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….