Trigger efficiency

Since the KLM trigger and hit information are not required in $\mu$-pair event selection criteria, the KLM trigger efficiency was studied using $\mu$-pair event samples. . Fig.  shows track by track trigger efficiencies as functions of cos$\theta$, $\phi$, and run number.

The two dips at cos $\theta$ = - 0.6 and 0.9 in Fig.  (a) are caused by the KLM geometry. This is due to the chimney hole of the superconducting solenoid around cos $\theta$ = - 0.6. Muon tracks which go through the hole do not yield trigger signals. When one track goes through the chimney hole, the other track tends to go to the forward end-cap region. The dip at cos $\theta$ = 0.9 is caused by tracks which go to a sector gap of forward end-cap KLM. The cause for the dips shown in Fig.  (b) is the same. Since the chimney hole is located around $\phi$ = 1.2 to 1.9, the dip at this region is directly caused by tracks which go through this region and the dip around $\phi$ = - 1.5 is caused by the other tracks of $\mu$-pairs.

Except for this geometrical effect, the trigger efficiency is kept about 98% in average. Fig.  (c) shows the long term stability of the KLM trigger efficiency from Jan. 2000 to Jun. 2000. Most of this period, the trigger efficiency has been kept above 97.5% and its fluctuation has been within statistical errors.

Figure: Trigger efficiency using $\mu$-pair events: (a) cos $\theta$ dependence, (b) $\phi$ dependence, and (c) run number dependence