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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

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Contents

1. Introduction

2. PHENIX detector and data set

3. Method of prompt photon measurement

4. Result

5. Summary

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1-1. Introduction• Production of prompt photon

– Quark - gluon scattering is dominantsub-process at pp collision in√s = 62.4 GeV

• The related experiment– PHENIX √s = 200 GeV– 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 calcul

ation• pQCD calculation in qg scattering :

– Once the applicable range of pQCD is determined, the framework of pQCD can be used to calculate other quantities of interest, in particular ALL

prompt photon

proton

gluon

quarkproton

€

dσ pp(qg )→γX

dpT

≅ dx1 ⋅dx2 ⋅q(x1)∫ ⋅g(x2) ⋅ ˆ σ qg →γX

q(x), g(x) : PDF for quark, gluon

　 : sub-process cross section

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ˆ σ

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1-2. Introduction• Double helisity asymmetry ( ALL )

• Comparing to ALL in 200 GeV, large Bjorken’s x can be reached at 62.4 GeV

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0.02 0.04 0.06 0.08 0.1 xT

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xT =pT

s /2→ x

√s = 200 GeV

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Proton beam

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

prompt γ

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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

N prompt γ = Nall γ- (1+A)*(1+R)*Nγ tag

By measuring Nall γand Nγtag, one can extract small Nprompt γsignal

( tagging method )

detecting two photon from pi0 decaymissing one photon from pi0 decay eta, eta' and omega decay photonprompt photon

Nγ tag

N prompt γ

Nall γ

x R* Nγ tag

x A*(1+R)* Nγ tag

’ ωdecay photon– The ratio of ’and ω0 production to

production – The ratio of branching ratio of photon of

’and ω0 to the branching ratio of photon of

• A = Σσiσ Br i γ / Br -> γγ

i : ’ ω

decay photon– Detecting two photon ( N

γ tag )• Reconstruction invariant mass

– Missing one photon• Evaluated by fast MC simulation• The ratio ( R ) of the missing one ph

oton to detecting two photon

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4-1. Result - cross section

• Cross section :

• pQCD calculation with NLO and CTEQ6M PDF agrees with experiment within theoretical uncertainty and experimental uncertainty

€

Ed3σ

dp3=

1

2πpT

⋅1

L⋅

ΔNγcorr

ΔpTΔη

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4-2. Result - comparison of PHENIX √s = 200 GeV and ISR experiments

• 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

Open black circle : PHENIX at √s = 200 GeV ( 2005 year )

The others : ISR experiments at √s = 63 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 γ = Nall γ- (1+A)*(1+R)*N

γ 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

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are needed to see this picture.

3-1. Method of Prompt Photon Measurement

• Main issue of prompt photon measurement– Evaluation of systematic

uncertainties0 extraction and so on

– Prompt photon yields is small signal of all photon

Pro

mp

t p

hot

on /

All

ph

oton

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4. Result - systematic errors

• Error in Nγ tag

– Fit ( Gauss + pol.3 ) to the region of pi0 mass peak to extract 0 photon with 3 ( 105 < Mγγ < 165MeV/c2 )

– Difference of between Nγ tag with pol.2 and N

γ tag with pol.3 and between N

γ tag with 3 and Nγ tag w

ith 4 is assigned as the error

– 3.4 % to the Nγ tag with pol.3 and 3at 2 Ge

V/c

2.8 % to the Nγ tag with pol.3 and 3 at 3.75

GeV/c

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€

Nγcorr =

N promptγ

ε acceptance ⋅ε energysmering ⋅ε conversion ⋅ε BBCtrigger ⋅ε EMCaltrigger

Ed3σ

dp3=

1

2πpT

⋅1

L⋅

Nγcorr

dpT

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4. Result - systematic errorsNeutral hadron contamination

Secondary origin

Error in 1+A

Dalitz decay pi0 partner photon conversion loss

Error in Nγtag

1+R (acceptance)

1+R (Minimum E cut)

1+R (Pi0 cross section slope)

Energy scale uncertainty

Luminosity uncertainty

Conversion error

BBC bias uncertaintyQ

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W = prompt / inclusive Error of C is scaled by 1/W - 1

16x

xDG (x)

present x-range

GS-C

GS-C, ΔG = 1