# Presentation: 035243

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

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

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## Outline

- Data set: ~5 million Au+Au 200 GeV fullfield minimum bias events
-  Motivation
-  TPC photons from conversion
-  BEMC photons from new cluster finder
-  Direct photon HBT study from EMC-TPC photon pairs
-  Conclusions

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## 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.

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## 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.

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## Cuts for TPC photon 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

- 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
- Two photons can not share the same electron or positron. We keep one with smaller DCAxy at conv.

- (∆ dip and phi are the angular difference between the photon momentum and the vector from the primary vertex to the conversion point)

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## TPC Photon Spectrum

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

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## π0 signal from TPC-TPC Photon Pairs

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

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## 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

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## New Cluster Finder (1)

- Motivation:
-  Old cluster finder searches for clusters within each single module, so it might underestimate photon’s energy and cause cluster splitting.
-  New cluster finder will ignore the module boundary. We try to obtain pure photons and investigate the unknown peak of the correlation function at low Qinv.

| 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:

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## 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 be 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.

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## 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
-  no two photons are in the same tower or in two neighboring towers

- In StEpcMaker:
-  match SMDE and SMDP clusters under every 2×2 tower square.
-  energy can still be split if more than one smd clusters underneath.

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## EMC Photon Spectrum

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

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## π0 signal from EMC-TPC Photon Pairs

- new cluster finder

- old cluster finder

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## Correlation Function C2-1

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## TPC Photon E - Qinv

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## EMC Photon E - Qinv

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## Opening Angle - Qinv

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## C2 – EMC Photon E 

- All TPC photons are used.
- The energy of EMC photons does not affect the correlation function.

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## C2 – TPC Photon E

- All EMC photons are used.
- Do the high energy TPC photons bring the angular correlation?

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## 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.

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## Slide 21

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