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. Λc 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 Λc 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 Λc→Λ∗(1520)+ν differential and integrated decay rates as well as angular observables. Furthermore, by combining the recent BESIII measurement of the Λc→Xe+νe inclusive semipositronic branching fraction [Phys. Rev. Lett. 121, 251801 (2018)PRLTAO0031-900710.1103/PhysRevLett.121.251801] with lattice-QCD predictions of the Λc→Λe+νe, Λc→ne+νe, and Λc→Λ∗(1520)e+νe 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 Λc→Λ∗(1405)e+νe branching fraction to be very small, which may be another hint for a molecular structure of the Λ∗(1405).