Antineutron

The antineutron is the antiparticle of the neutron in the standard model. According to the quark model, it consists of two anti-down quarks and one anti-up quark.

The antineutron was discovered in 1956, one year after the discovery of the antiproton, by Bruce Cork et al. Like the neutron, it has a mass of about 939.6 MeV/c2, is electrically uncharged with a spin of ½ and is thus a fermion. The masses of the neutron and antineutron are identical with an uncertainty less than (9 ± 5) - 10-5. The CPT theorem predicts certain properties of particles/antiparticles, which can be tested experimentally using the antineutron, for example.

Antineutrons can be produced, for example, in the annihilation of accelerated high-energy electrons and positrons:

Although the antineutron has the same electric charge and spin as the neutron, it is a different particle because it is composed of antiquarks. The free antineutron decays to an antiproton, a positron, and an electron-neutrino, while the free neutron decays to a proton, an electron, and an electron-antineutrino. The lifetime and gyromagnetic ratio of the free antineutron have not yet been experimentally determined. According to the CPT theorem, theoretically the lifetimes of and coincide and the gyromagnetic ratio of the antineutron has the negative value of the gyromagnetic ratio of the neutron.

The experimental search for neutron-antineutron oscillations is considered as a further test of the quark theory. In vacuum, the free neutron fluctuates to the antineutron with a time constant greater than 2.7 - ¹⁰⁸ seconds (just under 10 years). Neutron-antineutron oscillations have been proposed theoretically and presuppose an as yet undiscovered process that violates baryon number conservation.

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