Transcript Measurement of direct photon in \sqrt{sNN}=200GeV Au+Au

Direct Photon
Production at RHIC
Tadaaki Isobe (CNS, Univ. of Tokyo)
Heavy Ion Café
2007, Jan. 13th
Motivation for direct photon
measurement
• For study of the hot and dense matter created in
relativistic heavy ion collisions.
• Direct photon: not from hadron decay.
• Penetrating probe for the bulk matter produced by Au+Au
collision.
– Don’t interact strongly, long mean free path.
• Can be described with elementary processes.
– Compton, Annihilation, Bremsstrahlung.
• High-pT direct photon
– Hard scattering
• Low-pT direct photon
– Thermal emission from QGP.
• Azimuthal anisotropy of direct photon
– Another way to extract component
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Thermal radiation from QGP
• Expected to be dominant at
mid-pT region
– 1GeV/c<pT<3GeV/c
• Strong signature of the
existence of QGP
• Carry information on thermodynamical state of medium
(temp., degrees of freedom)
• There are also contributions
from hadron gas interaction
– dominant at pT<1GeV/c
S. Turbide et al. PRC 69(2004)014903
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
High-pT direct photons
as hard probes
• p+p: agreement with NLO
pQCD.
• Run2 high-pT direct photons
in Au+Au: consistent with
binary scaled NLO pQCD.
• Binary scaling is not obvious.
Phys. Rev. Lett. 94, (2005) 232301
– 70% prompt photon + 30% jet
fragmentation at high-pT.
– Fragmentation photons are
suppressed by jet-quenching
effect at RHIC?
– Compensated by other
contribution?
30% contribution
from jet frag.
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Direct photon measurement at
RHIC-PHENIX
• Au+Au 200GeV Run at Year-4
– Sampled Luminosity 241b-1
• 1.5B events
• p+p 200GeV Run
g
– @Year-3 0.35pb-1
– @Year-5 3.8pb-1
• Photon measurement using
PHENIX- Electromagnetic
Calorimeter (EMCal)
– Lead Scintillator Type (PbSc, 18X0)
•
sE/E = 8%/√E + 2%, DfDh = 0.01x0.01, |h|
< 0.35
– Lead Glass Type (PbGl, 14.4X0)
• Min. Bias trigger from BBC-ZDC
coincidence
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Direct photon measurement at
RHIC-STAR
• BEMC
– sE/E = 16%/√E + 2%
– -1<h<1, 0<f<2, DhDf = 0.05 x 0.05
• BSMD
– wire-proportional counter
– DhDf = 0.007 x 0.007
– 0 reconstruction at high pT
• high ET trigger
– d+Au 2003: 1720 b-1 (4.5 GeV)
– p+p 2005: 2830 nb-1 (3.5 GeV)
– both datasets have 50% of BEMC
installed and triggering
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Method for direct photon
measurement
• Subtraction method
g
Direct
g
All
g
Hadronic
– Measure 0 and h with
good accuracy
– Estimate hadronic decay
background with MC
0/h
• 0, h, K0s, h’, w..
– Main contribution from
0->2g decay
• Reconstruct the slope with
mT scaling.
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Direct photon cross section
in p+p s = 200GeV
• PHENIX Year-3 final
result.
– PRL 98, 012002 (2007)
• Good agreement with
theoretical calculation
(NLO pQCD) within
the error and
theoretical
uncertainty.
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Fraction of fragmentation
photons
• Fraction of fragmentation
photons is evaluated in terms of
the fraction of isolated photons
in 200GeV p+p collisions.
• If energy of cone around a
target photon is much less than
the energy of photon, the photon
is defined as isolated photon.
Jet
g
Theoretical predictions where PHENIX
acceptance is taken into account
– Isolation cut removes
fragmentation (Bremsstrahlung)
photons.
• Data are consistent with
theoretical expectation within
the error.
– hep-ex/0609031
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
RHIC-PHENIX year-5
direct photon cross-section
• PHENIX Year-5
preliminary result.
• pT region is
extended up to
25GeV/c.
• Good reference for
the evaluation of
nuclear effect for
high-pT direct
photon production
at RHIC.
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Parameterization of the p+p data
• For the calculation of
nuclear modification factor,
p+p reference is
parameterized using a
fitting function.
• The fitting works well to
parameterize the p+p
photon data.
• The parameterized photon
data show enhancement
comparing with NLO
pQCD expectation by a
factor of more than 20%.
Data
Fit
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Direct photon spectrum
in d+Au sNN =200GeV
R = 1 + gdir / gdecay
NLO pQCD calculations from W.Vogelsang
Both data agrees with pQCD
predictions
in entire pt range
=> No indication for nuclear effects
(such as kT, EMC)
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Direct photon spectra
in Au+Au sNN = 200GeV
scaled232301
p+p cross-section
Phys.Blue
Rev. line:
Lett. N
94,
coll(2005)
• Run4 large amount
of Au+Au data
make it possible to
extend the pT
range up to
20GeV/c.
• Large error at lowpT region.
– Hard to see
thermal component.
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
New Approach
“internal conversion”
1 dN ee
2

Ng dm ee
3
2
2
4me2
2me2
1
mee
2
1
(1 
)
F(mee ) (1 
)3
2
2
2
mee
mee mee
M
Kroll-Wada Formula
200-300
÷
÷
÷
140-200
90-140
0-30
Rdata
•
•
•
•
phase space factor 1 for high pT g
any source of real photons
produces virtual photons with
very low invariant mass
Ratios of Minv bins to lowest one
(Rdata)
If no direct photons: the ratios
become exactly what can be
calculated from Dalitz decay
formulae above
If excess over calculation: direct
photons
+
Compton
q
g
Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
e
g*
q
e-
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The Direct photon spectrum
at mid-pT region
•
Direct photon ratio from internal conversion
method, and inclusive photon come from
EMCal measurement
g direct
g *direct
 g incl. 
g *incl.
•
NLO pQCD
•
A particular thermal Model
L.E. Gordon, W. Vogelsang
Phys. Rev. D48, 3136 (1993)
– 2+1 hydro, t0=0.15 fm/c
– T0ave=360 MeV(T0max=570 MeV)
D. d’Enterria, D. Peressounko
nucl-th/0503054
– The data are consistent with thermal +
pQCD
– pQCD questionable down to low pT
• needs confirmation from analysis of p+p data
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Medium induced high-pT
direct photons
• Interaction of hardscattered parton with
dense matter.
– Compton scattering (jet-photon
conv.)
Hard scattered
parton interacts
with gluon in
bulk matter
• R.J. Fries, Phys.Rev. C72
(2005) 041902
– Over estimation at high-pT
• S. Turbide, Phys.Rev. C72
(2005) 014906
– Reproduce data well
– Bremsstrahlung
• B.G. Zakharov, JETP Lett. 80
(2004) 1
• If it is observed, it can be
used as hard probe of bulk
matter.
All
NLO-pQCD
Jet-photon
QGP-Thermal
Jet-quenching effect for fragmentation 16
photons
are estimated with AMY formalism.
Tadaaki Isobe, Heavy Ion
Cafe, 01/13/07
Direct photon RAA
• Nuclear modification factor
– Yield from hard processes can be
scaled with Ncoll (# of collisions).
– divided by expected (binary
scaled) yield from p+p data.
RAA with pQCD
d N / dpT dh
RAA ( pT ) 
TAAd 2s pp / dpT dh
2
AA
TAA  N coll / s inel.
RAA with p+p data
• RAA is consistent with Ncoll
scaled p+p reference.
• RAA of very high-pT direct
photons is below one.
– A hint of jet-quenching effect for
fragmentation photons?
– RAA using pQCD shows different
tendency.
First data using p+p
data as reference.
Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
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Comparison of RAA with
theoretical calculation
• Turbide et al.
– Phys. Rev. C72 (2005)
014906 + Private
communication.
• AMY formalism for jetquenching effect for
fragmentation photons.
– Systematically data points are
below theoretical prediction.
• F. Arleo
– JHEP 0609 (2006) 015
• High-pT suppression due to
isospin effect, in addition to
jet-quenching and shadowing.
– BDMPS for jet-quenching.
• Medium induced jet-photon is
not taken into account.
– The suppression of very highpT photon is well reproduced.
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Other approach to extract
medium induced photon
• Measurement of direct photon elliptic flow
–
–
–
–
Jet-photon conversion, Bremsstrahlung (v2<0)
Jet-fragmentation (v2>0)
Thermal photon (v2>0)
pQCD (v2=0)
Thermal photon (HG+QGP): 6% v2 S. Turbide et al.
Phys.Rev.Lett.
96 01/13/07
(2006) 032303
R. Chatterjee et al. nucl-th/0511079
Tadaaki Isobe,
Heavy Ion Cafe,
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v2 measurement at PHENIX
• Standard reaction plane method
v2
direct

R  v2
 v2
R 1
inclusive
R
hadronic
• First measurement of direct
photon v2
PHENIX
Preliminary
– large error due to the large
systematic error on direct photon
excess ratio “R”
• Support the contribution from
jet-quenching effect
• New reaction plane detector
improve RP resolution
– will be installed for RHIC Year-7
run
PHENIX
Preliminary
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Summary
• Direct photons in 200GeV p+p collisions.
– Published final year-3 p+p data.
– Preliminary year-5 data.
– Consistent with NLO pQCD within uncertainty.
• Direct photons in 200GeV Au+Au collisions.
– Spectra up to 20GeV/c.
– Mid-pT photon spectrum shows the enhancement comparing with
NLO pQCD prediction.
• First direct photons RAA in 200GeV Au+Au collisions with
p+p reference.
– Almost consistent with binary scaled p+p data.
– RAA seems to be below one at very high-pT region.
• Azimuthal anisotropy of direct photon.
– Seems to be positive v2.
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
backup
Systematic error on direct photon
spectra in Au+Au 200GeV
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Systematic error on direct photon
spectra in p+p 200GeV
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Tadaaki Isobe, Heavy Ion Cafe, 01/13/07
Direct photon excess ratio
in sNN = 200GeV Au+Au collisions
Compared with
pQCD
calculation.
(dashed lines:
theoretical
uncertainty,
blue bands:
including also
Ncoll
uncertainty)
• Clear extraction of high-pT direct photon signal
thanks to the strong
suppression
of hadrons.
Tadaaki Isobe,
Heavy Ion Cafe, 01/13/07
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