In prior experiments, scientists have reported two disparate measurements of the lifetime of the neutron with a statistically significant difference. The conflicting findings prompted scientists at Oak Ridge National Laboratory (ORNL) to test for “mirror neutrons,” one of the particles in a theoretical “mirror dark sector” of matter, as a potential explanation of the discrepancy in a series of new experiments. A paper published in Physical Review Letters by the research team at ORNL reported that no such particle was found in the experiments, casting doubt on the validity of the mirror dark sector theory and keeping the cause of the discrepancy a mystery. See also: Neutron
Neutrons are subatomic particles that, together with protons, make up atomic nuclei. Outside of an atomic nucleus, neutrons are unstable particles. The amount of time that a neutron can exist outside a nucleus before breaking down into a proton, electron, and an anti-neutrino [in a process called beta (β) decay] is referred to as the neutron lifetime. A disparity in the neutron lifetime measurement results from two different methods of measurement. In the first method, called the “bottle” method, a gravitational or magnetic potential energy well is used to trap neutrons. Once the number of neutrons is ascertained, scientists can count the disappearances of neutrons and obtain an average lifetime. In the “beam” method, a neutron beam is passed through a magnetic field generated by a solenoid. As the beam passes through the magnetic field, the positively charged protons resulting from β decay will be redirected by the magnetic field and able to be detected and counted. Comparing this proton generation rate with the number of neutrons passing through the magnetic field per unit time allows for finding an average neutron lifetime. These two methods produce a difference of roughly nine seconds, with the beam method having the longer average lifetime. See also: Antimatter; Atomic nucleus; Electromagnetic field; Neutrino; Proton; Radioactivity; Solenoid (electricity)
One potential explanation of the discrepancy comes from the mirror dark matter hypothesis. Dark matter is an as-yet-unobserved type of matter that theoretically comprises the overwhelming majority of all mass in the universe, yet only interacts with ordinary matter through the force of gravity. The mirror dark matter hypothesis posits that dark matter is made up of “mirror” counterparts to each particle in the standard model, which is the theory describing all elementary particles and their interactions. For instance, the neutron, n, would have a mirror counterpart, n’, that is dark (that is, it only interacts with ordinary matter through gravity). Furthermore, it would be possible for any free particle to transform into its dark mirror counterpart and vice versa. This oscillation between ordinary and dark matter opens up an explanation for the neutron lifetime discrepancy. In the beam measurement of neutron lifetime, if a n ↔ n’ transformation were to occur as the neutron passed through the magnetic field, and then the n’ mirror neutron were to decay, it would be undetectable to the apparatus used in the experiment, as the resulting particles from the decay would be dark matter. This missing observation of an instance of decay would then make the neutron lifetime appear longer. See also: Dark matter; Elementary particle; Gravity; Matter; Standard model
The new experiments performed at ORNL sought to test this hypothesis by passing a neutron beam through a magnetic field towards a boron carbide neutron absorber. While the neutrons would be almost entirely absorbed by the boron carbide, the mirror neutrons would not interact with the absorber and pass through it. The purpose of the magnetic field was to supply potential energy to the system in order to compensate for any possible mass discrepancy between neutrons and mirror neutrons, thus encouraging n ↔ n’ transitions. The strength of the magnetic field at various distances from the boron carbide wall was arranged in a particular way to stimulate partial transitions of the neutrons to n’ before the wall, and entirely transition back to n after the wall. A neutron detector past the boron carbide wall was positioned to detect how many neutrons penetrated the barrier. See also: Mass; Particle detector
The experiment found no statistically significant difference in neutron detections from when the magnet was on, off, or on in a reversed direction. These results cast serious doubt on mirror neutrons as an explanation for the bottle and beam lifetime discrepancy, as well as the mirror dark matter theory as a whole. The null results do, however, further limit the options for what dark matter could be, and potentially get researchers one step closer to a verifiable observation of dark matter in a laboratory setting. As for the neutron lifetime discrepancy, it remains an unsolved problem, and scientists must search elsewhere for a fitting explanation. See also: Experiment