Speaker
Description
Observation of neutrinoless double-beta $(0\nu\beta\beta)$ decay, a beyond the standard model process that violates lepton number conservation, would imply that neutrinos are Majorana fermions. In order to draw reliable conclusions from the current experimental limits and potential future discoveries, it is important to reduce the uncertainties in the theoretical predictions of its decay rate. A major contribution to these uncertainties comes from the effective field theories (EFT) matched ab initio nuclear many-body calculation of its nuclear matrix element. It was pointed out recently that the leading order EFT amplitude of the subprocess $nn\to pp(ee)$ in the simplest scenario of light neutrino exchange remains undetermined due to an unknown contribution from a newly identified short-range operator. Lattice quantum chromodynamics (LQCD) is the only way to directly and reliably determine the associated low-energy constant. We provide here a formalism to obtain the physical $nn\to pp(ee)$ decay amplitude, and hence the missing contribution, from the LQCD calculation of the correlation function for this process. The complications arising from the Euclidean and finite-volume nature of the corresponding correlation functions are fully resolved, and the result of this work, therefore, can be readily employed in the ongoing LQCD studies of this process.
