[Halld-offline] [EXTERNAL] new hdgeant4 ability to generate mu+mu- pairs in GlueX target

[Richard Jones]

Hello all,

For some who may be interested, hdgeant4 has the capability to generate
mu+mu- pairs from electrons and protons in the GlueX target via the
standard model gamma conversion process. The treatment includes all
tree-level diagrams exactly, with screening of the proton by atomic
electrons based on the ground-state form factor of the hydrogen atom.
Atomic screening is much more important for e+e- pairs than for muons, so
dependence on the model for the atomic form factor should be negligible. In
case anyone cares, no attempt has been made to include modifications to the
H atomic form factor due to medium effects in the cryogenic liquid
environment, but these are expected to be small because the
vanderWalls interaction energy is small compared to the n=1 to n=2
transition gap of hydrogen, and anyway these corrections are important at
q^2 values that are smaller than the range that mu+mu- conversion samples.

On the other hand, the proton form factor is much more important. For that,
I am using a standard dipole form with a beta factor of 0.71 GeV for F1,
and similarly for F2.

Capability exists to generate either weighted (by the cross section) or
unweighted events, with support for linear polarization of the incoming
photon, either given by the coherent bremsstrahlung polarization spectrum
or fixed by the user. For details on how to set up a run of hdgeant4 as a
Monte Carlo generator, instead of its normal role as a detector simulation,
see the following HOWTO. To use this feature, you need to build hdgeant4
from the Master branch as of June 30, 2021.

https://halldweb.jlab.org/wiki/index.php/HOWTO_use_hdgeant4_as_a_beam_Monte_Carlo_generator

-Richard Jones
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