1. The h double helicity asymmetry and cross section
Frank Ellinghaus
for the PHENIX Collaboration
09/30/06
RSC Meeting

2. Introduction
Access to polarized gluon distribution function via double helicity asymmetry
Status: A_LL for eta unmeasured, eta fragmentation function unknown > no predictions for A_LL yet
Naïve expectation: increased relative contribution of strange quarks and glue (when compared to the pi0) should lead to a different asymmetry (when compared to pi0)
-> Observation of difference could help to disentangle the contributions from \delta q (u,d,s) and \delta g

3. PHENIX Detector Photon identification:
Photon detection:
Electromagnetic calorimeter: PbSc + PbGl, Acceptance eta <|0.35|, phi = 2*90
Photon trigger: Threshold ~ 1.4 GeV
Luminosity:
Beam-Beam Counter (BBC), 3.1 < eta < 4.0
Photon identification:
E_PbSC > 0.1 GeV, E_PbGl > 0.2 GeV
Cut on Shower shape of cluster
(Charge veto cut: closest track in PC3 is considered, should be far away or very close (pair production originated from photon))
(TOF cut)

4. Eta reconstruction via 2 Photon decay
Invariant two photon mass: 0.48 GeV < M < 0.62 GeV
Energy asymmetry cut: E1-E2/ E1+E2 < 0.7
Pt > 2 GeV
Fit with Gauss+Pol3
|Z_vertex| < 30 cm

5. Invariant cross section
L = integrated Luminosity, based on Vernier scan using the BBC
BR= Branching ratio: eta -> 2 photons = \pm
f_Acc = acceptance function from MC (includes smearing)
eff_Trig (MB data) = Trigger efficiency of MB trigger
eff_Trig (ERT data) = (Trigger effi. MB) x (Trigger effi. ERT4x4c)
eff_rec = Correct for eta loss due to photon conversion (~6% in PBSC, ~8% in PbGl) x loss due to cut on shower shape (~4%)
N = N_measured

6. f_acc from fastMC Acceptance and smearing correction from MC
(also accounts for dead regions in EmCal, minimum cuts on photon energies,…)
Up to 10% acceptance in PbSc for high p_T etas

7. Photon/MB Trigger Efficiency
MB trigger efficiency about 80 %
Photon trigger efficiency roughly stable from 4 GeV on (single photon trigger threshold is set to 1.4 GeV)

8. Eta cross section
Maybe the relative contributions of quarks and gluons are similar after all?
-> Need more Statistics -> Run 5 eta cross section analysis in progress
-> Extraction of frag. functions from e+e- and pp in progress

9. Asymmetry calculation
Relative Luminosity R from counts in BBCs
Asymmetry in bands around eta peak is consistent with each other

10. Background corrected Asymmetry

11. Single Spin Asymmetries

12. Comparison to pi0 and Outlook
Including Run 6 eta will result in comparable statistical power compared to present Run 5 preliminary pi0 data.

13. Summary Asymmetry of eta in Run 5 has been extracted (Step zero)
Run 3 cross section available, Run 5 result soon
Promises for FF’s have been made….