Selection of $\pi ^-p\rightarrow \gamma \gamma n$ Events

The background sources as identified in the previous section, were rejected using a set of cuts. A two-photon event was selected from the reconstructed photon-pair events by requiring the events to pass:

• $C$ counter timing cut that required that the two photons fire the $C$ counters within $\pm 4$ ns of each other. This time information is obtained from the FBTDC's reading out the $C$ counters.
• Beam telescope amplitude cut that rejected events due to multi-$\pi$ accidentals based on the pulseheights recorded by the four beam telescopes viewing the incoming pion beam entering the spectrometer.
• Opening angle cut that restricted the opening angle spectra to the region where the $\pi^0\rightarrow\gamma\gamma$ background is small.
• Lower energy cut that removed the small number of events with one or both photons misidentified from Dalitz $e^+e^-$ decay originating from the target.

Following the procedure described in Section 5.2, at least four good tracks were required, and two such good photons meeting the tracking and photon cut requirements were selected as a candidate photon-pair. A total of $2.3 \times 10^{6}$ photon pairs passed both the tracking cuts and photon cuts from a total of $3.1~\times~10^{11}$ pion stops.

Figure 5.8: Two-photon energy spectrum dominated by background contributions from $\pi ^o$ decays and multi-$\pi$ stops.

These photon pairs are shown in Figure 5.8 and are dominated by the backgrounds from $\pi ^o$ decays and multi-$\pi$ stops. The multi-$\pi$ background is clearly seen in the summed energy spectrum as events with $E > 150$ MeV. We have already seen the $\pi ^o$ background events in the previous Section in Figure 5.6 in the opening angle spectrum as events with $\cos{\theta} < -0.76$.