# Presentation: 658195

## Photon Identification via Conversion in 62 GeV Au+Au Collisions

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

- Guoji Lin        Yale University

## Outline

-  Data set.
-  Au+Au 62 GeV: 8M events after event cuts, minium bias
-  Photon reconstruction via V0 structure (idea from Gene van Buren).
-  cuts
-  results
-  Photon reconstruction via global track combination (lots of help from Alexei Chikanian, mainly refer to the work of Ian J. Johnson and Alexander Wetzler).
-  cuts
-  results
-  Preliminary results on gamma-gamma HBT
-  Conclusions

- Guoji Lin        Yale University

## V0 Structures

- Typical V0 structure: particle identification for neutral particle which weakly decays into two oppositely charged particles.

- *

- open angle<30 degrees

- DCAV0Daughters
- <1.0cm

- decay length>2cm

- DCAV0PVertex
- <1cm

- DCADaughterPVertex
- >0.7cm

- Guoji Lin        Yale University

## Topological Cuts of V0 Structure

-  Distance of closest approach (DCA) of two daughter tracks < 1cm.
-  Decay length of photon > 2cm.
-  DCA between the reconstructed photon and the primary vertex < 1cm.
-  DCA between the daughter tracks and the primary vertex > 0.7cm.
-  The open angle between the two daughter tracks < 30 degrees.
-  Number of hits to reconstruct the daughter tracks > 15.
-  The momentum magnitudes of two daughter particles > 0.1 GeV.
-  The absolute value of the photon rapidity < 0.5.

- Guoji Lin        Yale University

##       Armenteros (      ) Cut (1)

- First introduced in: J. Podolanski and R. Armenteros, Phil. Mag 45, 13 (1954).

- Guoji Lin        Yale University

##       Armenteros (      ) Cut (2)

**We set the        cut for photons as:        < 0.01 GeV.**

- Guoji Lin        Yale University

## dE/dx Cut

-  Plots are from primary tracks.
-  We require that               .

- Guoji Lin        Yale University

## Mass Cut

- and

- Black ones are photons without mass cuts. Red ones are actually
-                  that pass the Pta and dE/dx  cuts.

- Guoji Lin        Yale University

## Photon        Distribution

- Guoji Lin        Yale University

## Photon Spectra

-  Raw photon spectra
-  About 10 times less than Alex’s result.

- Guoji Lin        Yale University

##    Peak

- The two photons of each pion0 have different positive and negative daughter track IDs.

- Guoji Lin        Yale University

## Cuts for Global Track Reconstruction

- Event:
- vertex has to be found
- abs(vertexZ) < 30 cm
- Track:
- -2 < dE/dx-dev < 4
- fitPoints >= 12
- Hits/PossHits > 55%
- if (pt < 0.3) SDCA > 0
- Pion0:
- PosTrackID1≠PosTrackID2
- NegTrackID1≠NegTrackID2

- Photon:
- 10cm < 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        Yale University

## Photon        Distribution

- Now we have more than 1 photon per event.

- Black line is the result of the global track combination method. Red line is the rescaled result of the V0 structure method.

- Guoji Lin        Yale University

## photon reconstruction efficiency

- From Alexander Wetzler’s result:

- Guoji Lin        Yale University

## Photon Spectra

-  With photon reconstruction efficiency correction.
-  Consistent with Alex’s result.

- Guoji Lin        Yale University

##    Peak

- About 70 times as many as that from V0 structure method.

- Guoji Lin        Yale University

## Spatial Distribution of Conversion Point

- Guoji Lin        Yale University

## Q Variables and 

- Guoji Lin        Yale University

- Guoji Lin        Yale University

- Guoji Lin        Yale University

- We have to further check why it is not symmetric.

- Guoji Lin        Yale University

- Guoji Lin        Yale University

- We don’t know what the peak around the origin is. Is it the HBT signal??

- Guoji Lin        Yale University

## Conclusions

-  The method using global track combination is much more effective than the one using V0 structure.
-  There is a strong peak around the origin in the C2-1 plot, which still needs further check.
- We will continue to work on the Calorimeter photons and the Au+Au 200 GeV data.

- Guoji Lin        Yale University