Measurements of the Neutron Lifetime

Abstract

Free neutron decay is a fundamental process in particle and nuclear physics. It is the prototype for nuclear beta decay and other semileptonic weak particle decays. Neutron decay played a key role in the formation of light elements in the early universe. The precise value of the neutron mean lifetime, about 15 min, has been the subject of many experiments over the past 70 years. The two main experimental methods, the beam method and the ultracold neutron storage method, give average values of the neutron lifetime that currently differ by 8.7 s (4 standard deviations), a serious discrepancy. The physics of neutron decay, implications of the neutron lifetime, previous and recent experimental measurements, and prospects for the future are reviewed.

Precision experiments with muons and neutrons

“…Given its importance in particle physics and cosmology, the neutron lifetime τ n has been measured by more than 20 experiments since the 1950's. There is now a persistent disagreement between the two classes of methods to measure the lifetime, the so called "beam" and "bottle" methods [9]. In the "beam" method [10], one counts the rate of appearanceṄ of protons in a neutron beam within a fiducial volume containing N neutrons.…”

Alternative to the application of PDG scale factors

“…For a state of the art review of both types and more details, see Ref. [11]. The first type are beam experiments [12][13][14], which measure the number of protons or electrons from decays of cold neutrons in a beam passing through a magnetic or electric trap.…”

Measurement of the neutron lifetime and inverse quantum Zeno effect

“…There is an additional peculiar fact related to the neutron decay that deserves to be mentioned. To date [30], the averages of τ n as computed by considering all the beam experiments τ beam n ¼ 888.1 AE 2.0 s and all the trap experiments τ trap n ¼ 879.37 AE 0.58 s separately show a ∼4σ discrepancy, with the latter being ∼8 s shorter than the former. More precisely, the mismatch reads Δτ ¼ τ beam n − τ trap n ¼ 8.7 AE 2.1 s or, in terms of the ratio of decay widths, Γ trap n =Γ beam n ¼ 1.0098 AE 0.0024.…”

“…To summarize, the discrepancy between τ beam n and τ trap n is most likely due to complicated systematics in the beam measurements as discussed also in Refs. [16,30]. New measurements with the beam technique would be clearly very important to definitely rule out any possible discrepancy.…”