The neutron lifetime anomaly and charge exchange collisions of trapped protons**Dedicated to the memory of the late John M Robson, ScD.

Abstract

Measurements of the neutron lifetime have improved dramatically in accuracy in recent decades and provide data of crucial significance to particle physics and cosmology. Although experimental results with standard deviations of order 0.1% have been claimed, nevertheless the two main measurement techniques, the beam method and the bottle method, give results that differ by about 1% or four standard deviations. This troubling discrepancy could either be a hint of new physics, or of unidentified event losses in the two different types of particle trap that have been employed. A possible source of proton losses in the beam method arises from charge exchange collisions between residual gas molecules and protons stored in a quasi-Penning trap. Using published charge exchange cross sections, evaluations were made of the effectiveness of a number of gases commonly present as residual gas in high vacuum systems for bringing about proton loss. It is demonstrated that in many cases, these losses are significant for neutron lifetime measurements in vacua down to 10−11 mm Hg when 0.1% accuracy in the neutron lifetime is sought. Methods to correct for charge exchange losses are proposed.