[Halld-offline] DBCALTruthShowers

[Richard Jones]

Hello Justin,

You are right to be surprised by this behavior. It was not there when I
first set up this "truth shower" recording mechanism in hdgeant. I looked
into it, and was able to reproduce what you are seeing.

The problem is unique to the bcal. It came about primarily (there was
another minor issue that I fixed at the same time, that will reduce the
fragmentation of the fcalTruthShower info as well) because at some point we
added a relatively thick aluminum structure to the front of the bcal, which
serves as a primary mechanical support for the module. Because of its
thickness, a large fraction of the incident gammas convert and preshower in
this aluminum bar, and so what actually reaches the first active layer of
the bcal is often a gazillion low-energy shower particles clustered around
the incident gamma direction. That is why you were seeing fragmentation in
the bcalTruthShower tags.

I added a fix to gustep.F in the HDGeant directory in both the trunk and
commissioning branches. Please check out the latest update of whichever you
are using and try it. This code catches any particles incident on the bcal
and records the TruthShower info before they enter the aluminum support
beam. There are still a few conversions that take place in the CDC and the
FDC cables around the CDC, but you should see that most of your 2 GeV
gammas at 50 degrees now generate just a single bcalTruthShower in hdgeant.
For those that shower in the CDC and FDC cable material, I think we should
not try to merge them into a single bcalTruthShower because the magnetic
field will have a significant dispersive effect on them.

Thank you for reporting this problem. I hope this helps with bcal
reconstruction.

-Richard Jones

On Tue, Oct 14, 2014 at 3:30 PM, Justin Stevens <jrsteven at mit.edu> wrote:

> Hi Richard,
>
> I was hoping to follow up on what we talked about briefly at the
> collaboration meeting regarding BCAL truth showers.  I'm looking at some
> simulation events with single photons generated at theta = 50 degrees and E
> = 2 GeV.  From an earlier thread about hadronic showers on the offline list
> https://mailman.jlab.org/pipermail/halld-offline/2014-May/001677.html
>
> > Once a primary particle has entered the BCAL and had its TruthShower
> recorded, it is marked with a spot of red dye (colloquially). Any
> secondaries that are generated from interactions by that particle inherit
> its red spot, even if they escape from that module and enter another one.
>
>
> I was expecting there should be only one BCAL truth shower in each event
> which contain a single photon.  This is the case for some events like this
> one from hd_dump:
>
> DBCALTruthShower:
>  ptype: track: primary:    phi:      r:     z:     t:     p:     E:
> -------------------------------------------------------------------
>      1      1        1  -1.419  65.042  119.6  2.832  2.000  2.000
>
> However, for most events there are many DBCALTruthShowers listed, like in
> the example below:
>
> DBCALTruthShower:
>  ptype: track: primary:   phi:      r:     z:     t:     p:     E:
> ------------------------------------------------------------------
>      1      1        1  0.686  65.042  119.6  2.832  2.000  2.000
>      3      3        0  0.686  66.617  120.9  2.901  1.595  1.595
>      3      3        0  0.686  66.979  121.2  2.917  0.001  0.001
>      1      3        0  0.686  66.766  121.0  2.907  0.054  0.054
>      2      2        0  0.686  66.617  120.9  2.901  0.405  0.405
>      1      2        0  0.686  66.894  121.1  2.913  0.018  0.018
>      1      2        0  0.686  66.806  121.1  2.909  0.005  0.005
>      1      2        0  0.686  66.709  121.0  2.905  0.201  0.201
>      3      2        0  0.686  67.005  121.2  2.918  0.158  0.158
>      1      2        0  0.686  67.021  121.2  2.918  0.068  0.068
>      2      2        0  0.686  67.005  121.2  2.918  0.042  0.042
>      1      2        0  0.686  67.060  121.3  2.920  0.016  0.016
>      1      2        0  0.686  67.021  121.2  2.918  0.001  0.001
>
> So first of all, is this the expected behavior?  That there are many
> DBCALTruthShowers where many of them appear to be secondaries of the
> primary single generated photon in the event?
>
> And if the answer to the questions about is yes, for a general bggen event
> (where there are many particles incident to the BCAL) how should I best
> associate reconstructed BCAL showers with these DBCALTruthShowers?
>
> Thanks,
> Justin
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