the antineutrino is the antiparticle of the neutrino. A comparison of our data with pn data at the same momentum allows us to estimate the annihilation contribution to each multiplicity, and hence …

But to conserve also the lepton number a neutrino has also to be produced. the antineutron is the antiparticle of the neutron.

the antiproton is the antiparticle of the proton.

The positron for example has the same mass as an electron but it has a positive (+) charge whereas and electron has a negative (-) charge.

No problem of energy conservation due to the much smaller masses of the pions to the two nucleons. Anti-neutron may annihilate only with other baryons while electron belongs to a family of particles called leptons. From an exposure of the 30-inch deuterium filled bubble chamber to a 100 GeV/c antiproton-enriched beam at FNAL, we have calculated the topological cross sections for antiproton-neutron interactions with three or more charged particles in the final state. Thus, if the neutron were to decay into an antiproton and a positron (plus, I presume, a neutrino to conserve the lepton number), it would conserve the total charge and the lepton number ($-1$ for the positron and $+1$ for the neutrino), but it would violate the conservation of total baryon number.

In case of anti-neutron annihilation with a proton a positron has to be created to conserve charge.

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 .