Photons at RHIC

Henner BüschingFIAS – University of Frankfurt

Jyväskylä - March 2007

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In a perfect world…

RHIC: 200 GeV pp Run over

RHIC: 200 GeV Run Started

Hoax! News: RHIC is running!

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…temperature of the QGP can be measured by thermal photon radiation…

…photons don’t interact with the medium…

…good probe of the medium…

… gamma- jet correlations…

XXX’s Diploma Thesis

…in a simple world…

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…in the real world…

PHENIX

Experiment: Problem to extractLarge uncertainties

Theory: Do we understand all sources?

measured / backgroundmeasured / (background

Difficult measurement !

Experiment: Need statisticsUnderstand detector?

Theory: Do we understand all sources?

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In the real world…a wide field!

PHENIX Direct v2 -jet Shapes

Virtual

Intermediate pT Studies

Statistical MethodExt. Conversions

Photon Tagging

Reaction Plane Dependence

Thermal

Bremsstrahlung

Jet Medium Interactions

Isolation Cut

Precision!

Status

So far unfulfilled:

Initial temperature via thermal photonsEnergy loss via g-jet correlations

7Run4 data

Direct photon spectrum for 1 < pT < 4.5 GeV

via internal conversion

Punchline first: Where we are 1

Low pT

Punchline first: Where we are 2

Run2 dataHigh pT

Punchline first: Where we are 3

Run4 dataHigh pT - Proton Reference

Who ordered all these photons?

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Compton Annihilation

Hard direct photons:Direct component

q + g + q q + q + g

q

g q

q

q_

g

photons in A+A

direct photons decay photons

hard

direct fragmentationbremsstrahlung

Photon Sources in Au+Au

Un-affected by medium

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Hard direct photons:bremsstrahlung and fragmentation

Affected by medium

q

g

g

q

q

g

q

photons in A+A

direct photons decay photons

hard

direct fragmentationbremsstrahlung

Photon Sources in Au+Au

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photons in A+A

direct photons decay photons

hard thermal jet-plasma-interact.

QGP hadron gasdirect fragmentation

pre-equilibrium

jet--conv.

medium-induc. bremsstr.

Photon Sources in Au+Au

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Jet-plasma interactionjet-photon-conversion

hard QGPq + q + g

hard QGPq + g + q

photons in A+A

direct photons decay photons

hard thermal jet-plasma-interact.

QGP hadron gasdirect fragmentation

pre-equilibrium

jet--conv.

medium-induc. bremsstr.

Photon Sources in Au+Au

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Jet-plasma interaction:medium-induced bremsstrahlung q

QGP

photons in A+A

direct photons decay photons

hard thermal jet-plasma-interact.

QGP hadron gasdirect fragmentation

pre-equilibrium

jet--conv.

medium-induc. bremsstr.

Photon Sources in Au+Au

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photons in A+A

direct photons decay photons

hard thermal jet-plasma-interact.

QGP hadron gasdirect fragmentation

pre-equilibrium

jet--conv.

medium-induc. bremsstr.

Photon Sources in Au+Au

Created in all phases of the collisionOnce created, they survive

But this makes measurements hard to interpret

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Spectra - Schematic

• Thermal photons relevant below pT ~ 3 GeV/c

• Hard Photons for pT > ~ 6 GeV/c

T

1np

hard:

/ E Tethermal:

Decay photons(0→, →, …)

Jet-Plasma-Konv.

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C. Gale, hep-ph/0609301

Spectra – Recent Theory

Let’s measure

202005 p+p

0STAR: Welcome to the Club

2003 d+Au

0

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• Get clean inclusive photon sample

• Measure pT spectrum of and mesons with high accuracy

• Calculate number of decay photons per

– Done with Monte-Carlo

– mT scaling for (), ’, , …

• Finally:Subtract decay background from inclusive photon spectrum

The traditional wayall1backgroundalldirect )R1(

calc0all

meas0all

background

all

)/(

)/(R

/0

Subtraction

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Inclusive Photon Spectrum

Fraction of charged clusters

Fraction of neutral background(neutron, anti-neutrons)

efficiency

acceptancephoton conversion

Subtraction

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Uncertainties

Many systematic uncertainties of and photon measurements are highly correlated! Subtraction

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New ways I

• Advantage– Good momentum resolution

for low pT charged particles (ee) allows photon measurement at low pT

– Less trouble with background from hadrons

• Disadvantage– Radiator needs to be thin:

low photon detection probability

– Necessary to separate conversion pairs from 0 and Dalitz decays (0 e+e)

Conversion

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•Tag all photons, which combined with a photon from a second (less clean) sample can be identified as pion decay product•Correct direct photon candidate sample for contribution from decay photons with missing partner and for direct photons with fake partners•Subtract contribution from , etc. decays

TaggingNew ways II

• In central A+A collisions the number of random associations is too high for this approach

• Can work at high pT

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Results in Au+Au

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Direct photons in d+Au

It works…

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Internal Conversion

differentminv-distribution

• Any source of real produces virtual with very low mass

• minv distribution depends on process

• Rate and mass distribution given by same formula

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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

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222

2

2

2

2

)1()(1

)2

1(4

13

21

M

mmF

mm

m

m

m

dm

dN

Nee

eeeeee

e

ee

e

ee

ee

ee

ee

dm

dN

N

1 32

2

)1(M

meeeeee

e

ee

e

mm

m

m

m 1)

21(

41

3

22

2

2

2

22 )( eemF

Internal Conversion

“Thermal”

Discussion of results

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Low pT – internal conversion

2+1 hydro,d’Enterria, Perresounko

Good p+p is missingTi ~ 300 – 400 MeV

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Int. Conversions: d+Au

Not as successful as inAu+Au:

High background in Run3

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Comparison: d+Au & Au+Au

We still have to wait for Run5 p+p

p+p and d+Au

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Direct photons in p+p

Run 3Run 5 Phys. Rev. Lett. 98, 012002 (2007)

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Direct photons in p+p

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Initial state: d+Au

No indication for nuclear effects

NLO pQCD from W.Vogelsang

Run 3

High pT – Au+Au

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RAA at high pT

Energy loss

No energy loss

Reference for photons: pQCD

ddp/NdT

ddp/Nd)p(R

TNN2

AA

TAA2

TAA

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RAA at high pT

Reference for photons: measurement

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RAA with pQCD

RAA with p+p data

RAA at high pT

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RAA– Centrality Dependence

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Scenario: Isospin • Shadowing

• Isospin (p+n vs. p+p)

• Energy loss: • Suppression of direct photons

from fragmentation/bremsstrahlung

• BDMPS

• No medium induced jet-photon conv.

From fragmentation:~ 30% at pT = 15 GeV/c

F. Arleo, JHEP09 (2006) O15

direct

fragment

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Scenario: Isospin II

)Z)-(A Z)-2Z(A (Z)(1/A /N nn2

pnpp22

collAA

No problem?“primordial” photons unaltered?“medium-induced” photons can be enhanced or suppressedHelp from 62 GeV?Lower pT

Unclear effects? Jet- conversion

Vogelsang

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Scenario: Jet--Conversion + Energy loss

• AMY formalism for jet-quenching effect for fragmentation photons.• Systematically data points are below theoretical prediction.

Turbide et al.Phys. Rev. C72 (2005) 014906 + Private communication

isolation

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Isolation Cut Expectation for direct photons:

N+N: isolated

Jet-Photon conversion: isolated

Jet-fragmentation: non isolated

Bremsstrahlung: non isolated

Jet

at 200 GeVNNp p s hep-ex/0609031

Flow

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Direct-Photon v2

reaction plane

Spatial anisotropy Momentum anisotropy

v2 > 0 for hadrons

Expectation for direct photons:

N+N: v2 > 0

Jet-fragmentation: v2 > 0

Jet-Photon conversion: v2 < 0

In-medium bremsstr.: v2 < 0

Net effect: v2 < 0

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Expectation for v2: negative, |v2| 3% − 5%

Direct Photon v2 Turbide, Gale, Fries, Phys.Rev.Lett.96:032303,2006

Here: Assuming no energy loss: jet-g conversion dominates v2 : v2<0

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PHENIX PreliminarysNN=200GeV Au+Au

Direct Photon v2

inclusive decay direct 2 22 1

Rv vv

R

g gg -

=-

decay directinclusive

decay decay

Rg gg

g g

+= =

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PHENIX PreliminarysNN=200GeV Au+Au

Direct Photon v2

Run7: PHENIX new Reaction Plane Detector

53Statistical errors only.

Direct Photon v2

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hadrons

The ‘royal’ analysis

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decay

inclusivehdecayhinclusivehdirect RY

RY

RY

)1

(/11

1

Inclusive -h

Decay -h contribution(via 0-hadron)

Direct -h !

p+p at 200 GeV

hdirectinclusive

directhdecay

inclusive

decayhinclusive YYY

-jet

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-jet

First results in p+p

Consistent with expectations from Pythia

First results in Au+Au

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Summary

• Direct photons are a difficult measurement– New analysis techniques– STAR started contributing

• High pT reach in p+p: agrees with pQCD within uncertainty.

• Direct photons in 200GeV Au+Au: spectra up to 20GeV/c.

• First direct photons RAA in 200GeV Au+Au collisions with p+p reference.– RAA seems to be below one in very high-pT region– Isospin?– Strong Jet-Medium enhancement ruled out

• First results on direct photon v2 (v2>0 ?)

backup

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Systematic error on direct photon spectra in AuAu 200GeV

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

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More direct photon sources

dir

ect

photo

n R

AA

Bremsstrahlung photons through bulk matterB.G. Zakharov, JETP Lett. 80 (2004) 1