Neutron-Axion Scattering and the Neutron Lifetime Puzzle

Neutron lifetime anomaly is a discrepancy of results between two main types of experiment: the beam and the bottle methods. The difference between the most precisely measured results of each of the types is 10 ± 2 s, exceeding the 3σ bound. We consider a possible explanation for the anomaly, in which the neutrons in the experiments gain small recoil energy and are ejected from an apparatus due to the interactions with dark matter particles, as the solar system moves through the dark matter halo of the galaxy, with a mean velocity of 250 km/s. This could lead to an uncontrolled loss of neutrons, resulting in a shorter neutron lifetime in bottle experiments. Due to the unique capabilities of the UCNτ experiment to measure tiny recoil energies (∽10 neV), we base our calculations on the light dark matter models, using axions as convenient candidates. We present a calculation of the neutron-axion scattering cross-sections and use the lifetime differences to constrain various axion models, putting bounds on the dark matter's mass and its average density around the Earth's surface.