Find PV Offset
For some reason,plastic veto did not line up exactly with wirechambers.
One way to fine the offset is to analyze large amount single hit events.
For each of 20 plastic veto, caculate the average angle of all particls
that hit it. Subtract the average angle from the theoretical value, we
can get the offset for one veto. Average over 20 offsets got from 20 plastic
veto we can get a reasonable offset for the whole plastic veto.
By analyzing data from ten run(about 1000000 events),ying find the offset
is about 0.56 degree for the whole plastic veto.
The graph below show the histogram of six plastic veto giving the angle
distribution of the particles that hit each plastic veto.