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


2 Answers 2


This is a bubble chamber antineutron event

antineutron antineutrongraph

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.

  • $\begingroup$ Thank you! It was a very enjoyable answer to read. It seams to me that this shows a reaction between antineutron and a proton, not neutron-antineutron. Did I understand this correctly? Also, I am not very clear on what you meant by "ends up in a neutron antineutron pair". I would appreciate a clarification. Thanks! $\endgroup$
    – Milad P.
    May 24, 2015 at 7:02
  • $\begingroup$ @MiladP. Yes, the five pions have in sum charge 1, the charge of the proton on which the antineutron annihilated. Charge exchange means that the antiproton exchanges its charge with the proton, it becomes an antineutron and the proton becomes a neutron: teachers.web.cern.ch/teachers/archiv/HST2002/feynman/… $\endgroup$
    – anna v
    May 24, 2015 at 8:20

Anti-neutron generator

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

  • $\begingroup$ Hmm, that picture that you've used without attribution looks awfully familiar. $\endgroup$ May 22, 2015 at 5:00
  • $\begingroup$ @JohnRennie Oops, I forgot to add the picture citation. I added one, but I will take down the answer if you don't want me using your picture. $\endgroup$
    – Jimmy360
    May 22, 2015 at 5:05
  • $\begingroup$ Hi Jimmy. You're welcome to use the picture. Anything posted on the SE accepts the creative commons licence, and I'm perfectly happy with this or I wouldn't be posting here. The answer the picture came from mentions antineutron-neutron annihilation only in passing, so it isn't an answer to this question, but I would have mentioned it as a starting point then gone on to give a more detailed account of antineutron-neutron observations. $\endgroup$ May 22, 2015 at 5:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.