# Presentation: 035247

## Direct Photon HBT from 200GeV Au+Au Data

- Guoji Lin (Yale)
- Haibin Zhang(BNL)

- Guoji Lin        YaleUniversity

## Outline

- Data set: ~5 million Au+Au 200 GeV minimum bias events
-  Motivation
-  TPC photons from conversion
-  cuts
-  results
-  BEMC photons from new cluster finder
-  cuts
-  results
-  Results from EMC-TPC photon pairs
-  Conclusions

- Guoji Lin        YaleUniversity

## Motivation

- HBT signal for 1 billion simulated 200 GeV Au+Au central events with TPC&ECal from Alexei Chikanian.
- Big one: HBT correlation function C2, where |yγ|<1, Eγ>0.5 GeV. Solid line is a fit by function
- C2= P1·exp(-(P2*Qinv/0.19733)2)+P3.
- Small one: Efficiency of the EMC and TPC detector.
- Resolutions:
- EMC:
- TPC:

- We use the direct photon spectrum from PRC 69 (2004) 014903 multiplied by three.

- Guoji Lin        YaleUniversity

## Algorithm for Photon ID in TPC

-    Photons can not be detected directly in TPC.
-    A high energy photon hitting a charged particle might convert into an electron positron pair, which can be detected in TPC.

-  The algorithm we are using is that:
- Loop over all global tracks in TPC and only keep the electrons and positrons.
- For each pair of electron and positron tracks, apply cuts to check if they are qualified for the two daughter particles of a photon.
- Combine those qualified electrons and positrons and reconstruct the photons before conversion.

- Guoji Lin        YaleUniversity

## Cuts for Global Track Reconstruction

-   TPC photons are reconstructed from conversions by pairing two oppositely charged e+/e- global tracks.

- Event:
- vertex has to be found
- abs(vertexZ) < 30 cm
- standard minimum bias trigger cut
- Track:
- -2<NSigmaElectron<4
- fitPoints >= 12
- Hits/PossHits > 55%
- if (pt < 0.3) SDCA > 0
- Pion0:
- Two photons can not share the same electron or positron. We keep one with smaller DCAxy at conv.

- Photon:
- 5cm < conv. r < 200cm
- DCAxy at conv.<1.5cm
- DCAz at conv. < 1.3cm
- dip angle diff. < 0.03 rad
- full opening angle < 0.4 rad
- ∆ dip < 0.035 rad
- ∆ phi < 0.05 rad
- imass < 0.012 GeV
- (∆ dip and phi are the angular difference between the photon momenum and the vector from the primary vertex to the conversion point)

- Guoji Lin        YaleUniversity

## TPC Photon Spectrum

-  raw spectrum
-  a little higher than the 62 GeV spectrum

- Guoji Lin        YaleUniversity

## π0 signal from TPC-TPC Photon Pairs

- Fitted by Gaussian function + linear background.
- Resolution is about 10 MeV.

- Guoji Lin        YaleUniversity

## Correlation Function C2-1

- Suggested criterion for close tracks in mixed event:
-  have equal *p**z* and *p**T*, so two helices are parallel.
-  the conversion point of one track is on another track.

- Guoji Lin        YaleUniversity

## EMC Photon Reconstruction

- StMuDstEvent

- StEvent

- St_db_Maker

- StEmcADCtoEMaker

- StPreEclMaker

- StEpcMaker

-  StEmcADCtoEMaker: Calculate hit energy values from ADC signal.
-  StPreEclMaker: Reconstruct clusters from hits.
-  StEpcMaker: Reconstruct points from clusters.
-  StPhotonMaker: Apply some cuts and save information for offline analysis

- StPhotonMaker

- Guoji Lin        YaleUniversity

## New Cluster Finder (1)

- Old cluster finder searches for clusters within each module, which might underestimate photon’s energy and cause cluster splitting.
- New cluster finder will ignore the module boundary.

| Detector | Det ID | Eseed (GeV) | Eadd (GeV) | Eall (GeV) | Max size |
| --- | --- | --- | --- | --- | --- |
| BEMC | 1 | 0.4 | 0.001 | 0.7 | 4 (2×2) |
| SMDE | 3 | 0.08 | 0.001 | 0.4 | 5  |
| SMDP | 4 | 0.08 | 0.001 | 0.4 | 5  |

- Cluster parameters:

- Guoji Lin        YaleUniversity

## New Cluster Finder (2)

- For BEMC:
- Look for towers with energy deposited greater than all neighboring eight towers. We call them peaks.
- We require that every two peaks are at least three towers away.
- Add neighboring towers from the most energetic one to a peak up to the maximal size.

- For SMDE and SMDP:
- Sort all hits by energy.
- Loop over all hits from the most energetic one and group neighboring hits as a cluster, up to its maximal size.

- The only difference between SMDE and SMDP is the definition of neighboring hits. For SMDP the last hit in one tower and the first hit in the next tower are also neighbors.

- Guoji Lin        YaleUniversity

## StEpcMaker and StPhotonMaker

- In StPhotonMaker:
-  correct the *η* with the real VertexZ
-  E > 0.8 GeV
-  BEMC tower : SMD *η* cluster : SMD φ cluster = 1:1:1
-  remove EMC points with an associated TPC global track: ∆phi<0.05, ∆eta<0.05

- In StEpcMaker:
-  match SMDE and SMDP clusters under every 2×2 tower square.
-  require only one EMC point under a 2×2 tower square.

- Guoji Lin        YaleUniversity

## EMC Photon Spectrum

-  raw spectrum
-  in low pT region, central collision has less photons because our strict cut.
-  more high pT photons than TPC.

- Guoji Lin        YaleUniversity

## π0 signal from EMC-EMC Photon Pairs

- The π0 signal is invisible because of the tight cuts.

- Guoji Lin        YaleUniversity

## Correlation Function C2-1

- It goes all the way down and the π0 signal is invisible.

- Guoji Lin        YaleUniversity

## π0 signal from EMC-TPC Photon Pairs

- new cluster finder

- old cluster finder

- Guoji Lin        YaleUniversity

## Correlation Function C2-1

- Guoji Lin        YaleUniversity

## Opening Angle - Qinv

- Guoji Lin        YaleUniversity

## TPC Photon E - Qinv

- Guoji Lin        YaleUniversity

## EMC Photon E - Qinv

- Guoji Lin        YaleUniversity

## TPC Photon Eta - Qinv

- Guoji Lin        YaleUniversity

## EMC Photon Eta - Qinv

- Guoji Lin        YaleUniversity

## TPC Photon Phi - Qinv

- Guoji Lin        YaleUniversity

## EMC Photon Phi - Qinv

- Guoji Lin        YaleUniversity

## Conclusions

-  We discuss the methods of TPC and EMC photon identification.
-  A new BEMC cluster finder is introduced.
-  The peak of the correlation function at low Qinv is studied. It possibly comes from some angular correlations between TPC and EMC photons.

- Guoji Lin        YaleUniversity

## Slide 26

- Guoji Lin        YaleUniversity