A new neutron lifetime experiment with cold neutron beam decay in superfluid helium-4

The puzzle remains in the large discrepancy between neutron lifetime measured by the two distinct experimental approaches—counts of beta decays in a neutron beam and storage of ultracold neutrons in a potential trap, namely, the beam method versus the bottle method. In this paper, we propose a new experiment to measure the neutron lifetime in a cold neutron (CN) beam with a sensitivity goal of 0.1% or sub-1 s. The neutron beta decays will be counted in a superfluid helium-4 scintillation detector at 0.5 K, and the neutron flux will be simultaneously monitored by the helium-3 captures in the same volume. The CN beam must be of wavelength λ > 16.5 Å to eliminate scattering with superfluid helium. A new precise measurement of neutron lifetime with the beam method of unique inherent systematic effects will greatly advance in resolving the puzzle.