Transcript Preliminary Result on Direct Gamma-Gamma HBT from Half Field 200GeV Au+Au Data

Preliminary Result on Direct
Gamma-Gamma HBT from Half
Field 200GeV Au+Au Data
Guoji Lin (Yale)
Haibin Zhang(BNL)
5/25/2016
STAR Collaboration Meeting
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Outline
• Data set: Half field Au+Au 200 GeV, about 3M events
after event cuts
• Cuts for TPC and EMC photon identification
• Preliminary results on gamma-gamma HBT
• Some further study on cuts
• Conclusions and what to do next
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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:
E
15
(
 1.5)%,   10 mrad
E
E
TPC:
E
 (2  E )%
E
We use the direct photon spectrum from PRC 69 (2004) 014903 multiplied by three.
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Cuts for TPC Photon Identification
TPC photons are reconstructed from conversions by pairing two oppositely
charged e+/e- global tracks.
Photon:
Event:
• vertex has to be found
• 5cm < conv. r < 200cm
• abs(vertexZ) < 50 cm
• DCAxy at conv.<1.5cm
• standard minimum bias or central
• DCAz at conv. < 1.3cm
trigger cut
• dip angle diff. < 0.03 rad
Track:
• -2<NSigmaElectron<4
• full opening angle < 0.4 rad
• fitPoints >= 12
• ∆ dip < 0.035 rad
• Hits/PossHits > 55%
• ∆ phi < 0.05 rad
• if (pt < 0.3) SDCA > 0
• imass < 0.012 GeV
Pion0:
• Two photons can not share the same (∆ dip and phi are the angular difference
electron or positron. We keep a
between the photon momenum and the
better one
vector from the primary vertex to the
conversion point)
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EMC Photon Reconstruction
StMuDstEvent
StEvent
StEmcADCtoEMaker
St_db_Maker
StPreEclMaker
StEpcMaker
StPhotonMaker
• StEmcADCtoEMaker: Calculate hit energy values from ADC signal.
• StPreEclMaker: Reconstruct clusters from hits.
• StEpcMaker: Reconstruct points from clusters.
In StPhotonMaker:
• correct the η with the real VertexZ
• E > 0.8 GeV
• BEMC tower : SMD η cluster : SMD φ cluster ≠ 1:0:0
• remove EMC points with an associated TPC global track: ∆phi<0.05,
∆eta<0.05
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 Peak from TPC-TPC Photon Pairs
0
200 GeV
minbias
200 GeV
central
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 Peak from TPC-TPC Photon Pairs
0
200 GeV
minbias
62 GeV
minbias
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 Peak from EMC-EMC Photon Pairs
0
200 GeV
minbias
200 GeV
central
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 Peak from EMC-EMC Photon Pairs
0
200 GeV
minbias
62 GeV
minbias
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 Peak from EMC-TPC Photon Pairs
0
200 GeV
minbias
200 GeV
central
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 Peak from EMC-TPC Photon Pairs
0
200 GeV
minbias
62 GeV
minbias
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C2  1 Plots
200 GeV
minbias
62 GeV
minbias
TPC-TPC
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EMC-EMC
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EMC-TPC
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How Strict Shall We Be in Global Track Matching?
200 GeV minbias:
∆phi<0.01, ∆eta<0.01
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∆phi<0.03, ∆eta<0.03
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∆phi<0.05, ∆eta<0.05
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What SMDs Tell Us?
EMC-EMC
EMC-TPC
BEMC tower : SMD η cluster : SMD φ cluster
BEMC tower : SMD η cluster : SMD φ cluster
≠ 1:0:0
= 1:1:1
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Conclusions and What to Do Next
• We analyze the half field 200 GeV Au+Au data and compare it
with the 62 GeV data.
 Minbias: very similar to the 62 GeV result.
 Central: difficult to see the π0 signal from EMC photons.
• Try to modify the code to deal with all towers together
to avoid the photon splitting.
• Find a tradeoff between statistics and noise.
• Improve the mixing for the TPC-TPC photon pairs.
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