Has neutron-antinutron reaction been observed? If so, can you please provide a documentation (experimental observation) for neutron antineutron reaction?
This is a bubble chamber antineutron event
This picture, taken in the Berkeley 30-inch propane bubble chamber in 1958
Antiprotons enter from the top with momentum 684MeV/c . At the arrow one of the antiprotons in the beam disappears, shown with purple on the right. Then a vertex appears out of nothing where an annihilation event is obseved into five pions. The only hypothesis that can fit the energy and momentum balances of the event is that the antiproton undergoes a charge exchange with a proton in the liquid and ends up in a neutron antineutron pair. As the energies are low the neutron has no detectable scatterings as it is neutral , and leaves the chamber. The antineutron meets a proton and annihilates into pions.
It is interesting to note that the incoming kinetic energy is too low to allow the energies appearing in the five pions. The energy comes from the masses of the target proton and the antineutron.
Since then antineutron beams have been created and experiments with antineutrons performed as here
The operating performances of a low-momentum (< 270 MeV/c) tagged beam are reported. The beam is obtained by means of the charge exchange reaction on a liquid hydrogen target. The neutron associated to the antineutron in the two-body reaction is used for the determination of the energy and direction. The measured total rate of tagged antineutrons is 8.02 ± 0.03 × 10^−5 per incident antiproton at 300 MeV/c.