[Halld-physics] Primakov eta hadronic background

[Alexander Somov]

Hello Ashot,

Finally, I took a quick look on the hadronic bg for Primakov
eta. I considered bg from inclusive eta photoproduction
predicted by Pythia ( The inclusive eta background should
dominate combinatorial bg)


I put a few plots to

http://www.jlab.org/~somov/primex




---MC sample----

The MC sample corresponds to 2 10^7 Pythia events generated
in the beam energy range of 9 GeV < E_gamma < 12 GeV
(I selected events containing at least one eta in the final
state  with an angle < 1.6 deg, and processed these events
through Geant). The magnetic field was switched OFF.


This sample corresponds to about 32 reconstructed 'Primakov'
eta's in the eta->2gamma decay channel.

N_prim_eta(0 - 0.5 deg)_produced =  2e7 * 7.2e-4 / 122 = 118
N_prim_eta_rec = 118 * 0.71 * 0.39 ~ 32,

where 7.2 ub -  is the Primakov production Xsection p(0. - 0.5 deg)
       122 ub -  is the total photoproduction Xsection
       0.39   -  is the branching fraction for eta -> 2 gamma
       0.71   -  is the prim eta acceptance (according to Aram).


--Eta reconstruction--

  - considered events with only 2 clusters in the FCAL
  - eta p elastic decays were excluded from the sample
  - Primakov signal region was defined as:
                     E_beam - E_2gamma > -0.4 GeV
   (more than 3 sigma region  according to the eta mass constrained fit).
                     Angle(gamma gamma) < 0.5 deg.


Below are descriptions to plots
-------------------------------

      angle_vs_energy.pdf
      -------------------

      Reconstructed angle of 2 photons versus ( E_beam - E_2gamma )

      top right plot: veto events with the bcal total energy (summed
                      over all reconstructed photons) larger than
                      60 MeV. This cut removes most background from
                      p eta pi0 decays. Remaining bg originates mainly from
                      n eta pi+

   Note: 99% of the p eta (eta->gamma gamma) elastic decays survive this
         cut.


      bottom left plot: Exclude events where the reconstructed eta and
                        a track candidate having a hit(s) in the SC are
                        positioned NOT back-to-back in phi, i.e.,
                        delta_phi < 170 deg or delta_phi > 190, where
                        delta_phi = Phi(2gamma) - Phi(HIT_SC).
                        ( If there are no hits in the SC, we keep this
                         event ).

                        The delta_phi distribution for the reconstructed
                        p eta decays is shown in the file  eta_dphi.ps.
                                                            -----------
                        The remaining event close to the signal region is
                        p eta pi+; I need large statistics to study it in
                        detail. We can presumably measure p eta pi+ decays
                        and subtract them from the 'Primakov' signal
                        region.


All in all, I don't see large background in the Primakov signal region
(If here is no bug in my code, of course ). I hope that you know the ABC 
store close  to the lab... :)

We can discuss more details during the meeting. I will need to check 
evrth. and generate a large sample...


Cheers,
       Sascha