The neutron lifetime anomaly and charge exchange collisions of trapped protons

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.

Neutron’s dark secret

“…76 An experimental explanation suggests proton losses in the beam method arising from charge exchange collisions between the residual gas molecules and protons stored in the quasi-Penning trap. 77…”

Search for explanation of the neutron lifetime anomaly

“…For the first time, errors induced by residual gas were considered in ref. [12] and our goal is to extend this analysis and increase its accuracy.…”

“…The second part of the possible corrections is associated with the motion of protons inside the trap and the influence on this motion of the residual gas in the vacuum system. This problem was first discussed in [15], but we want to expand the research presented there. As in the previous section, we consider the configuration of the experimental setup from [1].…”

Neutron's Dark Secret

“…Other theoretical ideas for the neutron lifetime discrepancy involve an increased rate of neutron-mirror neutron oscillations due to the presence of a magnetic field in beam experiments [60], the existence of a large Fierz interference term [75] and the quantum Zeno effect [76]. An experimental explanation suggests proton losses in the beam method arising from charge exchange collisions between the residual gas molecules and protons stored in the quasi-Penning trap [77].…”