[Halld-offline] [EXTERNAL] Re: Progress with genBH

Richard Jones richard.t.jones at uconn.edu
Sat May 23 23:43:51 EDT 2020

Hello Mark, Rory and all,

At the end of our meeting yesterday, I agreed to have a look at the mods
that Lubomir made to genBH to make it work the GlueX photon spectrum and
not just a single energy. Several of his other changes I have incorporated
into my master branch, but I decided that support for a bremsstrahlung
spectrum should be done differently. The reason for that is that it depends
too much on the beam line geometry, and we want to be able to change things
about the beam line and see the results in the BH distributions. Instead
of building a mini-beamline simulation into genBH, I have incorporated the
internals of genBH into hdgeant4 itself. Now you just run a special run of
hdgeant4, and it tracks beam photons through the collimator down to the
liquid hydrogen target, then forces Bethe-Heitler interactions in the
liquid hydrogen target. This is how simulations of the TPOL are done. Now a
similar functionality exists for the liquid hydrogen target, except that
with the Bethe-Heitler generator you have control over the minimum
invariant mass of the pairs that you want to generate. In the TPOL sims you
get all pairs without any pre-selection. I took the proton form factors
that Lubomir implemented and copied them into the new code.

The output is just like what comes out of genBH. There are no hits in it,
just the <reaction> tag showing a beam photon coming from upstream of the
collimator and ending in a pair production vertex in the liquid hydrogen
target. You then take that output file and feed it back into hdgeant4 with
normal settings restored in control.in and it will simulate those pairs in
the detector, just like it would if it were reading from a file produced by
any other event generator.

There is a pull request on github HDGeant4 master branch with these
changes. Please have a look, and if this approach meets with your approval
go ahead and execute the PR. This update does not affect the way hdgeant4
works on normal simulation runs, without the GENBEAM card present and

I have written a short page on the gluex wiki explaining how to run a
Bethe-Heitler simulation from beginning to end, using this new tool. I have
also included some sample plots that allow you to check that you are seeing
what you should in your output.


-Richard Jones

On Thu, May 21, 2020 at 6:30 PM Mark-Macrae Dalton <dalton at jlab.org> wrote:

> *Message sent from a system outside of UConn.*
> Hi All,
> In preparation for our meeting tomorrow at 11 am, I thought I would send
> you my progress with using genBH.
> I run the generator for a BH spectrum between 3 GeV and 11.6 GeV.  The
> generator outputs the following text.
> est. total cross section after 100000000 events : 13440.8 +/- 1.87554 ub
> The figure shows the cross section that I determine as a function of beam
> energy.  Top left is for all events.  Here I recover the est. total cross
> section output by the generator.  The other panels show what happens if I
> require a trigger or a tagger hit or both.
> The thing that confuses me a little is that even requiring a trigger
> leaves about 160 microbarns of cross section which is a bit larger than the
> total hadronic cross section.  Since we don’t get half of the GlueX rate
> from BH events, this can’t be right.  I wonder whether this might indicate
> a problem with the trigger emulation in the simulation.
> Best,
> Mark
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