Proposal for Measurement of the Two-body Neutron Decay using Microcalorimeter

The bound beta-decay (BoB) of neutron is also known as the two-body neutron decay, which is a rare decay mode into a hydrogen atom and an anti-neutrino. The state of neutrino can be exactly inferred by measuring the state of hydrogen atom, providing a possible pathway to explore new physics. However, this rare decay mode has not yet been observed so far since it was predicted in 1947. The challenge in observing this decay is not only that its cross section is extremely low, equivalent to about branching ratio of the order of $10^{-6}$ of the three-body decay, but also that the final-state hydrogen atom is neutral and has extremely low kinetic energy, which cannot be effectively detected. In this study, we propose a microcalorimeter-based scheme for measuring the kinetic energies of hydrogen atoms produced from BoB of ultracold neutrons, which has a great advantage in terms of accuracy of the energy measurement. In this study, first, several important issues that require rigorous considerations for the decay measurements and possible solutions are discussed. Then, the requirements of the neutron flux and the appropriate structure design of the microcalorimeter are present by theoretical calculations. In short, this paper outlines our proposed novel experimental scheme for observing the BoB mode, addressing the possible solutions to all the necessary problems.