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Antineutron, antiparticle of the neutron. The neutron has no electric charge, and thus so does the antineutron. However, the neutron consists of one up quark (charge +2/3) and two down quarks (charge –1/3), and the antineutron consists of one anti-up quark (charge –2/3) and two anti-down quarks (charge +1/3). The baryon number is the number of quarks minus the number of antiquarks divided by 3. Thus, the neutron, made up of three quarks, has a baryon number of 1, and the antineutron, made up of three antiquarks, has a baryon number of –1. A free antineutron decays into an antiproton, a positron, and an antineutrino. The antineutron was first produced in 1956 at the Bevatron particle accelerator at the University of California, Berkeley, by passing an antiproton beam through matter. Antineutrons were created when antiprotons in the beam exchanged their negative charge with nearby protons, which have a positive charge. The antineutrons were detected through their annihilation reactions with neutrons and protons.

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