σ meson exchange effect on non-mesonic hypernuclear weak decay observables

Abstract

We analyze the influence of σ meson exchange on the main non-mesonic hypernuclear weak decay observables: Total rate Γ_NM, neutron-to-proton branching ratio Γ_n/p, and proton asymmetry parameter a_Λ. The σ meson exchange is added to the standard strangeness-changing weak ΛN → NN transition potential, which includes the exchange of the complete pseudoscalar and vector mesons octet (π, η, K, ρ, ω, K*). Using a shell model formalism, the σ meson weak coupling constants are adjusted to reproduce the recent Γ_NM and Γ_n/p experimental data for ⁵_ΛHe. Numerical results for the remaining observables of ⁵_ΛHe and all the observables of ¹²_ΛC decays are presented. They clearly show that the addition of the σ meson improves the agreement with the data. However, its effect it is not enough to reproduce well the a_Λ values, which means that other improvements of the model must be explored to explain the puzzle posed by the experiments.