Neutrinoless double beta decays of hyperons in covariant chiral perturbation theory
Neutrinoless double beta ($0νββ$) decays of spin-1/2 hyperons are investigated in a covariant baryon chiral perturbation theory framework, extended by a $ΔL=2$ operator proportional to the Majorana neutrino mass, where $L$ denotes the lepton number. Within the light Majorana neutrino exchange mechanism, the decay amplitudes are found to emerge at the one-loop level, representing the long-range contribution. The extended-on-mass-shell scheme is employed to renormalize the one-loop amplitudes and restore consistent chiral power counting. Consequently, the differential decay rates for all accessible hyperon $0νββ$ channels are predicted and the corresponding branching ratios are more than 20 orders of magnitude smaller than the current experimental upper bounds. Interestingly, it is found that the leading contribution to hyperon $0νββ$ decay is actually from short-range counterterm operators, as required by the renormalization argument. Neutrinoless transition form factors are proposed to determine this leading contribution through future lattice QCD simulations.