Σ0Mass

The mass of the ${\Sigma }^0$ hyperon is obtained from a single associated production event in which both strange particles are observed to decay. The event is unusual, first because the ${\Sigma }^0$ hyperon decays directly into an electron-positron pair plus a $\Lambda$, and secondly because all the tracks are long and of higher-than-average quality. The value of $Q_{\Sigma }$ obtained from this event is 76.4±3.3 Mev, where $Q_{\Sigma }\equiv M_{{\Sigma }^0}-M_{\Lambda }$. For $M_{\Lambda }=1115.2\mathrm{}\pm 0.14$ Mev the mass of the ${\Sigma }^0$ is 1191.6±3.3 Mev. Upon averaging this value with the existing measurements of Alvarez et al. and Eisler et al., one obtains $Q_{\Sigma }={{75.3}_{-1.4\mathrm{}}}^{+0.9\mathrm{}}$ Mev and hence $M_{{\Sigma }^0}={{1190.5}_{-1.4\mathrm{}}}^{+0.9\mathrm{}}$ Mev.