EEMC PhotonsPibero Djawotho (IUCF)

Wednesday, October 17, 2007

STAR Analysis Meeting (BNL)

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

qg

γ

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

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

L. Bland (?)

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Functional form of fit function

12 5.3

Real data (run=7155062/ev=254105)

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 (?)

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

Background rejection vs. signal efficiency

75% eff @75% rejection

Use perp distancefrom quadratic toproject in 1D

Not quite the 80-80from original proposalbut this simulation hasmost up-to-date detectorconfiguration.

Background rejection vs. signal efficiency

We start to loseefficiency withthis method athigher γ energies.

Pythia prompt γ productionPythia 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

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?

Isolated e to study SMD response(Based on work by Jan, Naresh, Justin)

L2-gamma isprefered trigger ID!

A quick look at the BSMD

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…