Luminosity vs Crossing (Bunch) Number
There are 57 bunches in each beam. It is possible that the luminosity will
vary with each bunch number. This requirement is to keep a record of the
luminosity for each bunch.
It is possible that trigger efficiency will be a function of instantaneous
luminosity, especially for p-p collisions. Thus a histogram of the triggers
for each trigger type (including min bias triggers) will be useful in
disentangling these effects. The sum over bunch number is an independent
way of measuring luminosity over a time interval short compared to the rate
of change in luminosity in time.
The number of collisions per crossing (10*-4 for Au-Au) is a measure of
instantaneous luminosity. A threshold condition should be set for each trigger
detector and a file written for each bunch that is incremented on every
crossing. Minimum bias triggers are events where anything at all has
happened. See Effective Luminosity.
In order to measure a cross section for each class of trigger it is necessary
to record the luminosity over a time interval that is short compared to the
rate of luminosity change ( several minutes). Since each trigger class has
its own live time as a function of what detectors are required a record must
be kept of live time/total time for each trigger class. Then two sources of
luminosity information (beam current from RHIC and trigger counter rates from
luminosity vs crossing number circuitry) can be used to calculate effective
integral luminosity for each trigger class. A special event must be recorded
periodically that contains only these numbers and a beam crossing luminosity
This information is essential if actual cross sections for various physics
processes are to be calculated.
A difference between luminosity derived from beam current measurements and
luminosity derived from trigger counter measurements would indicate a problem
that needs to be understood.
Luminosity Affected by Acceptance
Each of the detectors has a finite acceptance (the SVT, for example, may have
acceptance for only 25% of the events). Therefore, it is required that as
part of the event stream some unbiased events be recorded so that actual
acceptances for each detector can later be applied to the effective
luminosity data so that true cross sections can be calculated.
This requirement is essential if actual cross sections are to be calculated.
June 29, 1994