[Halld-offline] fixes to the CDC acceptance holes

[Eugene Chudakov]

Gianni got credit for speeding up the tracking code...
Has he also contribited to the GEANT4 tracking?

Eugene

On Thu, 16 Jan 2014, Richard Jones wrote:

> Hello Offliners,
>
> I have diagnosed and fixed the bug that Simon and Paul pointed out back in
> November, that after my geometry updates related to the Geant4 development,
> there appeared large holes in the acceptance for charged tracks at backward
> angles in the CDC.  The changes have been checked into subversion.  I
> invite anyone who is interested to check out the updated code and verify
> that the holes have been healed.  The problem was entirely with Geant3, not
> with mcsmear and not with the reconstruction code.
>
> The source of the bug was code that Simone (Gianni, not Taylor) introduced
> in Geant321, related to his scheme for virtual divisions.  That code
> employed a short-cut for determining the transverse extents of volumes with
> cylindrical symmetry when being cut along a cartesian axis.  He assumed
> that if the cos(theta) of the rotated volume is greater than 0.99 then it
> should be treated as if theta=0.  His code is full of approximations like
> this, and in most cases they are conservative ones that result in intervals
> that are larger than the actual volume, which is safe.  But in the case of
> the stereo straws, we have cos(6 deg) = 0.995, which results in the
> projection of the stereo staws onto the x,y plane having a width of only
> one straw diameter.  This is wrong by a whole order of magnitude, and
> results in straws not being checked for intersections with tracks if the
> track intersects the straw layer at a point in x that is far from its
> shunken image in x,y.  The image was being taken at the downstream end,
> which explains why the problems got worse at the upstream end, but really
> there were problems everywhere.
>
> That was one bug I found, and there was another one that I am not sure
> impacts us very much, but which I also fixed.
>
> Someone might be wondering what changed to make this bug suddenly impact us
> in such a clear way after the recent geometry updates.  The reason is
> somewhat arbitrary, because the straws were changed from straight tubes to
> "profiled tubes" in order to make them "neck down" to the diameter of the
> feedthroughs where they punch through the end plates.  This change made the
> code form the "shunken image" of the straw x-y profile (that I referred to
> above) at the downstream end instead of in the middle of the straw, as it
> did formerly.
>
> We were LUCKY THIS HAPPENED, because the bug was always there in the
> simulation code, but the effects were more subtle before, and were only
> happening near the two end plates, which made them more difficult to notice
> and diagnose.  Now with this fix, any bogus inefficiencies that were seen
> in the stereo layers near the end-plates are now removed.
>
> Thanks to all of you for your patience with this.  We might have gotten
> away with ignoring it and moving forward with the G4 development, but I
> think that would have been a mistake.  We need both simulations to work
> during the transition period, and this information may be useful for
> explaining artifacts in prior simulations that were never fully understood.
>
> -Richard Jones
>