Author: Shun Zhou (Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China)
Despite its great success in describing fundamental interactions in nature and the evolution of the Universe, the standard model (SM) of elementary particles and cosmology is currently facing quite a number of challenges. Among them, the most important ones are the origin of neutrino masses, the matter-antimatter asymmetry and the existence of dark matter. During the past few decades, tremendous efforts have been devoted to solving all these challenges in a consistent and unified theoretical framework, which extends the symmetries and particle content of the SM. In a class of models with a continuous global symmetry broken spontaneously, an axion-like particle (ALP) arises naturally, serving as a possible candidate for dark matter and leading to rich phenomena for the ongoing and forthcoming experiments.
A recent study [1] explores systematically the baryon-number-violating (BNV) decays of nucleons with an ALP in the final state. Without resorting to any specific model, the BNV decays of nucleons are governed by a complete set of dimension-eight BNV operators involving three light quarks within the axion-extended low-energy effective theory. These operators are first decomposed into the irreducible representations of the QCD chiral symmetry group, and then matched onto the recently-developed chiral framework to derive the nucleon-level effective interactions among the ALP, octet baryons and octet pseudoscalar mesons. The presence of the ALP opens a variety of exotic BNV decay modes of hyperons and nucleons, and the momentum distributions for three-body decays are found to be dramatically different from those in the literature due to the newly-identified chiral structure of the effective operators. The existing data from the Super-Kamiokande experiment have been implemented to set stringent upper bounds on the BNV proton decay rates. In particular, the projected bounds on the BNV decays of neutrons are within the future reach of the next-generation neutrino experiments such as JUNO and DUNE. The present study lays important theoretical groundwork and guidance for experiments searching for signatures of baryon number violation. The BNV interaction itself is required for dynamically generating the matter-antimatter asymmetry in the Universe, and the experimental discovery of the BNV decays of neutrons will definitely shed light on which kind of new physics beyond the SM should be.
References
[1] W.Q. Fan, Y. Liao, X.D. Ma and H.L. Wang, Chin. Phys. C 50, xxxx (2026), arXiv:2507.11844 [hep-ph].
