The antineutron is the antiparticle of the neutron. It was discovered by Bruce Cork in the year 1956, a year after the antiproton was discovered. An antineutron has the same mass as a neutron, and no net electric charge. However, it is different from a neutron by being composed of antiquarks, rather than quarks.
Since the antineutron is electrically neutral, it cannot be observed directly. Instead, the products of its annihilation with ordinary matter are observed.
Composed of anti-quarks down down up.
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