Trapping probability of strangeness via Xi− hyperon capture at rest in nuclear emulsion

Abstract

In the E373 experiment at KEK-PS, we have located nearly 2 $\times$ 10$^4$ stopping events of ${\Xi^-}$ hyperon candidates in nuclear emulsion. Among these, the identification of the ${\Xi^-}$ hyperon was performed with the constant sagitta (CS) method, by measuring multiple Coulomb scattering for 695 clear stopping events, $\sigma$-stop, with emission of nuclear fragments due to negatively charged hadrons. With use of a Geant4 simulation, the parameters for the CS method were optimized and we obtained the number of real ${\Xi^-}$ stopping events to be $432.3 \pm 7.6 ^{+~0.0}_{-14.0}$ with 3.2% systematic error by contamination by the $\Sigma^-$ hyperon. We have classified the events for ${\Xi^-}$ hyperon capture by light (C, N, O) and heavy (Ag, Br) elements in the emulsion. The trapping probabilities of two ${\Lambda}$ hyperons for light and heavy nuclei were found to be $5.0 \pm 1.7\%$ and $4.2 \pm 1.4\%$, respectively, for $\sigma$-stop via ${\Xi^-}$ hyperon capture at rest in the emulsion. For at least one ${\Lambda}$ trapping, the probabilities were $69.4 \pm 8.1\%$ for light nuclei and $51.1 \pm 5.7\%$ for heavy nuclei. These results allow us to present the trapping probabilities of ${\Lambda}$ hyperons via ${\Xi^-}$ hyperon capture at rest in emulsion for the first time, because the previous E176 experiment showed the lower limits due to fewer statistics.