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Ellis eta ecross section

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					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

              
     ALL           ~ a gg G 2  aqg qG  aqq q 2
             

•    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


                                                                          2
            PHENIX Detector
                                        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)
                                                                               3
    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




                                                          4
   Invariant cross section

  d 3 1 1 1 1              1             N (pT , y )
E 3 
  d p 2 pT L BR f Acc  effTrig  eff rec pT y

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


                                                                        5
          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



                                                                               6
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)




                                                         7
              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

                                                                               8
    Asymmetry calculation

       1 N   R  N                    L     
ALL                                    R  
      PB PY N    R  N                L
Relative Luminosity R from counts in BBCs




 Asymmetry in bands around eta peak is consistent with each other

                                                                    9
Background corrected Asymmetry

    Ah  BG  r  ALL
                   BG
                        r
                             N BG
Ah  LL                    Nh  N BG
          1 r
 LL




                                       10
Single Spin Asymmetries


                         1 N  RN
                    AL 
                         P N  RN
                      L
                    R 
                      L




                                       11
 Comparison to pi0 and Outlook




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




                                                             13

				
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posted:5/18/2013
language:English
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