[G12] vert2 function

[Michael C. Kunkel]

Greetings,

I looked into the workings of vert2. The method of obtaining the vertex 
for 2 tracks in this method is no different than the method I have been 
using for years. The problem with this method is that it will fail if 
the tracks are parallel, which is usually the case for conversions and 
low mass e+e- events (the problem are in which we are investigating).

Below is a the snipet of the code to prove it

   /*calculate dot product of direction unit vectors*/
   R = v3dot(line1.dir,line2.dir);

   if(R==1){ /*parallel to beamline*/
     /*returns a negative distance so know that it was not calculated*/
     return;
   }

In this case beamline is not used, this is a regurgitated function used 
for another program that calculates the vertex of a particles from the 
beamline if # of tracks ==1, otherwise all tracks in TBTR.

I will provide plots tomorrow, I wanted to give a heads up about the 
code, and why this might not be useful for either side of the conversion 
debate.

BR
MK
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