Prompt Photon Cross Section from Proton

Prompt Photon Production from Proton - proton
Collisions at √s = 62.4 GeV in PHENIX
( PHENIX実験における重心系62.4 GeVでの陽子-陽子衝突からの
直接光子の生成断面積の測定 )
JPS meeting
March 26th, 2008
26pZF-11
Kohichi Sakashita ( Tokyo Tech )
for the PHENIX Collaboration
1
Contents
1.
2.
3.
4.
5.
Introduction
PHENIX detector and data set
Method of prompt photon measurement
Result
Summary
2
1-1. Introduction
• Production of prompt photon
proton
– Quark - gluon scattering is dominant
sub-process at pp collision in s = 62.4 GeV
• The related experiment
gluon
proton
quark
prompt
– PHENIX s = 200 GeV
photon
– R806, AFS, CCOR and CMOR using ISR collider at s = 63 GeV in
CERN
• Test the applicability of perturbative QCD (pQCD)
– Comparing the cross section of measurement to the one of pQCD
calculation
• pQCD calculation in qg scattering :
d pp(qg)X
ˆ qgX
  dx1  dx2  q(x1)  g(x2 ) 
dpT
q(x), g(x) : PDF for quark, gluon
ˆ:

sub-process cross section
– Once the applicable range of pQCD is determined, the framework of
pQCD can be used to calculate other quantities of interest, in particular
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
A
LL
1-2. Introduction
• Double helisity asymmetry ( ALL )
√s = 200 GeV
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
pT
xT 
x
s /2
0.02
0.04
0.06
0.08
0.1
xT

• Comparing to ALL in 200 GeV, large Bjorken’s x can be
reached at 62.4 GeV
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2. PHENIX Detector and Data Set
• PHENIX central arm detector
  = 90° x 2, || < 0.35
• Data set
– 2006 pp run
– Integrated luminosity : 0.065 pb-1
• Basic analysis cuts
• EMCal&BBC trigger
• Vertex cut |z| < 30 cm
• Remove 2 edge towers, dead and hot
towers
– Event selection
• pT > 2 GeV/c
• Shower shape cut
• Charge veto with PC3
Proton beam
prompt 
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3-1. Method of Prompt Photon Measurement
• Main issue of prompt photon
measurement
– Evaluation of systematic uncertainties
 0 extraction and so on
– Prompt photon yields is small signal of all
photon
• About 10 % at 3 GeV/c
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3-2. Method of Prompt Photon Measurement
Nall 

–
N prompt 
x A*(1+R)* N tag
’   decay photon
–
N tag
x R* N tag
The ratio of ’and 0 production to
production
The ratio of branching ratio of photon of
’and 0 to the branching ratio of
photon of 
•

A =  σi
σ


i : ’ 
Br i  / Br ->
decay photon
–
Detecting two photon ( N tag )
•
Reconstruction invariant mass
detecting two photon from pi0 decay – Missing one photon
missing one photon from pi0 decay
• Evaluated by fast MC simulation
• The ratio ( R ) of the missing one
eta, eta' and omega decay photon
photon to detecting two photon
prompt photon
N prompt  = Nall - (1+A)*(1+R)*N tag
By measuring Nall  and N tag, one can extract small Nprompt signal
(  tagging method )
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
4-1. Result - cross section
• Cross section :
corr
d 3
1 1 N
E 3
 
dp 2pT L pT 
• pQCD calculation with NLO
and CTEQ6M PDF agrees
with experiment within
theoretical uncertainty and
experimental uncertainty
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are needed to see this picture.
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4-2. Result - comparison of PHENIX √s = 200
GeV and ISR experiments
Open black circle : PHENIX at √s = 200 GeV ( 2005 year )
The others : ISR experiments at √s = 63 GeV
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• Results of ISR agree with this result
within the experimental uncertainty
• Cross section slope at √s = 200 GeV is
gentler than one at √s = 62.4 GeV
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5. Summary
• Test the applicability of pQCD calculation
• Data set
– pp collision at 2006 year
 s = 62.4 GeV
– Integrated luminosity : 0.065 pb-1
• Prompt photon yields as a function of pT are extracted by the 0
tagging method ( N prompt  = Nall - (1+A)*(1+R)*N tag ) with
PHENIX central arm detector ( = 90° x 2, || < 0.35 )
• pQCD calculation with NLO and CTEQ6M PDF agrees with
experiment within theoretical uncertainty and experimental
uncertainty
• Results of ISR agree with this result within the experimental
uncertainty
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Back up
11
• Main issue of prompt photon
measurement
– Evaluation of systematic
uncertainties
 0 extraction and so on
– Prompt photon yields is small
signal of all photon
Prompt photon / All photon
3-1. Method of Prompt Photon Measurement
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4. Result - systematic errors
• Error in N tag
– Fit ( Gauss + pol.3 ) to the region of pi0 mass peak
to extract 0 photon with 3 ( 105 < M <
165MeV/c2 )
– Difference of between N tag with pol.2 and N tag
with pol.3 and between N tag with 3 and N tag
with 4 is assigned as the error
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
– 3.4 % to the N tag with pol.3 and 3at 2
GeV/c
2.8 % to the N tag with pol.3 and 3 at 3.75
GeV/c
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corr
N

N prom pt
acceptance energysm ering conversion BBCtrigger EMCaltrigger
corr
d 3
1 1 N
E 3
 
dp 2pT L dpT
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4. Result - systematic errors
W = prompt / inclusive
Error of C is
scaled by 1/W - 1
Error in 1+A
Neutral hadron contamination
Dalitz decay pi0 partner photon
conversion loss
Secondary origin
Error in Ntag
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Energy scale uncertainty
1+R (acceptance)
Luminosity uncertainty
Conversion error
BBC bias uncertainty
1+R (Minimum E cut)
1+R (Pi0 cross section slope)
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TIFF (Uncompressed) decompressor
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GS-C, ΔG = 1
present
x-range
GS-C
x
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