History: Livetime

In Hall A we typically refer to the "livetime" as the fraction of the time the experiment is sensitive to events of interest, and "deadtime" as then (1-livetime).

^Note: In other references the "deadtime" actually corresponds to the length-of-time per event that the experiment is "dead" to another event of interest.^

-=Types of deadtime=-

When an event of interest occurs and leaves a sufficiently large signal in our detector, it still might not be readout due to the front-end electronics or the DAQ system being "dead" at that time. Here I discuss two types of deadtime in our experiment:
[*] Electronic (or front-end) deadtime
[*] DAQ deadtime

They are important for the asymmetry measurement since they are a function of the trigger rate and could, in principle, be different for the two helicity states.



-=Electronic deadtime=-


-=DAQ deadtime=-
This is the simpler deadtime to measure and understand, especially since we ran in an unbuffered mode. When the Trigger Supervisor (TS) receives a trigger that passes the prescale-factor cuts, it will instruct the other ROCs to digitize the present event and send the data onto the CODA event builder. During this digitization process, the TS and ROCs send and hold a ___BUSY___ signal such that the TS does not accept another trigger until all the ROCs are ready again. This period of being BUSY was ~300-500 micro-seconds per event, during which the DAQ and TS were "dead" to new triggers -- hence the designation as the DAQ deadtime.

The DAQ deadtime is rather straight-forward to measure, since scalers are used to count each trigger for each helicity state irrespective of the TS state. DAQ livetime for T3 (our production trigger) is then just

::LT_DAQ_T3 = prescale_T3 * N_T3_datafile / N_T3_scaler::


-=References=-

W.R. Leo, __Techniques for Nuclear and Particle Physics Experiments: A How-to Approach__, Second Edition. (Springer-Verlag, Berlin, Heidelberg 1994)