Lifetimes, branching ratios, and asymmetries in Σ-hyperon decays

Abstract

Σ-hyperons produced by the capture of K−-mesons in nuclear emulsion have been studied. The mean lifetime of the Σ+-hyperons which decay via the proton and π+-meson modes has been measured. For the proton mode using 117 decays at rest and in flight we find\(^{\bar \tau } \sum {^ + } \to p(all) = (8.5_{ - 1.1}^{ + 1.4} ) \cdot 10^{ - 11} s\). Using the 49 decays in flight only and the method of Maximum Likelihood,\(^{\bar \tau } \sum {^ + } \to p(flight) = (9.5_{ - 3.8}^{ + 18.7} ) \cdot 10^{ - 11} s\). For these decays the calculated lifetime is strongly dependent upon the measured Σ-hyperons energies. Both statistical and systematic errors tend to lower the measured lifetime. The lifetime calculated from 37 Σ+ decays in flight into π+ mesons is\(^{\bar \tau } \sum {^ + } \to p(flight) = (6.0_{ - 2.0}^{ + 6.8} ) \cdot 10^{ - 11} s\) and is essentially unchanged when a cut-off of 5·10−11 s is used. No evidence exists for a short-lived component. The following branching ratios have been found:\(\frac{{\sum ^ + \to p + \pi ^0 }}{{\sum ^ + \to n + \pi ^0 }} = 1.00 \pm 0.09\) and\(\frac{{\sum ^ + \to p + \gamma }}{{all \sum ^ + }}< 0.75\% \). The values for the asymmetry parameters have been measured. When combined with previous data they are found to be\(\alpha ^0 \bar P = - 0.03 \pm 0.14\),\(\alpha ^0 \bar P = - 0.03 \pm 0.16\) and\(\alpha ^0 \bar P = - 0.14 \pm 0.75\). The superscripts refer to the charges of the decay π-mesons.