[Halld-offline] HDGeant4 Meeting Minutes, June 1, 2021

[Mark Ito]

Folks,

Please find the minutes here 
<https://halldweb.jlab.org/wiki/index.php/HDGeant4_Meeting,_June_1,_2021#Minutes> 
and below.

   -- Mark

      __________________________________________________


    HDGeant4 Meeting, June 1, 2021, Minutes

Present: Alex Austregesilo, Tegan Beattie, Sean Dobbs, Mark Ito (chair), 
Igal Jaegle, Richard Jones, Zisis Papandreou, Simon Taylor, Beni Zihlmann

There is a recording of this meeting 
<https://bluejeans.com/s/IlW47yET1A6/> on the BlueJeans site. Log into 
the BlueJeans site <https://jlab.bluejeans.com> first to gain access 
(use your JLab credentials).


      Review of minutes from the last meeting

We went over the minutes from the meeting on May 18 
<https://halldweb.jlab.org/wiki/index.php/HDGeant4_Meeting,_May_18,_2021#Minutes> 
without a lot of discussion.


      Charged Hadrons and BCAL Timing

Tegan showed several plots, which he previewed at the last meeting, 
comparing timing distributions for charged hadrons between data and 
Monte Carlo. On the simulation side he plots both Geant4 and 
GEANT/HADR4. He used exclusive ρ events for both data and MC. He plots 
the difference in the time predicted by charged particle tracking to 
that measured in the BCAL.

  * For protons the agreement in the core Gaussian part of the
    distributions is good for all three species; see this plot
    <https://halldweb.jlab.org/wiki/images/4/46/Proton_tegan.pdf>. The
    data does show non-Gaussian tails on the log plot shown that could
    be due to event selection. We suggested that he try a tighter cut on
    the χ^2 of the kinematic fit to see if that reduces the tails. Tegan
    reported that on a linear plot, these tails are not visible to the eye.
  * For pions the agreement among all three is very good, with a high
    side tail visible in all three species; see this plot
    <https://halldweb.jlab.org/wiki/images/a/a6/PiPlus_tegan.pdf>.

Mark suggested that if Tegan does not uncover further problems, we 
should go back to the original studies of event efficiency that launched 
us on this path. Those studies showed overall event reconstruction 
efficiency showing large differences between the various simulation 
engines. We have since concluded that GEANT/HADR1 does not give reliable 
timing distributions for charged hadrons in the calorimeters, but the 
comparison between Geant4 and GEANT/HADR4 is still relevant. There have 
been many changes in the code since those studies were performed and we 
should see where we stand with the latest versions. Zisis mentioned that 
Ahmed Foda's ωπ analysis shows good agreement in cross sections between 
Spring 17, Spring 18, and Fall 18 lately, a new development.


      The Schick Effect

We discuss the problem Andrew Schick discovered in his Bethe-Heitler 
analysis where the total energy of the B-H pairs in the FCAL were lower 
than the beam photon beam energy by 4 to 5%. Even in simulation some of 
this deficit is seen, though only by 1 to 2%. Beni mentioned that there 
is also a difference seen between electrons and positrons. Alex also 
reminded us that Drew Smith (Compton scattering) and Mark Dalton (ω 
production) both showed effects in the cross sections that pointed to 
inefficiencies in the inner blocks of the FCAL.

At this meeting, the question was whether there might be some insight to 
be gained by studying these effects in simulation in the future.


      Simulation GlueX Doc

Richard floated the idea of the group writing a GlueX Doc summarizing 
the knowledge gain by this group vis-a-vis the fidelity of our 
simulation so that the ideas don't get lost. This would be along the 
lines of the document being produced by the Beamline Working Group on 
beamline-related systematics.


      Exploiting New Features of Geant4

Richard reported on work he has been doing to incorporate circular 
polarization into the simulation. One aspect of his work is to use 
recently introduced polarization dependent cross sections from Geant4. 
He finds that it is sufficient to go to version 10.6.p1 to pick these 
up, but that requires GCC 5.3.1 (again sufficient if not necessary) 
which is beyond that provided by CentOS 7. He is using Devtoolset 4[?] 
to get the compiler. He stressed the need for GlueX to press forward 
with using more modern versions of Geant4 and other packages.


      Bethe-Heitler Event Generator

Richard has been working on a built-in Bethe-Heitler generator for 
HDGeant4. This is to support Andrew's work, mentioned above, with a more 
accurate physics model that the one Andrew has been using thus far.


      Action Items

 1. Re-do proton-in-BCAL timing study with tighter cuts on the χ^2 from
    the kinematic fit. (Tegan)
 2. Re-do comparisons of overall event reconstruction efficiencies.
 3. Think about how to study the Schick-Smith-Dalton effect(s). (all)
 4. Think about a GlueX Doc on simulation. (all)
 5. Think about how to get to modern versions of external software
    packages. (all)


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