclear Modification RAA
[Qin, Ruppert, Gale, Jeon & Moore (2009)]

Jet-medium photons roughly make up for the loss through jet
quenching, except for very large PT.
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Jet-Medium Dileptons

Jets can convert into virtual photons
[Srivastava, Gale & RJF, PRC 67 (2003)]

Dileptons w/o hadronic sources:
[Turbide, Gale, Srivastava & RJF, PRC 74 (2006)]

Possible signals at high transverse momentum.
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“Flavor” Conversions
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Hard Probes Revisited


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
F
I
2
qˆ 

= momentum transfer squared per mean free path.

Several calculations on the market using different sets of
assumptions, e.g.
AMY
AMY
Higher Twist
BDMPS
ASW
GLV
DGLV
Extrapolated from DIS
off large nuclei (e+A  h+X)
Perturbative plasma in
the high temperature limit
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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”:

F
I

Gluons g

Light quarks q = u,d

Strange quarks s

Heavy quarks Q = c,b

Real photons, virtual photons (dileptons) g
Measure flavor conversions  jet chemistry
Example: Schäfer, Wang, Zhang;
HT formalism
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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]
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What Can Chemistry Tell Us?

Measure equilibrium or rate of approach to equilibrium.

Low PT:

Intermediate PT:
recombination, ridge
vs jet etc.
INT 2010
inclusive Au+Au: M. Lamont
(STAR) SQM06 Cu+Cu: C. Nattrass
(STAR), QM2008
Au+Au: J.B. (STAR), WWND07
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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
q̂
:

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.
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Quark-Gluon Conversions

Gluon  (light) quark conversions

Available in some jet quenching schemes (HT, AMY, …)

[Ko, Liu, Zhang; Schäfer, Zhang, Wang; …]
Relative quenching of gluons and
quarks: color factor 9/4

Not explicitly observed in data

Shouldn’t be there in a system
short mean free path!
with

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
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functions!

Rainer Fries
Two Examples for Rare Probes

Example 1: excess production of particles which are rare in the
medium and rare in the probe sample
jet

photon
dN rare 1 jet
 N
dt


Example: photons

Need enough yield to outshine other sources of Nrare.

N rare, excess L

N jet

Example 2: chemical equilibration of a rare probe particle
g
s
e.g. g  s  s  g
 s 
w jet  
  5%
 u  d  jet
@ 10 GeV for RHIC
 s 
wce  
 50%

 u  d medium

Example: strangeness at RHIC

Coupling of jets (not equilibrated) to the equilibrated medium should drive
jets towards chemical equilibrium.
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Conversion Rates

Coupled rate equations for numbers of jet particles (flavors a, b,
c, …) in a fireball simulation.
a
dN
  ab ( pT , T ) N a   ca ( pT , T ) N c
dt
b
c


1
g2d 3 p2
d 3 p3
d 3 p4
f ( p2 )[1  f ( p4 )]
2 E1  2 3 2 E2 2 3 2 E3 2 3 2 E4
 M 1234 (2 )4  ( 4 )  p1  p2  p3  p4   M 1234
2
Here: reaction rates from elastic 2  2 collisions
qq  gg
qg  gq
Quark / gluon conversions
qq g  g
q g g q
Heavy quarks production?
Photons and dileptons;
inverse reaction negligible

Need to compare to 2  3 processes.

Non-perturbative mechanisms?
Rainer Fries
g Q Q  g
g  g Q Q
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2
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
[Liu, RJF, PRC (2008)]
p
(p/  ) AA RAA

   to cut uncertainties from

(p/ ) pp RAA
Use double ratios g p / 
fragmentation functions.

Recombination region
K 4
K 0
[Liu, RJF]
[Ko, Liu, Zhang]
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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?
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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
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[Liu, RJF, PRC (2008)]
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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!
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Elliptic Flow at High PT
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Elliptic Flow v2


Azimuthal anisotropy for finite impact parameter.
z
y
Three different mechanisms:
x
Initial
anisotropy
Final anisotropy
Bulk
pressure
gradient
collective flow
v2 > 0
saturated hard
path length
quenching
v2 > 0
path length
additional
production
v2 < 0
Elliptic flow v2
probe
rare hard
PT probe Rainer Fries
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[Turbide, Gale & RJF, PRL 96 (2006)]
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Photon Elliptic Flow

Have to add other photon sources
with vanishing or positive v2.


[Turbide, Gale & RJF (2006)]
Almost perfect cancellation, |v2| small
Status:

Large negative v2 excluded by experiment.

Large uncertainties from fireball model?
[Liu & RJF, PRC (2006)]
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[Chatterjee, Frodermann, Heinz, Srivastava; …]
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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 account
w/ conversions
w/o conversions
[Liu & RJF (2008)]
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Correlations at High PT
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Correlations with Photons

Photon-hadron and photon-jet correlations
can provide a handle on the initial energy of
a jet before quenching.
[Wang, Huang & Sarcevic (1996)]
g


“Gold Plated Measurement” for energy loss.
Caution: additional photon sources + radiative corrections
complicate the picture.
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Correlations with Photons

Dilution of kinematic correlation through different photon
sources!
[Qin, Ruppert, Gale, Jeon, Moore,(2008); (2009)]

NLO effects important.
[Arleo et al. (2004)]
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Spatial Fluctuations and Tomography
with Hard Probes
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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 fm
b=11 fm
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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
R
between emission and background
densities.
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|r2-r1|
40
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.
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Numerical Study: RAA

Numerical study using event-by-event jet quenching.

Events from GLISSANDO Glauber model using collision densities
[Broniowski, Rybczynski & Bozek, CPC (2009)]


Two quenching models (simple ~L2 deterministic energy loss
[sLPM], Armesto-Salgado-Wiedemann [ASW]).
Both models give less
quenching at all centralities
and momenta.
b = 3.2 fm
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Numerical Study: RAA


RAA can be refitted across all
centralities and momenta
after adjusting the quenching
strength.
smooth
Event-byevent
csLPM
0.055
0.085
cASW
1.6
2.8
Additional uncertainty to
extraction of q̂ from
geometry.
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Residual Signatures





Elliptic Flow reduced
After refitting: small residual
suppression.
Di-hadron pair suppression reduced.
b = 11 fm
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.
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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.
Smooth event
Asymmetric event
v2
v1
v4
v2
v3
v4
v3
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v1
45
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?
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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!
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