Is the process $n + \bar{n} \rightarrow \pi^{+} + \pi^{-} + \pi^{0}$ possible?
Answer
Within a few MeV, from isospin symmetry of the strong interaction, it is similar to the antiproton proton annihilation which happens even at rest: Role of Delta exchange for proton-antiproton annihilation into two-pion and three-pion channels .
No problem of energy conservation due to the much smaller masses of the pions to the two nucleons.
This is an actual bubble chamber photograph of an antiproton (entering from the bottom of the picture), colliding with a proton (at rest), and annihilating. Eight pions were produced in this annihilation. One decayed into a mu + and a neutrino. The paths of positive and negative pions curve opposite ways in the magnetic field, and the neutral leaves no track.
This is antiproton proton, but antineutron-neutron would look the same (impossible to get a beam of antineutrons). Average pion multiplicity at rest is five, two pi0 3 charged (note, average).
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