The neutron, an otherwise stable subatomic particle residing in the nucleus, is known to disintegrate into an electron, proton, and an anti-neutrino when isolated, in a process called “beta decay.” One of the most intriguing questions in particle physics is centered around the lifetime of an isolated neutron. The measured values obtained from two kinds of experiments, one of which measures the decay product and the other measures missing neutrons, differ significantly and it is not yet understood if the discrepancy is due to undiscovered systematic effects or new physics.

To explore the cause behind the discrepancy, scientists from Japan devised a new method to determine the neutron lifetime in which they compared the rate of neutron decay relative to that of a reaction between a Helium-3 nuclei and a flux of pulsed neutron beam obtained from the neutron source at the Japan Proton Accelerator Research Complex (or J-PARC). Specifically, they measured the decay and reaction rates by simultaneously detecting electrons obtained from neutron beta decay and protons from the reaction between Helium-3 and pulsed neutrons in a gas chamber filled with dilute Helium-3 gas. This offered several advantages over the conventional methods: the simultaneous detection got rid of some systematic uncertainties while measuring the decay electrons enabled sensitivity to decay modes with no proton emission.

With these measures in place, the scientists estimated a neutron lifetime of 898 seconds, a value that was greater than those reported previously but covered them within its uncertainty range—which made it consistent with previous findings. However, the lifetime puzzle remains to be solved and will inspire future experiments with improved statistical and systematic uncertainties.