direct photons in 200 gev p+p , d +au, au+au

Direct Photons in 200 GeV p+p, d+Au, Au+Au

Stefan Bathe UC Riverside

for the PHENIX collaboration

QM 2005, Budapest, August 4-9

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Why Direct Photons?

● p+p:

♦ Test of QCD

• Reduce uncertainty on pQCD photons in A+A

● d+Au

♦ Study nuclear effects

● A+A

♦ Photons don’t strongly interact with produced medium

♦ Hard photons

• Allow test of Ncoll scaling for hard

processes• Important for interpretation of high-pT hadron suppression at

RHIC

♦ Thermal photons

• Carry information about early stage of collision

• QGP potentially detectable via thermal photon radiation

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Hard Photons

p+p

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Direct Photons in p+p● good agreement with NLO

pQCD

● Important baseline for Au+Au

PbSc

at 200 GeVp p s+ =

PbSc

New for QM: PHENIX Preliminary

Poster O. Zaudtke

PbGlnew

Poster A. Hadj Henni

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Hard Photons

d+Au

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Direct in d+Au

● p+p and d+Au spectra compared to NLO pQCD

● ratio to NLO pQCD

● consistent with 1

● No indication for nuclear effects

2

Poster H. Torii

Poster D. Peressounko

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Hard Photons

Au+Au

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Direct Photons in Au+Au

PRL 94, 232301

Expectation for Ncoll scaling of

direct photons

Recently published

holds for all centrality classes

0 suppression caused by medium created in Au+Au collisions

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Thermal Photons

Au+Au

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Schematic Photon Spectrum in Au+Au

Decay photons

nT

1

phard:

/ E Tethermal:

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Going to low pT

● No significant excess at low pT

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● New data set

● Selection of most stable runs

● Re-evaluation of systematic uncertainties

New from Run4

● Stay tuned for more improvements Poster T. Sakaguchi

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Thermal Photons

Au+Au

A New Approach

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Opening up the phase space

Minv

pT

direct photon analysis

new dilepton analysis

conventional dilepton analysis

0

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phase space factorform factorinvariant mass of virtual photon

invariant mass of Dalitz pair

phase space factorform factorinvariant mass of Dalitz pair

invariant mass of virtual photon

32

222

2

2

2

2

)1()(1

)2

1(4

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21

M

mmF

mm

m

m

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dN

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meeeeee

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● Start from Dalitz decay

● Calculate invariant mass distribution of Dalitz pairs

● Now direct photons

● Any source of real produces

virtual with very low mass

● Rate and mass distribution given by same formula

♦ No phase space factor for mee<< pT

photon

0

0

e+

e-

Compton

q

g q

Compton

q

g q

e+

e-

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● Calculate ratios of various Minv bins to lowest one: Rdata

● If no direct photons: ratios correspond to Dalitz decays

● If excess: direct photons

Method

÷

÷÷

0-3

0

90-1

40

140-2

00 M

eV

200-3

00

Rdata

● Material conversion pairs removed by analysis cut

● Combinatorics removed by mixed events

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S/B=~1

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S/B=~1

R

R

Rdirect

0

0

direct

data

incl.

direct

*

*

RR

RR

calculated from Dalitz formula

measuredRdata ÷

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S/B=~1

0

0

direct

data

incl.

direct

*

*

RR

RR

calculated from Dalitz formula

measuredRdata ÷

R

R

Rdirect

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S/B=~1

0

0

direct

data

incl.

direct

*

*

RR

RR

calculated from Dalitz formula

measuredRdata ÷

R

R

Rdirect

incl.

direct

measured with EMCal

Here we are…

~25 % systematic error :

~20 % from measured 0 ratio

~10 % from inclusive

~5 % acceptance

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140-200 MeV0-20 %

Rdata

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

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*direct/*inclusive

Significant 10% excess of very-low-mass virtual direct photons

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

0-20 %

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Centrality Dependence

Indication for centrality dependence

more peripheral

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Comparison to Conventional result

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

( + 1 )

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direct

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

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The Spectrum

Compare to published Run2 result: PRL94 232301

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

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The SpectrumCompare to NLO pQCD

• excess above pQCD

• L.E.Gordon and W. Vogelsang

• Phys. Rev. D48, 3136 (1993)

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The Spectrum

Compare to thermal model

• data above thermal at high pT

2+1 hydro

T0ave=360 MeV(T0

max=570 MeV)

0=0.15 fm/c

• D. d’Enterria, D. Perresounko

• nucl-th/0503054

Compare to NLO pQCD

• excess above pQCD

• L.E.Gordon and W. Vogelsang

• Phys. Rev. D48, 3136 (1993)

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The Spectrum

Compare to thermal + pQCD• data consistent with

thermal + pQCD

Compare to thermal model

• data above thermal at high pT

• D. d’Enterria, D. Perresounko

• nucl-th/0503054

Compare to NLO pQCD

• excess above pQCD

• L.E.Gordon and W. Vogelsang

• Phys. Rev. D48, 3136 (1993)

2+1 hydro

T0ave=360 MeV(T0

max=570 MeV)

0=0.15 fm/c

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Conclusions

● Hard direct photons pT>4GeV/c

♦ p+p:

• Spectrum consistent with pQCD calculations

♦ d+Au:

• No apparent nuclear effects

♦ Au+Au:

• Confirms Ncoll scaling for hard processes

● Thermal (?) direct photons 1<pT<4GeV/c

♦ New EMCal measurement with reduced systematics

• Stay tuned for further improvements

♦ New measurement through very-low-mass virtual photons

• Significant 10% direct photon excess above decay photons

• Spectrum consistent with thermal model

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Backup

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WA98 Interpretation: T or kT ?

● QGP + HG rates convoluted with simple fireball model plus pQCD hard photons

● Data described with initial temperature Ti=205 MeV + some nuclear kT broadening (Cronin-effect)

● Data also described without kT broadening but with high initial temperature (Ti=270 MeV)

Turbide, Rapp, Gale, Phys. Rev. C 69 (014902), 2004

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WA98 Data: Conclusions

● Data consistent with QGP picture, but also with pure HG picture

● Large variations in extracted initial temperature Ti (however, most models give Ti > Tc)

Data can be described under a variety of different assumptions, e.g.:

Ti = 214 - 255 MeVQGP + HG + pQCD(Non-boost inv. hydro) Huovinen, Ruuskanen, Räsänen

(Nucl. Phys. A 650 (227) 1999)Ti = 213 - 234 MeVPure HG + pQCD

(Non-boost inv. hydro)

Ti = 335 MeV, = 0,2 fm/c

QGP + HG + pQCC(Bjorken hydro)

Svrivastava (nucl-th/0411041)

250 < Ti < 370 MeV,0,5 < < 3 fm/c

QGP + HG + pQCDRenk (Phys.Rev.C67:064901,2003)

Ti = 250 - 270 MeV, = 0,5 fm/c

QGP + HG + pQCD without kT

Ti = 205 MeV, = 1 fm/c

QGP + HG + pQCD with kTTurbide, Rapp, Gale

(Phys.Rev.C69:014903,2004 )

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PHENIX 0 RdA--Final

Cronin effect small!

New for QM: to be published

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90-140

90-140 MeV, 20-40 %

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Only Ncoll scaling?

● What about fragmentation photons?

frag

men

tati

on

con

trib

uti

on

(%

)

● fragmentation contribution substantial in p+p

● parton energy loss in QGP reduces fragmentation contribution in Au+Au

● compensated by induced photon bremsstrahlung in QGP

● Effects cancel?