Neutron lifetime beam anomaly and possible explanation

TL;DR

This paper reviews neutron lifetime measurements, highlighting a significant discrepancy between beam and storage methods, and discusses potential causes such as proton loss due to charge exchange collisions affecting experimental results.

Contribution

It provides a comprehensive comparison of recent neutron lifetime measurements and proposes a plausible explanation for the observed experimental discrepancy.

Findings

Storage method measurements agree within the Standard Model.

Beam method measurements show a significant discrepancy at 3.6σ.

Proton loss due to charge exchange collisions may explain the discrepancy.

Abstract

The review of experimental measurements of neutron lifetime is presented. Latest measurements with gravitational trap (PNPI NRC KI) and magnetic trap (LANL, USA) confirmed the result obtained by PNPI group in 2005. The results of measurements performed using UCN storing method are in good agreement; however, there is a significant discrepancy at 3.6{\sigma} (1% of decay probability) level with beam method experiment. The latest most accurate measurements of the neutron decay asymmetry and neutron lifetime measurements by storage method are in agreement within the Standard Model. This article discusses the possible causes of discrepancy in the measurements of the neutron lifetime. The most probable cause, apparently, is the loss of protons in beam method experiment during storage in a magnetic trap due to charge exchange collisions of protons with the residual gas. The proton becomes neutral and leaves the trap, which leads to a decrease in the number of registered protons, i.e. to a decrease in the probability of neutron decay or to an increase in the measured neutron lifetime.