Search for explanation of the neutron lifetime anomaly

Abstract

All results of the neutron lifetime measurements performed in the last 20 years with the ultracold neutron storage method are in a good agreement. These results are also consistent with the latest most accurate measurements of the neutron decay asymmetry within the Standard Model. However, there is a significant discrepancy at the $3.6\ensuremath{\sigma}$ (1% of the decay probability) level between the averaged result of the storage method experiments and the most accurate measurements performed with the beam method. This article addresses the possible causes of the neutron lifetime discrepancy. We focused on finding the spectrum of possible systematic corrections in the beam experiment. Four separate sources of the systematic errors that had not been properly addressed in articles dedicated to the beam technique were considered. Two of those sources are related to the motion of protons in an electromagnetic field and the elastic scattering by the residual gas. These problems are associated with the geometrical configuration of the setup and the proton detector size. The Monte Carlo simulation shows that these effects are negligible and do not affect measurement results. The third error concerns proton loss in the dead layer of the detector. It is shown that the correction for the dead layer requires a more detailed analysis than that given in the papers describing the beam measurement method. The fourth source of the systematic error is the charge exchange process on the residual gas. The influence of the residual gas was neglected in the beam method experiments. We present arguments showing that careful analysis of this issue is necessary since the proposed proton losses correction decreases the measured lifetime, bringing it closer to the storage method results. For the problem of proton charge exchange on a residual gas, the spectrum of possible corrections is investigated. It is shown that for an accurate calculation of the correction, it is necessary to directly measure the concentration and composition of the residual gas inside the proton trap. The analysis reveals that even the presence of only ${\mathrm{H}}_{2}$ molecules inside the proton trap can lead to the significant correction, which is the most probable explanation of the neutron anomaly.

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