[Halld-offline] vertex position from hit-based tracks

[Craig Bookwalter]

Many thanks, this will keep me busy for a while!

David Lawrence wrote:
>
> Hi Craig,
>
>     I just spoke with Simon and realized there was some confusion on 
> how DReferenceTrajectory::DistToRT() works. It is set up to assume 
> that the DCoordinateSystem object represents a wire and will find the 
> point of closest approach (POCA) to that wire. This works OK for a 
> plane when the track is more or less perpendicular to the plane, but 
> will be less accurate as the angle to the plane increases.
>
>     This needs to be fixed in the DReferenceTrajectory class and will 
> be soon. There is another solution though that currently exists in the 
> repository which uses DMagneticFieldStepper. Below is a snippet of 
> code to give you the idea of what needs to be done assuming you have 
> the JEventLoop and DTrack object pointers.
>
> // We must get a pointer to the magnetic field object so do a small 
> backflip to get it using DApplication
> DApplication* dapp = dynamic_cast<DApplication*>(loop->GetJApplication());
>
> // Create a DMagenticFieldStepper object
> DMagneticFieldStepper stepper(dapp->GetBfield(), track->charge());
>
> // Swim to a plane defined by "origin" and "norm". "origin" is any 
> point in the plane.
> double pathlen;  // Holds pathlength from starting point to 
> intersection with plane
> DVector3 pos = track->position();  // Holds starting position on entry 
> and intersection point on output
> DVector3 mom = track->momentum();  // Holds starting momentum on entry 
> and intersection point on output
> stepper.SwimToPlane(pos, mom, origin, norm, &pathlen);
>
> The DMagneticFieldStepper class also has a SwimToRadius method that 
> can be used to find the intersection point with a cylinder centered on 
> the beamline.
>
> I hope this helps.
>
> -David
>
>
> Simon Taylor wrote:
>> Hi, Craig.
>>
>> You can get the position of the closest approach to the beam line of the 
>> track using the position() method; e.g. track->position();
>>
>> You can track to any plane using one of the methods of the 
>> DReferenceTrajectory class ( a DReferenceTrajectory object is attached 
>> to the wire-based track).  Here's a code snippet:
>>
>>  DReferenceTrajectory *rt=const_cast<DReferenceTrajectory *>(track->rt);
>>  DCoordinateSystem st_plane;  // plane parameters
>>   double s; // path length
>>  double doca=rt->DistToRT(&st_plane,&s);     // Distance of closest 
>> approach between plane     // and track
>>
>> Simon
>>
>> Craig Bookwalter wrote:
>>   
>>> Hi folks,
>>>     I'm looking for a couple methods to help me determine the event 
>>> start time, namely:
>>>     - a method for getting the vertex position of a hit-based track in 
>>> the CDC or FDC
>>>     - a method for getting the path length along a hit-based track from, 
>>> say, the start counter to the target (this would require some 
>>> association between start counter hits and CDC/FDC tracks, which I'm not 
>>> sure is happening yet)
>>>
>>>     I've looked through the DOxygen docs and haven't seen anything that 
>>> stuck out to me, but I certainly could have missed something. If these 
>>> guys do not yet exist I would be happy to help implement them.
>>>
>>>     Thanks in advance!
>>>
>>> --cb
>>>    
>>>
>>>    
>>>
>>>   
>>>     
>>
>> _______________________________________________
>> Halld-offline mailing list
>> Halld-offline at jlab.org
>> https://mailman.jlab.org/mailman/listinfo/halld-offline
>>   
>
> -- 
>
> ------------------------------------------------------------------------
>  David Lawrence Ph.D.
>  Staff Scientist                 Office: (757)269-5567   [[[  [   [ [       
>  Jefferson Lab                   Pager:  (757)584-5567   [  [ [ [ [ [   
>  http://www.jlab.org/~davidl     davidl at jlab.org         [[[  [[ [[ [[[
> ------------------------------------------------------------------------
>
>   
> ------------------------------------------------------------------------
>
> _______________________________________________
> Halld-offline mailing list
> Halld-offline at jlab.org
> https://mailman.jlab.org/mailman/listinfo/halld-offline


-- 
-------------------------------------------------------------------------
 Craig Bookwalter			FSU Office:  (850) 644 3808
 Department of Physics			JLab Office: (757) 269 5465
 Florida State University		craigb at hadron.physics.fsu.edu
 Tallahasse, FL 32306			craigb at jlab.org		


"One toke? You poor fool. Just wait till you see those (expletive) bats."
-------------------------------------------------------------------------