Antineutron

An antineutron is the antiparticle of the neutron. It is a neutral baryon made up of three antiquarks, specifically anti-up, anti-down, and anti-down (ū d̄ d̄). It has the same mass as the neutron and zero electric charge, but opposite quantum numbers such as baryon number, which is -1 for the antineutron, and an opposite magnetic moment. Like the neutron, the antineutron is a spin-1/2 fermion.

In interactions, the antineutron participates in the same strong and electromagnetic processes as the neutron. A key difference is that an antineutron annihilates when it encounters ordinary matter (nucleons), typically producing a burst of pions and other mesons. When free, an antineutron can decay via the weak interaction into an anti-proton, a positron, and an electron neutrino (anti-neutron → anti-proton + e+ + νe) with a lifetime similar to that of the neutron, on the order of several minutes.

Antineutrons are produced in high-energy particle collisions and in cosmic-ray interactions. They are detected by observing their annihilation signatures in detectors, or, less directly, by tracking decay products if the antineutron decays before annihilating. The existence and properties of antineutrons are predicted by CPT symmetry, which ensures that particles and their antiparticles have identical masses and lifetimes (within experimental limits) but opposite charges and quantum numbers.

Antineutrons play a role in fundamental tests of symmetries and in studies of antimatter interactions, contributing to our understanding of baryon number conservation and matter–antimatter asymmetry.