Neutron antineutron reaction documentation Has neutron-antinutron reaction been observed? If so, can you please provide a documentation (experimental observation) for neutron antineutron reaction?
 A: 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.

A: 
A beam of anti-protons are fired into a charge-exchanger. If an anti-neutron is produced, S1 and S2 will not detect anything, but the final detector will. ([Picture courtesy of John Rennie])2 [Here is a book on the matter.]3
