Direct photon at RHIC-PHENIX
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Transcript of Direct photon at RHIC-PHENIX
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K.Okada 1
Direct photon at RHIC-PHENIX
Kensuke Okada (RBRC) For the PHENIX collaboration
Rencontres de Moriond March 13, 2012
3/13/2012
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Direct photon: a unique probe
• high pT: Known source, Penetrating probe – A good reference to jet quenching
• low pT : QGP production mechanism – Spectra : A thermometer– Azimuthal dependence to the reaction plane :
Probe for the formation mechanism
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A new state of the matter (sQGP)Produced by HI collision.
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PHENIX detector
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Fine segmented EMCalElectron identification, tracking +High DAQ rate Trigger
The central arm spectrometer
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Reference data (p+p)
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Measured up to pT=25 GeV/c.
The pQCD NLO calculation is in good agreement with data.
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xT scaling
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pp/p+pbar, inclusive/isolated
They are on a universal curveexcept one experiment, from LHC to fixed target experiments.
xT2pT/scross section scaled by s4.5
PHENIX e+e-
PHENIX
n=4.5 is in a range consistent with expectations from NLO pQCD.
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Direct photon RAA
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There is a difficulty in high pT region in the measurement. (=0 merging BG, cosmic BG in p+p.)
Various checks are done.Final results will come out soon.
Nuclear modification factor (R_AA) Photon vs. hadron
high pT
PRL101,232301 (2008) PRC 82,011902(2010)
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Photon = e++e-?
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They have the same quantum number. Kroll-Wada formula connects the real and (ee) yield.
Independent of the origin in the very low mass region (Mee < ~30MeV)No 0 Dalitz above the 0 mass. The key to improve the S/N ratio. Direct photon follows 1/Mee.
ProcedureCombinatorial and correlated pair subtractionCocktail and direct photon shapes Normalized in the very low mass regionFit and Extract the direct photon component
SdNMM
m
M
m
dM
Nd
eeee
e
ee
e
ee
121
41
3
22
2
2
22
22eeT Mp When , S=1 for photons.
m
q
g*
g q
e+
e-
low pT
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Virtual photon spectra
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p+p: Already shown in the previous xT plot.d+Au: Consistent with scaled p+p. Little or no nuclear effects.
Au+Au
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Au+Au: Large excess in low pT region. <T> is extracted from the inverse slope.
PRL 104,132301 (2010)
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Tc~170MeV from lattice QCD
Initial temperature & formation time
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PRC 81, 034911 (2010)
They are all above the critical temperature. However Tinit depends on 0 in the theory calculations.
Need additional experimental constraints
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Direct photon 2
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Hadron gas
sQGP
Parton+medium
Jet fragment
Hard scat.
pTThe ingredients are tuned by pT
v2 > 0
v2 < 0 Parton+medium
Jet fragment
q+gHard scatteringv2 = 0
sQGP Hadron gas
v2 0 Amplitude for 0, Relative ratio
(another knob)
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Direct photon 2
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.2.
2
R
vvRv
BGincdir
inc2
BG2
Inclusive photon v2
BG photon v2 (From 0 and other mesons v2)
B
BSR
From virtual photon measurement
From real photon in the EMCal with virtual photon measurement
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Direct photon 2
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At high pT, v2~0. It is consistent with the hard scattering source. At low pT (thermal region), an unexpectedly large v2 is observed!
It depends on the formation time.But theory predictions are too small.
early late small large
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arXiv:1105.4126
MB (0-92%) 0-20% 20-40%
calculation from PRC 79, 021901(R)
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Next step: 2 of virtual photon
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v2 from combinatorial BGand remaining Dalitz decaycan be well controlled.
virtual photon
real photonsys. uncertainty tablearXiv:1105.4126
Advantage (systematic)
Disadvantage (statistics)Total statistics goes down by a factor of ~200.Even in 1-3 GeV/c bin, it’s statistically challenging.
ee are induced in a strong magnetic field in HI collisions?PRC 83, 017901 (2011) Tuchin
If it is true, it affects the assumptionsin all virtual photon results. Anyway it is interesting.
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Future sPHENIX project
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What can we add to the current RHIC detectors?Large acceptance with high /0 separation.More tracking layers, hope eID is still powerful.Hadron calorimeter for +Jet analysis.
A plan to upgrade the detector.
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Summary
• Showed PHENIX (selected) photon results• p+p reference measurements are on the
universal curve of world’s measurements. • An unexpectedly large 2 was measured in the
low pT region. No theory explanation so far. • PHENIX upgrade discussion is on going.
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Backup
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Space time evolution
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Hadron contamination check
• Hadron contamination can be significant
• Check with an external conversion analysis– Identify photons via
conversion in material to dileptons
– No hadron contamination• Two measurements are
consistent• Hadron contamination in the
real photon (EMCal) measurement well understood
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STAR
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STAR vs PHENIX
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PHENIX has a little better S/B ratio. Acceptance STAR = 6*PHENIXDAQ STAR(~1kHz) vs PHENIX(~6kHz)
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PHENIX Au+Au Statistics
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Run species AuAu200 intL comment
4 AuAu200, 64.4 1.5G (0.241/nb)
7 AuAu200, 5.1G(0.813/nb) RXN, HBD prototype
8 dAu, p+p
9 p+p500, 200
10 AuAu200, lowE (1.25/1.07/nb) for 30/20cm
RXN, HBD
11 pp500,AuAu19.6, 200 (0.913/0.788/nb) for 30/12cm
VTX
In Run10, 11, the integrated luminosity is comparable to Run7. Can we remove Dalitz decay to reduce the combinatorial? Then small Mee will be cut out. It needs some trick for the mass shape fit.
STAR has an advantage if their claim for the combinatorial BG is true.
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RHIC advantage to LHC
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System size dependence of g fraction
• g fraction = Yielddirect / Yieldinclusive
• Largest excess above pQCD is seen at Au+Au.– Moderately in Cu+Cu also.
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No excess in d+Au(no medium)
Excess also in Cu+Cu
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• Later thermalization gives larger v2 (QGP photons)
• Large photon flow is not explained by models for QGP
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Comparison with models. No success..
Curves: Holopainen, Räsänen, Eskola., arXiv:1104.5371v1
thermal
diluted by prompt
Chatterjee, Srivastava PRC79, 021901 (2009)
Hydro after t0
QM: Quark MatterHM: Hadronic Matter