Exclusive Charmonium and QED γγ→μμ

Contact:

Long Zhang (lzhang@phys.ualberta.ca)

Mike Albrow (albrow@fnal.gov)

James Pinfold (pinfold@phys.ualberta.ca)

 

    We present a preliminary measurement of the cross sections of 
    (1) QED dimuon production mediated by 2-photon exchange, 
    (2) J/ψ and ψ' photoproduction mediated by photon-pomeron exchangeand, and 
    (3) χc production mediated by pomeron-pomeron exchangeand.
    The three processes share same final state of a muon pair and nothing else 
    observabale in the detector, except χc events which contain an
    additional soft photon. 


 Data and Event Selection 
 

   The data is collected by a muon-plus-track trigger, which requires one low 
   Pt (1.5 GeV/c) CMU muon at Level 1 and an additional track at Level 3. The 
   data used corresponds to total integrated luminosity of 1484 pb-1. 
   We applied offline muon quality cuts, cosmic veto cuts and the exclusivity 
   cuts to finally select 402 signal events.



 The Exclusivity Cuts 

   The exclusivity cuts are a set of Et cuts applied on calorimeters and BSC
   outside two cones around the two muons, to make sure nothing else happened 
   in the detector. The cut values are determined by analyzing the zero-bias 
   data. The zero-bias data is divided as "interaction" or "non-interaction". 
   The separation of interaction and non-interaction peaks indicates the cut 
   values. 
   The picture below shows the exclusivity cut in BSC-1.
   


 402 Signal Events after all cuts. 
 

   The picture below is the dimuon mass of the 402 signal events. 
   

   The above mass plot is fitted to 2 Gaussian plug a QED background. 
   


 Effective Luminosity 
 

   Sometimes due to beam noise or multi interactions, even a true exclusive 
   dimuon event happens, it will still be killed by the exclusivity cuts. 
   Thus only a fraction the 1484 pb-1 integrated luminosity corresponds to 
   the observed 402 signal events. That fraction is determined by using the 
   zero-bias data. 

   We apply the exclusivity cuts to zero-bias data of the same run periods. 
   The fraction of events that pass the cuts is exclusive efficiency, the 
   picture below shows exclusive efficiency varies with bunch luminosity.
   

   As shown in the picture below, the bunch luminosity histogram (open hist) 
   multiplied by exclusive efficiency curve gives the effective luminosity 
   (shaded hist).  
   


 Efficiencies 
 

   (1) The trigger efficiency is determined using the MuonUser package. 
   

   (2) The detector and reconstruction efficiency is determined by the LPAIR 
       Monte Carlo, STARLIGHT Monte Carlo data and CDF detector simulation. 
   

   (3) The offline muon quality cuts efficiency is determined by the LPAIR 
       Monte Carlo, STARLIGHT Monte Carlo data too. 
   


 The χc background for J/ψ photoproduction 
 

   The background is estimated by releasing the EM exclusivity cut in Central 
   and plug calorimetors, allowing more χc soft photons in, then extrapolate 
   the curve back to under the cut value. This give ~1% χc inside the J/ψ 
   signal events.

   


 Results: 
 

   (1) QED:   σ = 2.7  +/- 0.5 pb
       (pTμ>1.4 GeV/c, |ημ|<0.6, Mμμ∈[3.0,4.0] GeV/c2)
   (2) J/ψ:  dσ/dy(y=0) = 3.92 +/- 0.62 nb
   (3) ψ':   dσ/dy(y=0) = 0.54 +/- 0.15 nb
   (4) χc:   dσ/dy(y=0) = 75   +/- 14   nb