Track Swimming methods

[David Lawrence <davidl@jlab.org>]

Hi All,

    For those who are interested, I just committed some changes to the 
DMagneticFieldStepper class. I have filled in the SwimToRadius method 
which until now, was an empty place holder (thanks to Matt B. for 
motivating this). At the same time, I added an additional, optional 
argument to both SwimToPlane and SwimToRadius so that one can obtain the 
path length of the track on return.

    Both of these methods are not exact, but should be good to better 
than 1-2 mm.

    Below is an example of using the SwimToRadius method. Let me know if 
you have any questions or problems.

Regards,
-David

    // Instantiate DApplication object (if in a DEventProcessor, use 
loop->GetDApplication())
    DApplication app(narg, argv);

    // Create stepper object using the standard field map
    DMagneticFieldStepper *stepper = new 
DMagneticFieldStepper(app.GetBfield());

    // Set starting position/momentum
    DVector3 pos(0.0, 0.0, 65.0);   // center of target
    DVector3 mom;
    mom.SetMagThetaPhi(0.6, 60.0/57.3, 0.0); // 600MeV/c at 60 degrees
   
    // Swim to a radius
    double pathlength;
    stepper->SwimToRadius(pos, mom, 65.0, &pathlength);
    pos.Print();
    cout<<"pathlength="<<pathlength<<"  r="<<pos.Perp()<<"  
z="<<pos.Z()<<endl;

-- 

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  David Lawrence Ph.D.
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