Measurement of the neutron lifetime using an asymmetric magneto- gravitational trap and in situ detection

TL;DR

This paper reports a new neutron lifetime measurement using an innovative magnetic trap that minimizes loss mechanisms, achieving a more accurate and correction-free result crucial for physics and cosmology.

Contribution

It introduces a novel asymmetric magneto-gravitational trap with in situ detection, eliminating previous trap loss corrections and providing a more precise neutron lifetime measurement.

Findings

Neutron lifetime measured as 877.7 ± 0.7 (stat) +0.4/-0.2 (sys) seconds.

First modern measurement without large correction factors.

Reduces discrepancy between beam and trap measurements.

Abstract

The precise value of the mean neutron lifetime, $\tau_n$, plays an important role in nuclear and particle physics and cosmology. It is a key input for predicting the ratio of protons to helium atoms in the primordial universe and is used to search for new physics beyond the Standard Model of particle physics. There is a 3.9 standard deviation discrepancy between $\tau_n$ measured by counting the decay rate of free neutrons in a beam (887.7 $\pm$ 2.2 s) and by counting surviving ultracold neutrons stored for different storage times in a material trap (878.5$\pm$0.8 s). The experiment described here eliminates loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls and neutrons in quasi-stable orbits rapidly exit the trap. As a result of this approach and the use of a new in situ neutron detector, the lifetime reported here (877.7 $\pm$ 0.7 (stat) +0.4/-0.2 (sys) s) is the first modern measurement of $\tau_n$ that does not require corrections larger than the quoted uncertainties.