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Charm-baryon semileptonic decays and the strange $Λ^*$ resonances: New insights from lattice QCD

Understanding the properties of the strange $Λ^*$ baryon resonances is a long-standing and fascinating problem. $Λ_c$ charm-baryon semileptonic weak decays to these resonances are highly sensitive to their internal structure and can be used to test theoretical models. We have performed the first lattice-QCD computation of the form factors governing $Λ_c$ semileptonic decays to a $Λ^*$ resonance: the $Λ^*(1520)$, which has negative parity and spin $3/2$. Here we present the resulting Standard-Model predictions of the $Λ_c\toΛ^*(1520)\ell^+ν_\ell$ differential and integrated decay rates as well as angular observables. Furthermore, by combining the recent BESIII measurement of the $Λ_c \to X e^+ ν_e$ inclusive semipositronic branching fraction [Phys. Rev. Lett. 121, 251801 (2018)] with lattice-QCD predictions of the $Λ_c \to Λe^+ ν_e$, $Λ_c \to n e^+ ν_e$, and $Λ_c \to Λ^*(1520) e^+ ν_e$ decay rates, we obtain an upper limit on the sum of the branching fractions to all other semipositronic final states. In particular, this upper limit constrains the $Λ_c\toΛ^*(1405)e^+ ν_e$ branching fraction to be very small, which may be another hint for a molecular structure of the $Λ^*(1405)$.