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EEMC Photons

EEMC Photons. Pibero Djawotho (IUCF) Wednesday, October 17, 2007 STAR Analysis Meeting (BNL). Why photons?. Pros: p+p → γ +jet+X (qg Compton scattering)  large partonic a LL

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EEMC Photons

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  1. EEMC Photons Pibero Djawotho (IUCF) Wednesday, October 17, 2007 STAR Analysis Meeting (BNL)

  2. Why photons? • Pros: • p+p→γ+jet+X (qg Compton scattering)  large partonic aLL • Measurement of final state  reconstruct initial state kinematics  xgluon with high precision for pT(γ) ~1% (?) and typical pT(jet) resolution ~25% (?) • Access to ΔG at low xgluon with minimal theory input • Cons: • Rare process • Large background at low pT from π0/η→γγ q γ g q

  3. Background to direct γ • Aiming for threshold pT~5 GeV/c  γdirect/π0~1:40  challenging analysis! • High pT  less statistics • Heavily rely on clever software algorithms for γ/π0 separation and specialized subdetectors: SMD at low pT and PRS at high pT

  4. γ/π0 discrimination in the SMD • Basic idea: • Look at transverse shower profile in the SMD • γ and e transverse shower profile  single peak  narrow Gaussian+wide Gaussian with common centroid in each SMD plane (u and v) • π0→γγ double peak structure: main peak and peaklet (asymmetric π0 decay) • Fit main peak and compute residual=data-fit • For given energy E, π0 should have more residual than γ

  5. L. Bland (?) Functional form of fit function Real data (run=7155062/ev=254105)

  6. Roadmap to γ-jet analysis • Study performance of γ/π0 discrimination algorithm with: • Single γ • Single π0 • Pythia event (prompt γ subprocesses) • Real data • Isolated e as surrogate for γ SMD response • Embedding (?)

  7. Single thrown γ and π0 • 10k γ/π0 each sample • STAR y2006 geometry • z-vertex at 0 • Flat in pT=10-30 GeV/c • Flat in η=1.0-2.1 Quadratic y(x)=100+0.1x2

  8. 75% eff @ 75% rejection Background rejection vs. signal efficiency Use perp distance from quadratic to project in 1D Not quite the 80-80 from original proposal but this simulation has most up-to-date detector configuration.

  9. Background rejection vs. signal efficiency We start to lose efficiency with this method at higher γenergies.

  10. Pythia prompt γ production • Pythia prompt photon production subprocesses: • q+qbar → q+γ • f+fbar → γ+γ • q+g → q+γ (qg Compton scattering  dominant subprocess) • g+g → γ+γ • g+g → g+γ

  11. Run 6 p+p real data • Real data can be messy! • Makes it hard to obtain good parametrization for transverse shower shape • Nearby jet debris or detector noise?

  12. Isolated e to study SMD response(Based on work by Jan, Naresh, Justin) L2-gamma is prefered trigger ID!

  13. A quick look at the BSMD

  14. Conclusion and outlook • Much progress made on γ in EEMC • StGammaMaker (Jason, Renee, Pibero) stable for some time now • Still some ways to go to extract γ or γ-jet cross section • Next step: add LDA framework to SMD moment-based analysis…

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