# Presentation: 602954

## 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*

## Outline

- Motivation
- Analysis
- Photon identification
- Inclusive photon v2
- Decay photon v2
- Conclusion & outlook

- RHIC & AGS Users' Meeting

- 06/07/2006

## Motivation (I)

-  QGP is an important source of direct photons. Studying direct photon v2 involving 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.

- RHIC & AGS Users' Meeting

- 06/07/2006

## 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.

- RHIC & AGS Users' Meeting

- 06/07/2006

## Motivation (III)

- 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.

- RHIC & AGS Users' Meeting

- 06/07/2006

## Data Set

- 3.2M 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 pt distributions from high tower events and minbias events using the same cuts.

- Black: high tower event
- Red: minbias event

- RHIC & AGS Users' Meeting

- 06/07/2006

## Photon identification

- The 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<1 GeV.
-  High tower trigger is required: each EMC photon has at least one tower hit with energy > 3 GeV*sin(θ).

- RHIC & AGS Users' Meeting

- 06/07/2006

## Inclusive Photon v2 Calculation

- Non-flow effect has to be considered carefully in high pt range.
- 4 particle cumulant method.
- Scalar product method.

- Non-flow effect

- <uQ*>

- <uQ*> of charged particles in different collision systems.
- STAR, Phys. Rev. C 72 (2005) 014904

- RHIC & AGS Users' Meeting

- 06/07/2006

## Correction of Inclusive Photon v2

- The 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<φ<2pi, 0<η<1), their average v2 is v2;ch, then the charged particle distribution is:

- For one charged particle, the probability that it is in a 0.1*0.1 box centered at (η0,φ0), which is the position of a photon, is:

- So for all charged particles, if none of them is in this 0.1*0.1 box, the probability is:

- So after the charged particle veto cut, the photon distribution will be:

- RHIC & AGS Users' Meeting

- 06/07/2006

## 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

- RHIC & AGS Users' Meeting

- 06/07/2006

## 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.

- RHIC & AGS Users' Meeting

- 06/07/2006

## Decay Photon v2--Simulation

- 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).

- RHIC & AGS Users' Meeting

- 06/07/2006

## 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 v2 looks systematically higher than decay photon v2.

- RHIC & AGS Users' Meeting

- 06/07/2006

## Conclusion & Outlook

- Conclusion
- 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 v2
- Add η contribution to decay photon v2 study.
- Other systematic errors.

- RHIC & AGS Users' Meeting

- 06/07/2006

- Back up

- RHIC & AGS Users' Meeting

- 06/07/2006

- RHIC & AGS Users' Meeting

- 06/07/2006