Re: Tracks with no hits.

[David Lawrence <davidl@jlab.org>]

Hi Richard,

    Great! Thanks for jumping on this so quick. I will take a look at it 
and let you know if it looks fixed for me too.

Regards,
-David

Richard Jones wrote:
> 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
>>
>

-- 

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