[Halld-physics] TOF MC improvements: new simulations needed

[Justin Stevens]

Dear Collaborators,

As reported at the last February Collaboration Meeting and followup at recent working group meetings, we have been tracking down a few issues with the simulation of the TOF (full details here https://github.com/JeffersonLab/halld_recon/issues/780).  The short summary is that the simulated TOF hits had a long tail in the timing distribution that caused our default charged track pre-selection cuts to have a lower efficiency in MC than data.  This clearly has an impact on any cross section measurement with charged particles that hit the TOF, as shown in the GitHub issue linked above.  HDGeant4 was updated with a fix to the TOF simulation and new smearing parameters were added to the CCDB for the GlueX-I periods (2017-2018) to match the data and MC response.  The new software versions that include this fix can be found in Alex’s recent announcement https://mailman.jlab.org/pipermail/halld-offline/2024-April/009013.html (or below the signature).

While we were considering this, we also investigated the BCAL timing distribution and the effect of charged track pre-selection there https://github.com/JeffersonLab/halld_recon/issues/792.  The conclusion was that the default pre-selection cuts in the BCAL do not significantly bias the efficiency from MC but have a systematic uncertainty of ~1% per track.  More restrictive cuts on the BCAL timing will likely require dedicated studies of the systematics.  No software or smearing parameters were changed for the BCAL from this study.

Long story short: the change in the TOF simulation has a significant effect on the efficiency determined from our MC, therefore cross section studies with charged particles in the TOF region should generate new MC samples with the latest libraries.  We have begun generating large MC samples for several of the high-priority publications under review with this new software and you’re encouraged to submit your simulations to MCWrapper via the web form or on the ifarm.

Thanks to the many people who helped tackle these issues and happy simulating :) .

-Justin


PS. Here’s a copy of the version set information from Alex’s message linked above where the TOF simulation is fixed in hdgeant4:

recon-2017_01-ver03_39.xml
recon-2018_01-ver02_31.xml
recon-2018_08-ver02_30.xml
recon-2019_11-ver01_12.xml
recon-2017_01-ver04_6.xml  (soft link to 2019-11)


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