PrimEx at GlueX

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There is an interest in continuing the PrimEx physics program with the GlueX detector with the 12 GeV beam. One can find links to meetings and minutes for a series of GlueX and PrimEx meetings here.

Physics Topics

  • Measurement of the γγ width of the η and η' using Primakoff Production
  • Measurement of rare η decay branching fractions

Monte Carlo Studies

A key item to evaluate with Monte Carlo is how well the detector allows separation of signal from the anticipated backgrounds. We are also interested in exploring how the photon reconstruction resolution can improve this signal separation.

Simulation and Reconstruction

Events from the physics generators are fed into a GEANT3-based model of the GlueX detector that includes all active detector elements and inactive material in the detector volume. Signal events can be passed in in the ASCII format described below. In addition a PYTHIA-based physics generator is used to simulate generic hadronic photoproduction. This cross section is about 120 μb at 9 GeV incident photon energy.

To optimize efficiency in generating large samples of background events, we require that genNeuETot (defined below) be greater than 6 GeV. It is expected that background that does not satisfy this cut will not produce enough energy in the forward calorimeter to pass any typical set of signal selection cuts. Events are passed through standard reconstruction packages. The main area of interest for these studies is in the forward calorimeter. To save time, reconstruction in the barrel calorimeter is turned off.

Monte Carlo Samples

A series of Monte Carlo samples have been generated and processed. (The file format is documented below.) All samples are available in this directory:

  • eta_rare_9gev_10k.root: rare η→π0γγ decays, 9 GeV beam, 10000 events
  • eta_rare_11gev_10k.root: rare η→π0γγ decays, 11 GeV beam, 10000 events
  • eta_prim_he4_22k.root: η→γγ decays from Primakoff η production, 22000 events

Note: The PYTHIA-generated background includes η's whose decays are handled by GEANT. GEANT does simulate a 7x10-4 branching fraction for η→π0γγ. All η decays can be removed by requiring neuType != 17 for all generated neutral particles in an event. (Later versions of the MC will be revised to zero out this branching fraction.)

Input Format for Signal Events

Signal events can be fed into the GlueX Monte Carlo using a simple ASCII format for the four vectors of each a event. Below is a sample event.

9000 1 6 
1 8 0.137
 1 -0.33816 -0.199603 1.50846 1.56474
2 9 0.137
 -1 0.0592538 -0.0287174 0.270175 0.309998
3 7 0.135
 0 0.125819 -0.0498192 0.20477 0.280122
4 8 0.137
 1 -0.117507 0.288612 1.31965 1.36284
5 7 0.135
 0 0.143892 -0.446216 5.27102 5.29355
6 13 0.93926
 0 0.126703 0.435744 0.425928 1.12674

Where the format is:

run event no_particles
particle_index particle_id mass
charge px py pz E

Output Data File Format

The reconstructed and analyzed data events are written out in a ROOT tree with the name "event". The tree contains several series of variable length arrays. A description of the entries appears below:

  • n: The number of reconstructed photons in the FCAL
  • E[n], px[n], py[n], pz[n]: The four momentum of the nth reconstructed photon
  • neuETot: The total reconstructed energy in the FCAL
  • M: The invariant mass of all FCAL photons
  • genNeuETot: The total generated energy of all η's, π0's; γ's, electrons, and positrons.
  • nNeuGen: The number of generated neutral particles
  • genNeuE[nNeuGen], genNeuPx[nNeuGen], genNeuPy[nNeuGen], genNeuPz[nNeuGen]: The four momentum of the nNeuGenth generated neutral
  • neuType[nNeuGen]: The GEANT particle type of the nNeuGenth particle
  • nChGen: The number of generated charged particles
  • genChE[nChGen], genChPx[nChGen], genChPy[nChGen], genChPz[nChGen]: The four momentum of the nChGenth generated neutral
  • chType[nChGen]: The GEANT particle type of the nChGenth particle