Electromagnetic Probes at RHIC
Stefan Bathe UC Riverside
ETD-HIC07, Montreal, July 19, 2007
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Em radiation: a probe of the qgp
● EM radiation not strongly interacting♦ Once produced, leaves
collision region unscathed♦ Carries information about
early times♦ Serves as baseline for
other probes
Electromagnetic Radiation: A Probe of the QGP
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Mass landscape of EMR
● 0 and Dalitz decay♦ thermal radiation from real
and virtual photons
● light vector mesons and low-mass continuum♦ mass shifts, broadening,
excess yield?
● open heavy flavor♦ thermal radiation♦ medium modification
● quarkonia♦ medium modification schematic dilepton mass distribution
Di-leptons via charged particle tracking & PID
Quarkonia: topic of its own
Mass Landscape of EMRad
0
e+
e-
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pT landscape of EMR
● Thermal radiation from the medium♦ Temperature of QGP
● Medium response to jets♦ Jet photons, induced
bremsstrahlung
● Prompt photons♦ Baseline for medium
modification of jets
Direct photons with EMCal
Thermal photons also by internal/external conversion!
Dileptons and direct photons:
● two topics
● linked at low mass♦ physics (thermal radiation)♦ measurement (via dileptons)
pT Landscape of EMRad
Ch. Gale, QM05
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1. Dileptons
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Chiral symmetry restoration● One of two fundamental properties of QGP
(besides deconfinement)● Low-mass dileptons most sensitive probe ● Primarily through meson decays
♦ =1.3 fm/c (fireball 10 fm/c)♦ Most decays in medium
Nucl. Phys. A774, 43 (2006) Phys. Rev. Lett. 96, 162302 (2006)
NA60
Low-mass excess at SPS:
observed by CERES (Pb+Au)
confirmed by NA60 in (In+In)
Chiral Symmetry Restoration
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Thermal radiation
● Promise: initial temperature of QGP
centralcollisions
M (GeV/c2)
R. Shahoyan, QM06
R. Shahoyan, QM06
NA60, In+In
Intermediate-mass excess at SPS:
observed by NA50 (Pb+Pb)
Shown to be prompt by NA60 (In+In)
Thermal Radiation
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Raw mass distribution
submitted to Phys. Rev. Lett
arXiv:0706.3034
Au+Au
200 GeV
Small S/B
Continuum, , J/ visible
J/
Systematic error
0.25% B/S (bg substraction)
9% S (cross pair subtraction,
below 600 MeV/c2)
Converter run• bg 2.5x larger • spectra consistent
Run-4
(improved analysis compared to previous preliminary result)
Raw Mass Distribution
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Invariant mass distributionInvariant Mass Distribution
● Low-Mass Continuum♦ enhancement in
150 <mee<750 MeV
● Intermediate-Mass ♦ Data consistent with
Pythia♦ PYTHIA single e-
softer for p+p, but coincide for Au+Au
♦ Angular correlations expected to weaken in Au+Au
♦ Room for thermal contribution
submitted to Phys. Rev. Lett
arXiv:0706.3034
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Low mass: comparison with theory
R.Rapp, Phys.Lett. B 473 (2000)R.Rapp, Phys.Rev.C 63 (2001)R.Rapp, nucl-th/0204003
Broad-range enhancement:150 < mee < 750 MeV3.4±0.2(stat.) ±1.3(syst.)±0.7(model)
Low-mass: Comparison with theory
Include chiral symmetry restorationand thermal radiation
submitted to Phys. Rev. Lett
arXiv:0706.3034
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Centrality dependence
• 0 production ~ Npart
• If in-medium enhancement from or qq annihilationyield should increase > ~ Npart
Low mass
Intermediate mass
Centrality Dependence
peripheral central
• charm follows binary scaling yield should increase ~ Ncoll
submitted to Phys. Rev. Lett
arXiv:0706.3034
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Invariant mass distribution in p+p
• Good agreement with hadronic cocktail except for intermediate-mass region•N.B.: PYTHIA known to be softer than p+p data from single e- analysis
Invariant Mass Distribution in p+p
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• p+p and Au+Au normalized to 0 region• Agreement at resonances ()• Au+Au enhancement for 0.2 < mee < 0.8 GeV
• Agreement in intermediate mass and J/ just for ‘coincidence’(J/ happens to scale as 0 due to scaling with Ncoll + suppression)
p+p – Au+Au comparison
p+p normalized to mee<100 MeV/c2
arXiv:0706.3034)
First evidence for radiation from
early stage of collision!
(measured p+p reference)
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The future: HBD● Dalitz & Conversion rejection via opening angle
♦ Identify electrons in field free region♦ Veto signal electrons with partner
The Future: Hadron Blind Detector
signal electron
Cherenkov blobs
partner positronneeded for rejection e+
e-
pair opening angle
~ 1 m
PHENIX from Run-7 on
projection, 10 B evtents
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2. Direct Photons
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The promise and the peril
Promises (two of them)● Initial temperature, transition
temperature via thermal photons● Jet energy scale, precise energy
loss via -jet correlations
The promise and the peril
Peril● Richer probe than originally thought
(medium-induced photons)● Old idea of unfolding photon sources
(starting from the clearly- understood high-pT spectra and moving down in pT) seriously challenged
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Photon sources in A+A
Photons in A+A
Direct Photons Decay Photons
Non-thermal thermal Hard+thermal
Initial hardscattering
Pre-equili-brium photons
QGP Hadron gas
Interaction of hard parton with QGP
1) and
2) Medium induced photon bremsstrahlung
gqq QGPhard qgq QGPhard
pQCD or prompt photons (as N+N, but modified)
from medium
Photon Sources in A+A
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Disentangling sources
log t
pT
1 10 107
(GeV)
(fm/c)
hadrondecays
hadrongas
sQGP
jet-thermal
hardscatt.
jetbrems.
<pT> vs. creation timein principle: working one’s way down from the highest pT (and excluding hadron decays) one might be able to disentangle different sources
Disentangling Sources
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Actual calculations
C. Gale, NPA 785, 93c (2007)
Very difficult to disentangle sources
Actual calculation
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Direct photon elliptic flowDirect Photon Elliptic Flow
● Jet-photons and induced photon bremsstrahlung: out-of-plane, negative v2
● Jet-quenching less fragmentation photons: in-plane, positive v2
Elliptic flow helps to disentangle various contributions
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Disentangling the contributions contribution softer v2 isol.
1 compton, annihi. =0 yes
2 jet-thermal <0 yes
3 fragmentation >0 no
4 induced. brems. <0 no
123
4
Gale, NPA 785, 93c (2007)
Disentangling++
v2 and isolation help to disentangle contributions
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P+p from Run-5
at 200 GeVp p s+ =
While data and pQCD calculation consistent, data tend to be higher than pQCD by at least 20%, increasing to low and high pT
Data Fit
p+p from Run-5
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Direct photon RAA
RAA with pQCD reference
RAA with p+p data
Direct Photon RAA
First direct photon RAA using p+p data as reference
RAA with data reference
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Comparison to pi0
Direct RAA with measured
p+p reference data
0
Direct photons
and 0 touch at
highest pT
Comparison to 0
● Still consistent with ♦ final state effect for 0 and initial state effect for direct photons♦ Modification of structure function?
• different for 0 and direct photons
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Theory comparison I, II
● Turbide et al.♦ Jet photons + pQCD + thermal♦ AMY formalism for jet-quenching of
fragmentation photons♦ Data systematically below theory♦ Phys. Rev. C72 (2005) 014906 +
private communication
● F. Arleo♦ pQCD photons only
♦ High-pT suppression due to isospin effect, shadowing, and energy loss
♦ BDMPS for jet-quenching♦ JHEP 0609 (2006) 015
Theory comparison I, II
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Theory comparison IIITheory comparison III
● B.G. Zakharov, ♦ Bremsstrahlung photons through bulk
matter♦ JETP Lett. 80 (2004) 1
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Is the suppression real?Can suppression be confirmed?● By going to smaller collision
energy (200 GeV->62 GeV), modification of structure function can be tested at smaller pT (24 GeV->~7.5 GeV)
● Advantage: independent detector systematics (no cluster merging)
● Caveat: other direct photons sources may come into play
● Result still suffers from large uncertainties● Needs higher-statistics run and p+p measurement at same energy● Ideally structure function measurement in p+A
(Run5) p+p data” to NLO pQCD
Au+Au, 62.4 GeV
minimum bias
● Other tests♦ PbGl (PbGl better spatial resolution than PbSc)♦ RxNP (shadowing should be RxNP independent)
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Towards thermal photons: conversion measurements
Towards thermal photons: conversion measurements
Internal conversion in Au+Au
Needs p+p reference measurement
Almost there (see dilepton result)
Confirmation from external conversion
Internal conversion in d+Au
Still too large uncertainties to draw a conclusion
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Photon-tagged jets
Hadrons
Because of energy loss, di-jet measurement suffers from surface bias
No trigger surface bias for direct photon tagged jets
Direct photon is measure of jet energy
Photon-tagged Jets
Hadrons
Trigger particle
Associated particle
di-jet
-jet
More on this in Saskia’s talk
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● Electromagnetic radiation in HI collisions comprises two subfields♦ Dileptons♦ Direct photons
● Coupled at low mass by physics and measurement techniques● PHENIX dielectron measurement in Au+Au (combined w/
baseline p+p) provides first evidence for early-time radiation at RHIC
● Interpretation of direct photon measurements challanged by previously non-thought-of sources (medium modifications)
● New measurements (conversion, v2, isolation) combined with future high-luminosity runs and detector upgrades will help to solve the puzzle
● STAR has entered the game, healthy competition for PHENIX
Summary and Outlook
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Extra Slides
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HBT
D1
D2
pd
L
R
p
(D1,D2)
(D1)(D2)
1
2
h/R
Enhancement for p R Rpd
L or d
c
E
L
R
f
p
(D1,D2)
(D1)(D2)
1
3/2
1+f 2/2
The Hanbury-Brown-Twiss method of photon interferometry works from stars to nuclei!
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Two-photon correlations
1
2
1
2rec
e-
e+
External conversion:• No cluster interference• No hadron contamination
C2 calculated from EMCal only and conversion+EMCal agree =>detector effects under control
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-Jet Correlations
decay
inclusivehdecayhinclusivehdirect RY
RY
RY
)1
(/11
1
Inclusive -h
Decay -h contribution(via 0-hadron)
Direct -h !
p+p collisions at 200 GeV
hdirectinclusive
directhdecay
inclusive
decayhinclusive YYY
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Comparison to Pythia
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-Jet Correlations in AuAu
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systematic from R
systematic from subtraction method
PHENIX Cu+Cu – jet E-scale
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The Spectrum
Compare to thermal + pQCD
Compare to NLO pQCD• L.E.Gordon and W. Vogelsang• Phys. Rev. D48, 3136 (1993)
• data consistent with thermal + pQCD
• above (questionable) pQCD
Compare to thermal model
2+1 hydro
T0ave=360 MeV(T0
max=570 MeV)
0=0.15 fm/c
• D. d’Enterria, D. Perresounko• nucl-th/0503054
• data above thermal at high pT
• Needs confirmation from p+p measurement
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Internal conversion: d+AuFit measured mass spectrum with function:
a - absolute normalization
b – fraction of direct photons:
decay
dir
N
Nb
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• p+p and Au+Au normalized to 0 region• Agreement at resonances ()• Au+Au enhancement for 0.2- mee < 0.8 GeV
• Agreement in intermediate mass and J/ just for ‘coincidence’(J/ happens to scale as 0 due to scaling with Ncoll + suppression)
p+p–Au+Au comparison both normalized to mee<100 MeV/c2
p+p multiplied by Npart/2 p+p multiplied by Ncoll
arXiv:0706.3034)
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Direct Photon RAA
First direct photon RAA using p+p data as reference
RAA with pQCD reference
RAA with p+p data
TNNfAA
TAAAA
d/d
d/ddir
dir
pT
pNR
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EEtttrigger trigger from 8.5GeV/c to 10.5 GeVfrom 8.5GeV/c to 10.5 GeV
First -jet results
● Both STAR and PHENIX have first -jet results● So far more proof-of-principle than precise measurements● Need higher-statistics run
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measurement
Leading Particle
Direct
Hadrons
gq
frag.
Measurement
0
● Large background from decay photons
● Three methods♦ 1: Statistical
• Measure inclusive photons
• Measure main sources of decay photons (0, )
• Calculate spectrum of decay photons from measurement
• Subtract decay photons from inclusive photons
• applied at all pT
♦ 2: Conversion• similar to dilepton
measurement
• Low pT only
♦ 3: Isolation• Can be applied in p+p and
very-high-pT Au+Au
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Current v2 results
Future improvements: more statistics, RxNP detector, internal conversion