Neutral Decay of theωMeson

Abstract

The reaction ${\ensuremath{\pi}}^{\ensuremath{-}}+p\ensuremath{\rightarrow}\ensuremath{\omega}+n$ at 1.2 $\frac{\mathrm{BeV}}{c}$ was detected by counter techniques. Recoil neutrons were detected in a scintillation counter array at ${\ensuremath{\theta}}_{n}={20}^{\ensuremath{\circ}}\ifmmode\pm\else\textpm\fi{}{2}^{\ensuremath{\circ}}$, and their velocity determined by a time-of-flight measurement. Measurement of the neutron direction and velocity determined the $\ensuremath{\omega}$ mass with resolution \ifmmode\pm\else\textpm\fi{}15 MeV, and enabled separation of $\ensuremath{\omega}$ production from background. Scintillation counters surrounding the hydrogen target distinguished charged from neutral $\ensuremath{\omega}$ decays, yielding a branching ratio: $\frac{{\ensuremath{\Gamma}}_{\mathrm{neutral}}}{{\ensuremath{\Gamma}}_{\mathrm{charged}}}=0.124\ifmmode\pm\else\textpm\fi{}0.021$. Neutral $\ensuremath{\omega}$ decays were directly observed by conversion of the resulting $\ensuremath{\gamma}$ rays in a leadplate spark-chamber array. Events with three observed $\ensuremath{\gamma}$'s were fitted to the decay $\ensuremath{\omega}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}+\ensuremath{\gamma}\ensuremath{\rightarrow}3\ensuremath{\gamma}$ (a two-constraint fit). Twenty-six fits with ${\ensuremath{\chi}}^{2}<5$ were found, of which five are estimated to be background. If $\ensuremath{\omega}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}+\ensuremath{\gamma}$ is the only neutral mode, then we expect 29 observed ${\ensuremath{\pi}}^{0}\ensuremath{\gamma}$ decays, as calculated from the recoil-neutron measurements and spark-chamber efficiency. Thus our data give clear evidence that the mode $\ensuremath{\omega}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}+\ensuremath{\gamma}$ represents a substantial fraction (consistent with 100%) of the $\ensuremath{\omega}$ neutral decays.