What if the disintegration of the neutron produces dark matter?

Neutrons, those neutral particles found in the nucleus of atoms, are not elementary particles, since they have structure. While they are stable when they are in the atomic nucleus, they disintegrate when they are free. A free neutron takes about 881.5 seconds to decay, a second and a half up or down, and the fact that a value is not available with a much greater precision is due to the fact that depending on how the experiment is done, some results or others.

B. Fornal and B. Grinstein / University of California, San Diego. Source: APS Physics

The source of the discrepancy between the two types of experiments carried out could be perfectly some kind of systematic error not yet identified. But another possibility could be that the neutrons disintegrate into those invisible particles in any part of the electromagnetic spectrum that we call dark matter, and that allow galaxies to exist as we know them.

This new hypothesis, as is logical, has aroused much interest, and experiments have already been carried out trying to prove it. The results of these do not rule out the idea, although they impose limits on which variants of the hypothesis are viable. Let's see.

Outside the nucleus, the neutron decomposes into a proton, an electron and an electronic neutrino. The studies of this disintegration process are of two types, which we can call "bottle" and "beam". In a bottle experiment, the researchers place a set of ultra-cold neutrons in a container (the bottle) and count how many remain after a certain interval of time. In a beam experiment, the researchers observe a neutron flux and count the number of protons created from the decays. The half-life of the neutron in a beam experiment can be up to 9 s longer than that measured in a bottle experiment.

Bartosz Fornal and Benjamin Grinstein of the University of California at San Diego (USA), propose a solution to this discrepancy that assumes that neutrons disintegrate 1% of the time in dark matter particles. Because beam experiments would not detect these decays, that would explain why these experiments measure a longer half-life of the neutrons than in the bottle ones, which would give values ​​closer to the correct value.

Neutron disintegration giving rise to a gamma ray. Source: APS Physics

Fornal and Grinstein investigated several possibilities with neutrons that decay giving different combinations of dark matter and visible particles. In one of these scenarios, "dark" neutron disintegrations are accompanied by the emission of a gamma ray. Inspired by this possibility, Christopher Morris of the Los Alamos National Laboratory, New Mexico (USA), and his colleagues controlled the possible emission of gamma rays in a bottle of ultra-cold neutrons. They found no signal, which seems to rule out this proposed decay channel in the photon energy range of between 782 and 1664 keV.

Neutron disintegration without producing a gamma ray. Source: APS Physics

But other possibilities of disintegration, which produce gamma rays of lower energy or that do not produce this type of radiation, are still possible and could be tested by looking for anomalies in the nuclear disintegrations.

Reference:

Bartosz Fornal & Benjamín Grinstein (2018) Dark Matter Interpretation of the Neutron Decay Anomaly Physical Review Letters doi: 10.1103/PhysRevLett.120.191801