$\pi ^o$ Decay Background

The $\pi ^o$'s were produced by either at-rest or in-flight pion charge exchange. The at-rest source yields $\pi ^o$'s with energy $T = 2.8$ MeV and decay photons with opening angles $\cos{\theta} < -0.91$, while the in-flight source yields $\pi ^o$'s with $T \leq 15$ MeV and photons with $\cos{\theta} < -0.76$.

For $\pi ^-p\rightarrow \gamma \gamma n$ the $\cos{\theta} < -0.76$ region was overwhelmed by $\pi ^o$ background. Furthermore, due to the finite resolution of the photon-pair spectrometer, the $\pi ^o$ background was also a potential problem for opening angles with $\cos{\theta} > -0.76$. In the $\cos{\theta} > -0.76$ region, the two-photon spectra has a two-photon opening angle tail consisting of $\pi^-p\rightarrow\pi^0n$ events having true opening angles $\cos\theta < -$0.76 yielding measured angles $\cos\theta > -$0.76.

Figure 5.6: The experiment measures an overwhelming number of background events with the $\pi ^o$ background evident in the $\cos\theta < -0.76$ region even after rejecting most of the back-to-back photon-pairs, while the random background is evident in the $\cos\theta > -0.1$ region. The tail between is also reproduced by the Monte Carlo generated $\pi ^o$ background shown overlaid as the shaded histogram.

This $\pi ^o$ decay background, that appears as a $\pi ^o$-tail in Figure 5.6, was removed by a two-photon opening angle cut which was determined from Monte Carlo simulations (30) as described in Section 5.4.3.