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 |×| 10|⁴| 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 ± 7.6 +~0.0-14.0| 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.