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 –Measurement of final state  reconstruct initial state kinematics  x gluon with high precision for p T (γ) ~1% (?) and typical p T (jet) resolution ~25% (?) –Access to ΔG at low x gluon with minimal theory input Cons: –Rare process –Large background at low p T from π 0 /η→γγ q qg γ

Background to direct γ Aiming for threshold p T ~5 GeV/c  γ direct /π 0 ~1:40  challenging analysis! High p T  less statistics Heavily rely on clever software algorithms for γ/π 0 separation and specialized subdetectors: SMD at low p T and PRS at high p T

γ/π 0 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) –π 0 →γγ  double peak structure: main peak and peaklet (asymmetric π 0 decay) –Fit main peak and compute residual=data-fit –For given energy E, π 0 should have more residual than γ

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

Roadmap to γ-jet analysis Study performance of γ/π 0 discrimination algorithm with: –Single γ –Single π 0 –Pythia event (prompt γ subprocesses) –Real data –Isolated e as surrogate for γ  SMD response –Embedding (?)

Single thrown γ and π 0 10k γ/π 0 each sample STAR y2006 geometry z-vertex at 0 Flat in p T =10-30 GeV/c Flat in η= Quadratic y(x)= x 2

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

Background rejection vs. signal efficiency We start to lose efficiency with this method at higher γ energies.

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

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 is prefered 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…