Direct Photon v2 Study in 200 GeV AuAu Collisions at RHIC

Guoji Lin (Yale)For STAR Collaboration

RHIC & AGS Users’ Meeting, BNL, June 5-9

06/07/2006 RHIC & AGS Users' Meeting 2

OutlineMotivationAnalysis

Photon identificationInclusive photon v2Decay photon v2

Conclusion & outlook

06/07/2006 RHIC & AGS Users' Meeting 3

Motivation (I)

3 4 5 6 7 8-0.05

0

0.05

0.1

0.15

0.2

0.25

v 2γ

jet-frag.+jet-brem.N-N + jet-th + th-th

3 4 5 6 7 8p

T (GeV/c)

inclusive (R+F)inclusive (F)PHENIX, inclusive

3 4 5 6 7 8

0-20 % 20-40 % 40-60 %

QGP is an important source of direct photons. Studying direct photon v2involving QGP is a remarkable step toward establishing its existence and would provide a stringent test of the reaction dynamics.

Different models predict different direct photon v2 behaviors in mid-pt range.

S. Turbide etc., PRL 96 (2006) 032303

Negative direct photon v2 is predicted:

• A mechanism works by absorption of particles or jets going through the medium (bremsstrahlung photon, Compton scattering).

• Positive strong space-momentum correlation.

06/07/2006 RHIC & AGS Users' Meeting 4

Motivation (II)

R. Chatterjee etc., nucl-th/0511079

Rich structure of thermal photon v2 is predicted:

At low pt a reduction of photon v2 around pt~0.4 GeV. Hadronic photon flow tracks the elliptic flow of thermal π (ρ) mesons when pt< (>) 0.4 GeV.

At high pt the overall photon v2 approaches the v2 of QGP thermal photons.

06/07/2006 RHIC & AGS Users' Meeting 5

Motivation (III)

0-10% 10-20%

20-30% 30-40%

(GeV/c)Tp0 2 4 6 8 10 12 14

60-92%

(GeV/c)Tp0 2 4 6 8 10 12 14

MinBias

B

ackg

rou

nd

)0 π/γ /

(M

easu

red

)0 π/γ(

012345601234560123456

-0.05

0

0.05

0.1

0.15

0.2

0.25 0-20%

R v

2(in

clu

sive

γ)

- v 2

(b.g

.),

v2(

incl

usi

ve γ

),

v 2(b

.g.)

20-40%

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 2 4 6

40-60%

0 2 4 6

Au+Au 200GeV 0-92%

v2(b.g.)v2(inclusive γ)

R v2(inclusive γ) - v2(b.g.)

pT (GeV/c)

PHENIX, PRL 96, 032302 (2006) PHENIX, PRL 94, 232301 (2005)

Below 4 GeV decay photons are dominating.

Difficult to extract direct photon v2 information at low pt.

Target for the middle or high pt range.

inclusive

decay

NN

06/07/2006 RHIC & AGS Users' Meeting 6

Data Set3.2M 200 GeV AuAu high tower events from year 4 productionLow&Mid&High.2.3m 200 GeV pp high tower events from year 5 pp production

Reason for high tower events:• Relatively more direct photon at high pt.• High pt photons are significantly enhanced by high tower trigger.

Inclusive photon ptdistributions from high tower events and minbias events using the same cuts.

(GeV)t

p0 2 4 6 8 10 12

evt

N t/d

pγ

dN

-610

-510

-410

STAR P

relimin

ary

Black: high tower eventRed: minbias event

06/07/2006 RHIC & AGS Users' Meeting 7

Photon identificationThe photons are identified from the energy deposited (BEMC points) in the STAR BEMC.

Both SMD η and φ clusters are required.

Charged particles are rejected within (Δη, Δφ)=(0.05, 0.05). Keep photon candidates if only low momentum charged tracks hit on it. Here my cut is Σptrack 3 GeV*sin(θ).

06/07/2006 RHIC & AGS Users' Meeting 8

Inclusive Photon v2 Calculation

* *AA ppAAp b p pu Q v v M u Q≈ +

Au+Au 100%-80%d+Au Minbiasp+p Minbias

Au+Au 60%-20%d+Au Minbiasp+p Minbias

Au+Au top 5%d+Au Minbiasp+p Minbias

0 2 4 6 8 10 12 0 2 4 6 8 10 12 0 2 4 6 8 10 12

0.1

1

(GeV/c)tp

〉))

iφ

-

tpφ

co

s (2

(iΣ 〈

Non-flow effect has to be considered carefully in high pt range.

• 4 particle cumulant method.

• Scalar product method.

2

" "

b

;

- Flow in a particle pt/eta "bin"

v - Average flow for particles used ("pool particles") to define RP

- # of charged tracks used to define RP

iii i

i pool

p

AA

Q u u e

v

M

φ

∈

= =∑

Non-flow effect

of charged particles in different collision systems.

STAR, Phys. Rev. C 72 (2005) 014904

06/07/2006 RHIC & AGS Users' Meeting 9

Correction of Inclusive Photon v2The measured inclusive photon v2 is smaller than the true value because of the charged particle veto cut: a photon is more likely to be rejected in the reaction plane direction.We assume the number of charged particle is N (0

06/07/2006 RHIC & AGS Users' Meeting 10

Correction of Inclusive Photon v2

Simulation result of 10-20% Average correction vs. centrality

The difference between the true v2 and the measured v2 is a constant, which only depends on the charged particle v2 and multiplicity. The actual correction is done by simulation because of the more complicated cut.

So the outcome of v2 should be

2; 2; 2;0.01

1 2measure real

chc Nv v v ccγ γ π

= − ≡−

Centrality0 1 2 3 4 5 6 7 8 9 10

> 2 vΔ<

0

0.005

0.01

0.015

0.02

0.025

0.03

STAR P

relimin

ary

(GeV)t

p0 2 4 6 8 10 12

(%

)2v

0

2

4

6

8

10

12

14

10-20%

input2voutput2v

input2-v

output2v

STAR P

relimin

ary

06/07/2006 RHIC & AGS Users' Meeting 11

Inclusive Photon v2

• Statistical errors only.

• A trend of decreasing as the increase of pt.

• The results using two different methods are consistent within error bars.

• Consistent with PHENIX results, extended to high pt.

06/07/2006 RHIC & AGS Users' Meeting 12

Decay Photon v2--Simulation / ndf 2χ 9.526 / 19

p0 1.066e+005± 4.49e+004

p1 0.2479± 0.5847

p2 0.4614± 8.987

/ ndf 2χ 9.526 / 19

p0 1.066e+005± 4.49e+004

p1 0.2479± 0.5847

p2 0.4614± 8.987

0 2 4 6 8 10 12-810

-710

-610

-510

-410

-310

-210

-110

1

10

/ ndf 2χ 1.159 / 10p0 0.5098± -0.9043 p1 0.5711± 11.3 p2 0.2652± -4.257 p3 0.1009± 1.185 p4 0.02422± -0.1746

/ ndf 2χ 1.159 / 10p0 0.5098± -0.9043 p1 0.5711± 11.3 p2 0.2652± -4.257 p3 0.1009± 1.185 p4 0.02422± -0.1746

0 2 4 6 8 10 12-20

-15

-10

-5

0

5

10

15

20

25

30

π0 generation:

• neutral pions are generated according to the spectrum from PHENIX.

• neutral pion v2 is assumed to be the same as charged particle v2, from the fitting of real data measurement (v2(4)).

• the upper and lower limits of v2 error bars are fitted and used to calculate the error bars of the decay photon v2.

• η distribution is flat.

Photon:

• the efficiency of smd response to photons is applied.

• the photon energy is smeared by 15%/sqrt(E).

06/07/2006 RHIC & AGS Users' Meeting 13

Decay Photon v2--Result

Statistical errors only.

20-60%: decay photon v2 and inclusive photon v2 are close together.

5-20% & 5-70%: inclusive photon v2looks systematically higher than decay photon v2.

(GeV)t

p0 2 4 6 8 10 12

(%

)2v

0

2

4

6

8

10

12

14

16

18

20

5-20%

2 v0π

2 vγdecay

2 vγinclusive

STAR

Prelim

inary

(GeV)t

p0 2 4 6 8 10 12

(%

)2v

0

2

4

6

8

10

12

14

16

18

20

5-70%

2 vγinclusive

2 vγdecay

2 v0π

STAR P

relimin

ary

(GeV)t

p0 2 4 6 8 10 12

(%

)2v

0

2

4

6

8

10

12

14

16

18

20

20-60%

2 v0π

2 vγdecay

2 vγinclusive

STAR

Prelim

inary

inclusive decaydirect decaydirect 2 2

2decay

, where 1

N NRv vv RR N

+−= =

−

06/07/2006 RHIC & AGS Users' Meeting 14

Conclusion & OutlookConclusion

Inclusive photon v2 is measured up to 10 GeV using scalar product and 4 particle cumulant methods. Results are reasonable and consistent.Decay photon v2 is calculated by simulation and compared to the inclusive photon v2.

What to do next?Use FTPC to measure the inclusive photon v2.π0 v2Add η contribution to decay photon v2 study.Other systematic errors.

06/07/2006 RHIC & AGS Users' Meeting 15

Back up

06/07/2006 RHIC & AGS Users' Meeting 16