Photon conversions

This routine is an ALPHA interface to the ALEPHLIB routine PAIRFD. Electrons from photon conversion initially will have parallel trajectories. This algorithm finds the point on each helix where the tracks are parallel in the X-Y plane and pass closest together; this point is called the materialization point. Note that photon conversions are also found in JULIA, and are available as V0s (see Sections 7.1 and 8.1.2).

Input arguments:

I1

ALPHA track number of a charged track.

I2

ALPHA track number of a another charged track.

Output arguments:

DXY

distance(cm) in the xy plane between the two tracks at the closest approach to the materialization point.

DZ0

Distance(cm) in z between the two tracks at the origin.

DZ2

The z separation of the tracks at the closest approach to the materialization point.

DTH

the theta difference of the two tracks.

RMA

the rho value at the materialization point.

ZMA

the z value at the materialization point.

XMA

The invariant mass of the tracks at the materialization point assuming they are both electrons.

NC1,2

Number of coordinates with radius less than RMA for track 1,2. 0 if no coordinate information is available or if there are no such coordinates.

DIN1,2

Radial distance between the coordinate closest to the origin and RMA for track 1,2; variable is 0. if no coordinate information is available or if there are no such coordinates.

P(3)

Summed momentum of the two tracks at the materialization point in the order x,y,z.

IER

= 0 if calculation is successful; 1 otherwise.