Photon Identification via Conversion in 62 GeV Au Au Collisions

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Photon Identification via Conversion in 62 GeV
AuAu Collisions

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

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Outline

• Data set.
• AuAu 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

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V0 Structures
Typical V0 structure particle identification for
neutral particle which weakly decays into two
oppositely charged particles.
DCADaughterPVertex gt0.7cm
DCAV0Daughters lt1.0cm
DCAV0PVertex lt1cm
open anglelt30 degrees

decay lengthgt2cm
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Topological Cuts of V0 Structure

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

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Armenteros ( ) Cut (1)
First introduced in J. Podolanski and R.
Armenteros, Phil. Mag 45, 13 (1954).
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Armenteros ( ) Cut (2)

We set the cut for photons as lt
0.01 GeV.
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dE/dx Cut

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

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Mass Cut
and
Black ones are photons without mass cuts. Red
ones are actually that pass the
Pta and dE/dx cuts.
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Photon Distribution
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Photon Spectra

• Raw photon spectra
• About 10 times less than Alexs result.

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Peak
The two photons of each pion0 have different
positive and negative daughter track IDs.
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Cuts for Global Track Reconstruction

• Photon
• 10cm lt conv. r lt 200cm
• DCAxy at conv.lt1.5cm
• DCAz at conv. lt 1.3cm
• dip angle diff. lt 0.03 rad
• full opening angle lt 0.4 rad
• ? dip lt 0.035 rad
• ? phi lt 0.05 rad
• imass lt 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)

• Event
• vertex has to be found
• abs(vertexZ) lt 30 cm
• Track
• -2 lt dE/dx-dev lt 4
• fitPoints gt 12
• Hits/PossHits gt 55
• if (pt lt 0.3) SDCA gt 0
• Pion0
• PosTrackID1?PosTrackID2
• NegTrackID1?NegTrackID2

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Photon Distribution
Black line is the result of the global track
combination method. Red line is the rescaled
result of the V0 structure method.
Now we have more than 1 photon per event.
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photon reconstruction efficiency
From Alexander Wetzlers result
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Photon Spectra

• With photon reconstruction efficiency
  correction.
• Consistent with Alexs result.

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Peak
About 70 times as many as that from V0 structure
method.
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Spatial Distribution of Conversion Point
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Q Variables and
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We have to further check why it is not symmetric.
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We dont know what the peak around the origin is.
Is it the HBT signal??
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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 AuAu 200 GeV data.