Shower merging
The calorimeter response simulation does not consider the problem of
merging of showers close together. Since this is a rather important
feature in eg. deciding if a neutral EM shower is a single gamma or
two gammas from a decay, a routine (ZAUSHO) to be called in the
analysis part of SGV is supplied to do this. The showers are merged if
the separation of the two showers (separation meaning separation in the
four shower-axis parameters simultaneously) is such that the probability is
greater than some user-defined value that two independent measurements
of the SAME shower would give that same observed separation (or less).
(Hence, big limiting probability implies many showers merged, and v.v.).
This is the multi-dimensional analog to the well known way to decide if
two single-number values will be possible to separate when measurement
precision is taken into account, ie. by checking the 'number of sigmas'
separating the true values.
Merged showers will have a seen energy equal to the sum of the those of
the contributing showers, and position and direction equal to the weighted
average of them.
Note that merging is done only within a calorimeter, not between different
ones. Hence, the possible m.i.p. signal from a hadron in an electro-magnetic
calorimeter will not be merged with the shower from the same hadron in
the hadron calorimeter behind.