[Halld-offline] [EXTERNAL] Progress with genBH

Mark-Macrae Dalton dalton at jlab.org
Tue May 26 10:26:54 EDT 2020

Hi Richard,

How would I configure a bremsstrahlung spectrum with circular polarization?


On May 23, 2020, at 11:43 PM, Richard Jones <richard.t.jones at uconn.edu<mailto:richard.t.jones at uconn.edu>> wrote:

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<https://urldefense.proofpoint.com/v2/url?u=http-3A__control.in&d=DwMFaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=XZ-uzdYsDwJAJMSlBfjvRw&m=3A0rKwZ6xgwlZoTgrEcgRk6L8Qm1hGY_yk6dFQBDLRI&s=FR-dVQ0XPZ-NwHqmQalLiAD5Qj2YgWRBw00Sk5GsmFo&e=> 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 enabled.

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<mailto: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.



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mailman.jlab.org/pipermail/halld-offline/attachments/20200526/c4394705/attachment-0002.html>

More information about the Halld-offline mailing list