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

In the E373 experiment at KEK-PS, we have located nearly 2 × 104 stopping events of Ξ−hyperon candidates in nuclear emulsion. Among these, the identification of the Ξ− hyperon was performed with the constant sagitta (CS) method, by measuring multiple Coulomb scattering for 695 clear stopping events, σ-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 Ξ− stopping events to be 432.3± with 3.2% systematic error by contamination by the Σ− hyperon. We have classified the events for Ξ− hyperon capture by light (C, N, O) and heavy (Ag, Br) elements in the emulsion. The trapping probabilities of two Λ hyperons for light and heavy nuclei were found to be 5.0±1.7% and 4.2±1.4%, respectively, for σ-stop via Ξ− hyperon capture at rest in the emulsion. For at least one Λ trapping, the probabilities were 69.4 ± 8.1% for light nuclei and 51.1 ± 5.7% for heavy nuclei. These results allow us to present the trapping probabilities of Λ hyperons via Ξ− hyperon capture at rest in emulsion for the first time, because the previous E176 experiment showed the lower limits due to fewer statistics.