Comprehensive investigation on baryon number violating nucleon decays involving an axion-like particle

Abstract

In this study, we systematically investigate baryon number violating (BNV) nucleon decays into an axion-like particle (ALP), within a low energy effective field theory extended with an ALP, which is referred to as aLEFT. Unlike previous studies in the literature, we consider contributions to nucleon decays from a complete set of dimension-eight BNV aLEFT operators involving light $u,\,d,$ and $s$ quarks. We perform the chiral irreducible representation (irrep) decomposition of these interactions under the QCD chiral group $\rm SU(3)_{\tt L}\times SU(3)_{\tt R}$, and match them onto the recently developed chiral framework to obtain nucleon-level effective interactions among the ALP, octet baryons, and octet pseudoscalar mesons. Within this framework, we derive general expressions for the decay widths of nucleon two- and three-body decays involving an ALP. Subsequently, we analyze momentum distributions for three-body modes and find that operators belonging to the newly identified chiral irreps $\pmb{6}_{\tt L(R)}\times \pmb{3}_{\tt R(L)}$ exhibit markedly different behavior compared to that in the usual irreps $\pmb{8}_{\tt L(R)}\times \pmb{1}_{\tt R(L)}$ and $\pmb{3}_{\tt L(R)}\times \bar{\pmb{3}}_{\tt R(L)}$. In addition, we reanalyze experimental data collected by Super-Kamiokande and establish bounds on the inverse decay widths of these new modes by properly accounting for experimental efficiencies and Cherenkov threshold effects because of the lack of direct constraints on those exotic decay modes. Our recasting constraints are several orders of magnitude more stringent than inclusive bounds used in the literature. Based on these improved bounds, we set conservative limits on associated effective scales across a broad range of ALP mass and predict stringent bounds on certain neutron and hyperon decays involving an ALP.