Researchers from the National Institute of Standards and Technology (NIST) and four universities have made the first experimental observation of rare particles of light emitted during the radioactive decay of the neutron, a key building block of matter. This work confirms theoretical predictions of this type of neutron decay and sets the stage for a new class of tests of basic theories in particle physics.

The neutron is stable only in the nucleus of a stable atom; a free neutron decays into other particles — a proton, electron, and antineutrino — within about 15 min. This "neutron beta-decay" has been studied for decades without proof of the occasional photon (light) emissions predicted by theory.

"This measurement is difficult because the neutron lifetime is very long, so hardly any neutrons decay at one time, and of those that decay, few emit a photon," says NIST physicist Jeffrey Nico. Background radiation is also intense and makes such observations even more difficult. The team, including researchers from NIST, Tulane University, the University of Michigan, the University of Maryland, and the University of Sussex (Brighton, England) determined that, on average, slightly more than three out of 1,000 neutron decays produce a photon above an energy level that is relatively low but still observable. The measured value has only about 10% uncertainty, which is considered remarkable given that this decay had never been observed before.

According to the team, the experiment will lead to new tests of the "standard model" of particle physics and the search for new physics beyond it. Future upgrades to the experimental setup will reportedly enhance measurement precision and open additional opportunities for pushing the boundaries of scientific knowledge. Read more details of the experiments at