When evaluating the performance of the segment finding, what
is most important from a trigger point of view is not
finding all the segments in an event but finding enough
segments to enable the high 
tracks to be found.
The minimum segment information required to find a track by
the present track finding algorithm is at least one found
segment in superlayer one and at least one found segment
in superlayer three.
We define the truth track as at least one found
segment in superlayer one and at least one found segment in
superlayer three from the same MC track.
The truth tracks represent what could be found using the
present algorithm if the track finder were 100%
efficient.
Measuring the efficiency using truth tracks de-couples the
track finding inefficiencies from the segment finding
performance and allow a useful means to judge the trigger
performance.