The h double helicity asymmetry and cross section Frank Ellinghaus for the PHENIX Collaboration 09/30/06 RSC Meeting
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
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)
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
Invariant cross section L = integrated Luminosity, based on Vernier scan using the BBC BR= Branching ratio: eta -> 2 photons = 0.3943 \pm 0.0026 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
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
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)
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
Asymmetry calculation Relative Luminosity R from counts in BBCs Asymmetry in bands around eta peak is consistent with each other
Background corrected Asymmetry
Single Spin Asymmetries
Comparison to pi0 and Outlook Including Run 6 eta will result in comparable statistical power compared to present Run 5 preliminary pi0 data.
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….