# Presentation: 739433

## Elliptic Flow of Inclusive Photons in CuCu and AuAu collisions at √sNN = 200 GeV

- Rashmi Raniwala
- for the STAR Collaboration
- Department of Physics
- University of Rajasthan, Jaipur

## PLAN OF THE TALK

- Introduction to Photon Multiplicity Detector at STAR
- Method of analysis to obtain v2 of photons
- Details of data analysed
- Results for AuAu & CuCu collisions
- Conclusions

- PMD

## Photon Multiplicity Detector

- PMD is at 539 cm from nominal interaction point, outside the magnet, covering pseudo-rapidity region  of       -3.7 to -2.3
- PMD has two sensitive layers Each layer is an array of 41472 hexagonal proportional chambers ( cells), with a 3 radiation thick Pb plate sandwiched in-between
- Photons convert in the Pb plate forming a pre-shower: detected in the downstream sensitive layer depositing energy over a group of cells.

## PMD at STAR

- For AuAu (CuCu) data, 67% (58%)of the total number of cells were used for further analyses.
- The ADC signal read in each hexagonal cell (hit) is normalized for gain and the hits are then grouped into clusters
- Charged particles falling on the PMD also deposit energy: forming clusters. Since there is no shower formation, these clusters are spread over fewer cells & deposit lesser energy compared to a photon
- Choosing clusters with Ncell > 1 & Cluster_ADC > 3*MIP_ADC gives a photon sample with  ~ 60% purity and ~ 60% efficiency which is called as N in this analysis

-     The dominant source of the photons on PMD is neutral pions

## Method of Analysis

- Event centrality determined using multiplicity of charged particles within one unit of rapidity at mid-rapidity measured in Time Projection Chamber
- v2 for photons is studied with respect to the event plane (2) determined using selected charged particles at mid-rapidity: measured in the TPC of STAR
- The rapidity gap between TPC and the PMD is more than 1.5 units. This minimises non-flow correlations between photons in PMD and the particles used for event plane determination from TPC
- Event plane resolution obtained by correlation between event planes of sub-events (sub-event planes): positive (negative) eta particles in one (the other) sub-event. Small eta gap between sub-events minimises non-flow correlation: Gap size affects event plane resolution correction factor (RCF)

## Data Statistics for the Present Analysis

- at √sNN = 200 GeV 2005 Data
- ( RHIC Run6)
- 1.38M events in centrality range 0-54% most central

- AuAu Collisions
-  at √sNN = 200 GeV
- 2007  Data
- 2.1M events in centrality range  0-60% most central

- In all following results, the statistical errors are within the symbol size.

## AuAu √s = 200 GeV: Event Plane Resolution Correction Factors 

- Used for present Analysis
- 2.4 rapidity away from PMD

## v2 for γ at Forward Rapidity

- Variation in v2 of with
- Rapidity in PMD coverage

- Variation in v2 of  with
- Event centrality

- v2 for γ at Forward Rapidity

- Statistical errors within the symbol size

## Systematic Errors in v2 for  in PMD (AuAu)

- Due to variation in event plane 2 determined from
- Full TPC
- Positive half of TPC
- Negative half of TPC
- A different eta-gap 
- Contributes ~2% systematic error

- Due to variation in efficiency and purity of 
- 3MIP→ ~60% purity
- 4MIP→ ~65% purity
- Contributes ~5% systematic error

## CuCu collisions at √sNN = 200GeV

- Variation in v2 of  with centrality

- Variation in v2 of  with rapidity

## v2 of  (in PMD) and v2 of charged particles 

- These are pT integrated values. The effect on photon v2 due to 0 decay needs to be estimated. The  results need to be corrected for efficiency and contamination.

## Comparing v2 for  in AuAu  and CuCu collisions at √sNN = 200 GeV 

- v2 in  for both AuAu and CuCu show similar variation with rapidity. The ratio is seen to be constant as a function of rapidity ~1.4

## Scaled v2 for inclusive  CuCu and AuAu collisions (√sNN = 200 GeV)

-  v2 scaled with two particle eccentricity reflects the strength of collective expansion, and as expected, is seen to increase with centrality.
-  For the same Npart, both CuCu and AuAu show same values.
-  For large values of Npart, scaled v2 tends to saturate, as expected in an equilibrium scenario.

## Conclusions

**Centrality and rapidity dependence of v****2 **** of photons has been studied in AuAu and CuCu collisions at √s****NN****=**** ****200GeV at forward rapidities. **
**v****2**** in photons increases from central to peripheral collisions up to 40% most central collision for both AuAu and CuCu collisions. **
**v****2**** of photons decreases with increasing y – y****CM**
**v****2****/********{2} for AuAu and CuCu collisions scales with number of participants. This scaling is similar to that seen in charged particles at mid-rapidity as well as at forward rapidity.**
** ****The scaled v****2**** shows little sensitivity to collision centrality for large values of N****part****, as expected in an equilibrium scenario.**

## Thank You

## Extra Slides

## Estimated v2 for  at = 0 

-  in AuAu

## Systematic errors in v2:due to variation in 2

## Systematic errors in v2:due to variation in purity of 

## CuCu 200GeV: Event Plane Resolution Correction Factor: RCF

## CuCu 200 GeV:Systematic error in v2 due to variation in 2

## CuCu 200 GeV:Systematic error in v2 due to variation in 2

## Two –particle eccentricity {2} v/s Npart for AuAu and CuCu collisions at 200 GeV

- Two –particle eccentricity {2} v/s Npart for AuAu and CuCu collisions at 200 GeV