Physicists' understanding of how free neutrons decay into other particles has taken a step forward with the aid of an experiment performed at the National Institute of Standards and Technology (NIST). The study involved a process called neutron beta decay (a process that assists in our understanding of the Big Bang Theory).

Notably, the team's findings help to confirm the Standard Model. This model explains the interaction between the basic building blocks of matter works and how the four fundamental forces govern them (in essence, it is the explanation of the fundamental physics of our universe).

The method used by the time also provides opportunities for more discoveries regarding the Standard Model and quantum electrodynamics (QED).

Credit: N. Hanacek / NIST

To break this down, protons and neutrons lump together in an atom's nucleus. But neutrons not bound in a nucleus, called free neutrons, decay in about 15 minutes. Usually, after undergoing beta decay, a neutron transforms into a proton (red), an antineutrino (gold) and an electron (blue), and a photon (white).

The team wanted to focus on the photons produced after decay. Specifically, they wanted to verify with precision the range of possible energies of a photon as predicted by QED.


The measurements were performed at the NIST Center for Neutron Research (NCNR). Two aspects of neutron decay were measured. One is the energy spectrum of the photons and the other is the branching ratio, which provides information on how frequent a photon accompanies a decay above a certain energy. This gave them branching ratio measurement values that are more than twice as accurate as the previous ones, and the first measurement of the energy spectrum.

With better detectors, the approach they used may be used to look for right-handed neutrinos, which have never been detected in nature before, and potential time-reversal symmetry violations.

The study is published in Physical Review Letters.

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