The double helicity asymmetry and cross section
Frank Ellinghausfor the PHENIX Collaboration
09/30/06RSC Meeting
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Introduction
• 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
22~ qaGqaGaA qqqgggLL
Access to polarized gluon distribution function via double helicity asymmetry
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PHENIX Detector
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)
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
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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
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Invariant cross section
yp
ypN
effefffBRLppd
dE
T
T
recTrigAccT
),(1111
2
13
3
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
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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
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Photon/MB Trigger Efficiency
Photon trigger efficiency roughly stable from 4 GeV on (single photon trigger threshold is set to 1.4 GeV)
MB trigger efficiency about 80 %
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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
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Asymmetry calculation
NRN
NRN
PPA
YBLL
1
L
LR
Asymmetry in bands around eta peak is consistent with each other
Relative Luminosity R from counts in BBCs
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Background corrected Asymmetry
r
ArAA
BGLL
BGLL
LL
1
BG
BG
NN
Nr
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Single Spin Asymmetries
NRN
NRN
PAL
1
L
LR
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Comparison to pi0 and Outlook
Including Run 6 eta will result in comparable statistical power compared to present Run 5 preliminary pi0 data.
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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….