More than a century after its discovery, the proton remains a source of intrigue, its charge-radius and spin posing puzzles that are the focus of intense study. But what of its mortal sibling, the neutron? In recent years, discrepancies between measurements of the neutron lifetime using different methods constitute a puzzle with potential implications for cosmology and particle physics. The neutron lifetime determines the ratio of protons to neutrons at the beginning of big-bang nucleosynthesis and thus affects the yields of light elements, and it is also used to determine the CKM matrix-element Vud in the Standard Model.
The neutron-lifetime puzzle stems from measurements using two techniques. The “bottle” method counts the number of surviving ultra-cold neutrons contained in a trap after a certain period, while the “beam” method uses the decay probability of the neutron obtained from the ratio of the decay rate to an incident neutron flux. Back in the 1990s, the methods were too imprecise to worry about differences between the results. Today, however, the average neutron lifetime measured using the bottle and beam methods, 879.4 ± 0.4 s and 888.0 ± 2.0 s, respectively, stand 8.5 s (or 4σ) apart.
We think it will take two years to obtain a competitive result from our experimentKenji Mishima
In an attempt to shed light on the issue, a team at Japan’s KEK laboratory in collaboration with Japanese universities has developed a new experimental setup. Similar to the beam method, it compares the decay rate to the reaction rate of neutrons in a pulsed beam from the Japan Proton Accelerator Research Complex (J-PARC). The decay rate and the reaction rate are determined by simultaneously detecting electrons from the neutron decay and protons from the reaction 3He → 3H in a 1 m-long time-projection chamber containing diluted 3He, removing some of the systematic uncertainties that affect previous beam methods. The experiment is still in its early stages, and while the first results have been released – τn = 898 ± 10(stat)+15–18 (sys) s – the uncertainty is currently too large to draw conclusions.
“In the current situation, it is important to verify the puzzle by experiments in which different systematic errors dominate,” says Kenji Mishima of KEK, adding that further improvements in the statistical and systematic uncertainties are underway. “We think it will take two years to obtain a competitive result from our experiment.”
Several new-physics scenarios have been proposed as solutions of the neutron lifetime puzzle. These include exotic decay modes involving undetectable particles with a branching ratio of about 1%, such as “mirror neutrons” or dark-sector particles.
K Hirota et al. 2020 Prog. Theor. Exp. Phys. 123C02.