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
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]]
INT 2010 3 Rainer Fries
Photons from Jets
INT 2010 4 Rainer Fries
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
INT 2010 5 Rainer Fries
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:
INT 2010 6 Rainer Fries
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
INT 2010 7 Rainer Fries
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)]
INT 2010 8 Rainer Fries
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.
INT 2010 9 Rainer Fries
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)]
INT 2010 10 Rainer Fries
Summary So Far Thermal + hard photons
Sufficient to give a decent description of RHIC data. [Turbide, Rapp & Gale, PRC (2004)] [d’Enterria & Peressounko (2006)]
INT 2010 11 Rainer Fries
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.
INT 2010 12 Rainer Fries
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)]
INT 2010 14 Rainer Fries
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]
INT 2010 15 Rainer Fries
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)]
INT 2010 16 Rainer Fries
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)]
INT 2010 17 Rainer Fries
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)]
INT 2010 18 Rainer Fries
“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.
INT 2010 19 Rainer Fries
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
INT 2010 20 Rainer Fries
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
INT 2010 21 Rainer Fries
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]
INT 2010 22 Rainer Fries
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
INT 2010 23 Rainer Fries
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
INT 2010 24 Rainer Fries
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!
INT 2010 25 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
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
INT 2010 26 Rainer Fries
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
INT 2010 27 Rainer Fries
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)]
INT 2010 28 Rainer Fries
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
INT 2010 29 Rainer Fries
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)]
INT 2010 30 Rainer Fries
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!
INT 2010 31 Rainer Fries
Elliptic Flow at High PT
INT 2010 32 Rainer Fries
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)]
INT 2010 33 Rainer Fries
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; …]
INT 2010 34 Rainer Fries
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
INT 2010 35 Rainer Fries
Correlations at High PT
INT 2010 36 Rainer Fries
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)]
INT 2010 37 Rainer Fries
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)]
INT 2010 38 Rainer Fries
Spatial Fluctuations and Tomography with Hard Probes
INT 2010 39 Rainer Fries
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
INT 2010 40 Rainer Fries
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|
INT 2010 41 Rainer Fries
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.
INT 2010 42 Rainer Fries
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
INT 2010 43 Rainer Fries
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
INT 2010 44 Rainer Fries
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
INT 2010 45 Rainer Fries
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
INT 2010 46 Rainer Fries
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?
INT 2010 47 Rainer Fries
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!