Direct Photon Production at RHIC

Tadaaki Isobe (CNS, Univ. of Tokyo)

Heavy Ion Café2007, Jan. 13th

Tadaaki Isobe, Heavy Ion Cafe, 01/13/072

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

Tadaaki Isobe, Heavy Ion Cafe, 01/13/073

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 thermo-dynamical 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

Tadaaki Isobe, Heavy Ion Cafe, 01/13/074

High-pT direct photonsas 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.– 70% prompt photon + 30% jet f

ragmentation at high-pT.

– Fragmentation photons are suppressed by jet-quenching effect at RHIC?

– Compensated by other contribution?

30% contribution from jet frag.

Phys. Rev. Lett. 94, (2005) 232301

Tadaaki Isobe, Heavy Ion Cafe, 01/13/075

• Au+Au 200GeV Run at Year-4– Sampled Luminosity 241b-1

• 1.5B events

• p+p 200GeV Run – @Year-3 0.35pb-1

– @Year-5 3.8pb-1

• Photon measurement using PHENIX- Electromagnetic Calorimeter (EMCal)– Lead Scintillator Type (PbSc, 18X0)

• E/E = 8%/√E + 2%, = 0.01x0.01, || < 0.35

– Lead Glass Type (PbGl, 14.4X0)

• Min. Bias trigger from BBC-ZDC coincidence

Direct photon measurement at RHIC-PHENIX

Tadaaki Isobe, Heavy Ion Cafe, 01/13/076

Direct photon measurement at RHIC-STAR

• BEMC– E/E = 16%/√E + 2%– -1<<1, 0<<2, = 0.05 x 0.05

• BSMD– wire-proportional counter = 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

Tadaaki Isobe, Heavy Ion Cafe, 01/13/077

Method for direct photon measurement

• Subtraction method

– Measure 0 and with good accuracy

– Estimate hadronic decay background with MC

• 0, , K0s, ’, ..

– Main contribution from

0->2 decay

• Reconstruct the slope with mT scaling.

0/

HadronicAllDirect

Tadaaki Isobe, Heavy Ion Cafe, 01/13/078

Direct photon cross section in p+p = 200GeVs

• PHENIX Year-3 final result.– PRL 98, 012002 (2007)

• Good agreement with theoretical calculation (NLO pQCD) within the error and theoretical uncertainty.

Tadaaki Isobe, Heavy Ion Cafe, 01/13/079

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.

– Isolation cut removes fragmentation (Bremsstrahlung) photons.

• Data are consistent with theoretical expectation within the error.– hep-ex/0609031

Jet

Theoretical predictions where PHENIX acceptance is taken into account

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0710

RHIC-PHENIX year-5direct 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.

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0711

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

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0712

Direct photon spectrum in d+Au =200GeV

Both data agrees with pQCD predictions in entire pt range

=> No indication for nuclear effects(such as kT, EMC)

NLO pQCD calculations from W.Vogelsang

R = 1 + dir / decay

NNs

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0713

Direct photon spectrain Au+Au = 200GeVNNs

Phys. Rev. Lett. 94, (2005) 232301Blue line: Ncoll scaled p+p cross-section

• Run4 large amount of Au+Au data make it possible to extend the pT range up to 20GeV/c.

• Large error at low-pT region.– Hard to see therma

l component.

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0714

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• 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 e

xactly what can be calculated from Dalitz decay formulae above

• If excess over calculation: direct photons

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Kroll-Wada Formula

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0715

The Direct photon spectrum at mid-pT region

• Direct photon ratio from internal conversion method, and inclusive photon come from EMCal measurement

• NLO pQCD

• A particular thermal Model– 2+1 hydro, 0=0.15 fm/c– T0ave=360 MeV(T0max=570 MeV)

– The data are consistent with thermal + pQCD

– pQCD questionable down to low pT• needs confirmation from analysis of p+p data

L.E. Gordon, W. VogelsangPhys. Rev. D48, 3136 (1993)

D. d’Enterria, D. Peressounkonucl-th/0503054

.. *

*

incl

directincldirect

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0716

• Interaction of hard-scattered parton with dense matter.– Compton scattering (jet-photon

conv.)• R.J. Fries, Phys.Rev. C72 (20

05) 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.

Medium induced high-pT direct photons

Jet-quenching effect for fragmentation photons are estimated with AMY formalism.

QGP-ThermalJet-photon

NLO-pQCD

All

Hard scattered parton interacts with gluon in bulk matter

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0717

Direct photon RAA

• Nuclear modification factor– Yield from hard processes can be

scaled with Ncoll (# of collisions).– divided by expected (binary scale

d) yield from 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 fra

gmentation photons?– RAA using pQCD shows different te

ndency.

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inelcollAA

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AA

TAA

TAA

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ddpdT

ddpNdpR

First data using p+p data as reference.

RAA with pQCD

RAA with p+p data

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0718

Comparison of RAA with theoretical calculation

• Turbide et al.– Phys. Rev. C72 (2005) 01490

6 + Private communication.• AMY formalism for jet-quenchi

ng 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 high-

pT photon is well reproduced.

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0719Thermal photon (HG+QGP): 6% v2

R. Chatterjee et al. nucl-th/0511079

Other approach to extract medium induced photon

S. Turbide et al.Phys.Rev.Lett. 96 (2006) 032303

• Measurement of direct photon elliptic flow– Jet-photon conversion, Bremsstrahlung (v2<0)– Jet-fragmentation (v2>0)– Thermal photon (v2>0)– pQCD (v2=0)

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0720

v2 measurement at PHENIX

• Standard reaction plane method

• First measurement of direct photon v2– 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

122

2

R

vvRv

hadronicinclusivedirect

R

PHENIX Preliminary

PHENIX Preliminary

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0721

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.

backup

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0723

Systematic error on direct photon spectra in Au+Au 200GeV

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0724

Systematic error on direct photon spectra in p+p 200GeV

Tadaaki Isobe, Heavy Ion Cafe, 01/13/0725

Direct photon excess ratioin = 200GeV Au+Au collisions

• Clear extraction of high-pT direct photon signal thanks to the strong suppression of hadrons.

NNs

Compared with pQCD calculation. (dashed lines: theoretical uncertainty,blue bands: including also Ncoll uncertainty)