Determination of the efficiency of the hodoscopes during the Drell-Yan run

The COMPASS trigger system is mainly composed of fast plastic scintillators working as hodoscopes. For the selection of events of physical interest during the COMPASS Drell-Yan run in 2015 it was configured to trigger single- and dimuon-events. Unbiased trigger decisions can be used to determine the efficiency of the six symmetrically oriented hodoscopes used in 2015. The unbiasedness of the trigger is based on the usage of the hadron calorimeter trigger (CT) which was used on two dates during the run: The first time in the beginning and the second one in the middle respectively. Due to different beam conditions between the two dates a direct comparison between the results is unfortunately difficult. Especially in the beginning of the run, most of the strips of different hodoscopes show between 90% and often even 100%-efficiency. Compared to the results from the middle of the run, the efficiency of several strips seemed to get much worse during this time. Furthermore it can be seen that the method of determining the efficiency for each hodoscope strip separately works only for those hodoscopes where the track resolution is several times smaller than the strip width. Unfortunately a confirmation of this trend for dates earlier and later cannot be created by using the same methods since no further data sets with CT trigger exist.