BCAL Low-Energy Photon Response

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One important unknown in the performance of the GlueX barrel calorimeter (BCal) is the minimum reconstructible photon energy. Monte Carlo studies show that photons below 50 MeV are needed to be reconstructed in the BCal in order to maintain a high degree of uniformity in the acceptance for important physics channels. The GlueX CDR contains a design goal of 20 MeV for this important parameter. The purpose of this study is to determine if that goal is achievable within the current design. According to the nominal resolution function from the Regina stand-alone BCal Monte Carlo simulation (ReSAM) [1],

{\frac  {\sigma _{E}}{E}}={\frac  {0.04}{{\sqrt  {E}}}}\oplus 0.01

20 MeV showers should have a 20% energy resolution, which is encouraging. In order to resolve them, however, two additional requirements must be met.

  1. The rate of background photons at that energy must be small enough to have be able to find primary photons of 20 MeV with good likelihood.
  2. The distribution of the sampling fluctuations for 20 MeV photons must be reasonably close to Gaussian.

The first question is best addressed with a full simulation with HDGeant. The second is best addressed with test beam data, or the ReSAM simulation if a reliable sample of triggered events cannot be found in the test beam data below 50 MeV.

Sampling Fluctuations with ReSAM

I would like to ask the Regina folks for a Monte Carlo simulation of single-photon showers into a single BCal block. In particular, I would like to see the following:

  • one million events,
  • original photons enter the middle of the block,
  • incidence angle are φ=0 and θ ~ Uniform [20°,90°],
  • incident photon energy ~ Uniform [10,50] MeV.

So that I can analyze the results, please create an output ntuple that contains (at least) the following elements per event:

  • incident photon energy [MeV],
  • incident photon polar angle θ [deg],
  • total energy deposition per readout cell [MeV],
  • summed energy deposition inside the fibers, per readout cell [MeV].

If this MC ntuple output file could be produced and placed somewhere on the web, I will offer to download and analyze it. -Richard Jones


  1. Gluex-doc-827