[Halld-offline] pid_hists plugin

[Nathan Sparks]

Hi All,

I added a plugin that fills some histograms that are useful in monitoring the quality of the PID. It is a work in progress, but I thought that it could be useful to those of you who are looking at PID now. 

The plugin uses the MC truth information (hit matching) to count how often the PID is wrong and in what way (this many pions were misidentified as kaons, etc.), and these counts are filled into a histogram. This plugin assumes that one is using the standard/normal PID (in DTrackTimeBased_factory.cc) and requires (includes a cut) that the pid FOM (chi2 probability) be larger than 1e-3.  Additionally, the plugin fills histograms with the time differences between the best PID hypothesis and the others, which is useful for visualizing the timing separation between the different mass hypotheses as well as the PID quality (because deviation from the known kinematic behavior occurs when the best hypothesis is wrong). I have not yet included a histogram that shows how the dE/dx contributes to the PID quality.

Does anyone have suggestions for additional histograms to add to this plugin?

Best Regards,
Nathan