Charm-baryon semileptonic decays and the strange ${\Lambda }^{\ast }$ resonances: New insights from lattice QCD

Abstract

Understanding the properties of the strange ${\Lambda }^{\ast }$ baryon resonances is a long-standing and fascinating problem. ${\Lambda }_c$ 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 ${\Lambda }_c$ semileptonic decays to a ${\Lambda }^{\ast }$ resonance: the ${\Lambda }^{\ast }(1520)$, which has negative parity and spin $3/2$. Here we present the resulting Standard-Model predictions of the ${\Lambda }_c\to {\Lambda }^{\ast }(1520){\ell }^{+}{\nu }_{\ell }$ differential and integrated decay rates as well as angular observables. Furthermore, by combining the recent BESIII measurement of the ${\Lambda }_c\to X{e}^{+}{\nu }_e$ inclusive semipositronic branching fraction [Phys. Rev. Lett. 121, 251801 (2018)] with lattice-QCD predictions of the ${\Lambda }_c\to \Lambda e^{+}{\nu }_e$, ${\Lambda }_c\to n{e}^{+}{\nu }_e$, and ${\Lambda }_c\to {\Lambda }^{\ast }(1520)e^{+}{\nu }_e$ 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 ${\Lambda }_c\to {\Lambda }^{\ast }(1405)e^{+}{\nu }_e$ branching fraction to be very small, which may be another hint for a molecular structure of the ${\Lambda }^{\ast }(1405)$.

Comments: 6 pages, 1 figure