STAR task force for evaluating tracking efficiency uncertainty

["Van Buren, Gene" <gene AT bnl.gov>]

A question at today's meeting was about the two ways to assign a sign to the 
XY DCA. These are documented in 
$STAR/StRoot/StarClassLibrary/StPhysicalHelix.cc as the functions:

geometricSignedDistance()
...where the sign is from whether the z component of the cross product of the 
DCA vector and the track momentum vector (this essentially says that the 
track is moving clockwise around the vertex at the DCA point, or 
counter-clockwise, regardless of the charge sign of the track):
    double cross = DCAVec.x()*momVec.y() - DCAVec.y()*momVec.x();
    double theSign = (cross>=0) ? 1. : -1.;
    return theSign*DCAVec.perp();

and
curvatureSignedDistance()
...where the sign is from whether the point is inside the curvature of the 
track, or outside the curvature, regardless of the direction the particle is 
moving. This is essentially the geometricSignedDistance() divided by the 
sign(-q*B) = mH:
        return (this->geometricSignedDistance(x,y))/(this->mH);

-Gene