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This would not show up in proton decay searches.","posed_since":"1970","precise":"Search for $\\Delta B = 2$ neutron-antineutron oscillations with free-neutron oscillation time $\\tau_{\\mathrm{nnbar}} > 8.6 \\times 10^{7}\\,\\mathrm{s}$ (ILL, 1994) and bound-neutron limits converting to $\\sim 4.7 \\times 10^{8}\\,\\mathrm{s}$ (Super-Kamiokande, 2021). 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