Re: Tracks with no hits.

[Richard Jones <richard.t.jones@uconn.edu>]

Dave and all,

This seems to be an optimization in the core tracking code of geant that 
causes mother volumes to appear empty of daughters under certain special 
conditions (floating-point rounding errors) that are usually rare, but 
apparently not so rare when all of the interactions are turned off and 
you are tracking with a magnetic field!  Bravo for finding such an easy 
way to reproduce it.  I have commented out the optimization line in 
gtrack.F that causes the fault, and the tracking seems to run at 
essentially the same speed, without the errors.  I think we should adopt 
the fix permanently, but please check it first and make sure it 
completely fixes the problems you found.  The patched gtrack.F can be 
found in the HDGeant source directory under the main trunk in svn.  I 
have incorporated it into the Makefile.orig that I use to build the 
hdgeant executables.  Dave, you will have to incorporate it into the BMS 
make system.

-Richard Jones


David Lawrence wrote:
>
> Hi Richard,
>
>    I have run across a mystery that I think you may be the only person 
> who can solve it. Recently (~ 1 month ago), I ran a simulation to map 
> out the efficiency and resolution for charged particle tracking using 
> the latest geometry and tracking code. One of the things that showed 
> up was an odd structure in the efficiency map (in ptot and theta) 
> where there was a several percent loss in efficiency in a thin band 
> that appears to be for a constant transverse momentum (though now, I'm 
> sure that is actually the case). Mark Ito repeated the exercise with 
> his independent fitter and found the same result.
>
>    I ran a few events with the particle gun aimed at the 
> low-efficiency area and I found that for some tracks (several percent) 
> there were no hits at all. I turned on the DMCTrajectoryPoints which 
> record the track parameters at every call to gustep, including the 
> radiation length of the material they are passing through. I could see 
> that the particles were going through the detector, but for the 
> problem events, the radiation length was always very large (30k cm) 
> indicating that it never left the gas. For good events, I could see it 
> getting smaller occasionally, presumably as it passed through the 
> straw tube walls. I'm wondering if this is a MANY/ONLY problem, but 
> I'm not sure where to begin in order to figure it out. Oh, and I also 
> saw this same thing for some forward going tracks that were passing 
> through the FDC.
>
>    I've attached a control.in file that shows the problem on events 6, 
> 25, 32, 46, ... I have verified that I see the problem on event 6 on 
> both Mac OS 10.5 and Fedora 8. I am using the source code tagged for a 
> special study that can be found here:
>
> https://halldsvn.jlab.org/repos/trunk/Studies/2009.02.track_resolutions/src 
>
>
> This contains the complete source tree including hdds and HDGeometry 
> and HDDM so you should be able to reproduce exactly what I'm doing.
>
> Could you maybe look into this when you get a chance?
>
> Regards,
> -David
>