Acceptance Studies

Figure 4.1 shows the measured magnetic field dependence of the two-photon acceptance. The data show a rapidly falling acceptance with increasing magnetic field due to the increasing $e^+e^-$ track curvature and decreasing $e^+e^-$ D counter hits.

Figure 4.1: The measured two-photon acceptance following $\pi ^-$ stops in a CH$_2$ target as a function of magnetic field and trigger condition. For definition of the trigger ID's see Table 4.1.

The logical definitions of the trigger conditions loose, 2-D, 4-D and tight are given in Table 4.1 and are denoted as 1, 2, 3 and 4 in Figure 4.1. As seen from the figure, requiring four valid D's (the 4-D and tight triggers) rather than two valid D's (the $loose$ and 2-D triggers) significantly reduces the acceptance. It is also evident that demanding two C's only (the loose trigger) rather than two or three C's (the 2-D trigger) also significantly reduces the acceptance. Both of these effects result from wrap-around e$^+$e$^-$ tracks which cause less than four D's and more than two C's to fire. Thus, from the point of view of maximizing the acceptance, low magnetic fields and low D counter multiplicities were found to be preferable.