Can an atomic nucleus contain both particles and antiparticles?

Question

Is it theoretically possible to make a "deuterium" atom containing a proton and an antineutron in its nucleus?

Would the strong nuclear force cause attraction between a proton and an antineutron?

Would such a nucleus be stable, or would the proton somehow annihilate the antineutron when close enough?

Answer 1

An antineutron ($\bar{u}\bar{d}\bar{d}$) is made up of antiquarks and these will annihilate with the quarks in the proton ($uud$).

Such a nucleus is therefore unstable.