Wikipedia explains:

The antineutron was discovered in proton–proton collisions at the Bevatron (Lawrence Berkeley National Laboratory) by Bruce Cork in 1956, one year after the antiproton was discovered.

Where is (preferably open accessible) literature about the discovery of the antineutron? Or more quick I ask:

  • Was the result with strong evidence of neutron-antineutron annihilation to pure energy?

  • Was the result in the way, that they get a particle shower (particle decay)? Which particles they discovered?


2 Answers 2


The paper announcing the discovery is Antineutrons Produced from Antiprotons in Charge-Exchange Collisions by Bruce Cork, Glen Lambertson, Oreste Piccione and William Wenzel. It was published in Phys. Rev. 104, 1193. This is restricted access, but the paper is also available on Google books.

The discovery was made by creating a beam of antiprotons and allowing these to create antineutrons by charge exchange, then diverting away the antiprotons to leave a beam of antineutrons. These were detected by measuring the scintillations created by their decay. This diagram shows a schematic illustration of the detectors used:

Antineutron detector

A beam of protons passes into the first scintillator, where charge exchange reactions such as:

$$ p + \bar{p} \rightarrow n + \bar{n} $$

create antineutrons. The mixture of antiprotons, antineutrons and other particles created in the reaction, such as $\pi$ mesons, pass through to a second scintillator where their annihilation is detected by the flash of light released. The intensity of light emitted indicates the mass of the annihilating particle, so it can distinguish $\bar{n}$ and $\bar{p}$ annihilation from the decay of lighter particles like $\pi$ mesons. Antiprotons and antineutrons can be distinguished because antiprotons create scintillations in two intermediate detectors $S_1$ and $S_2$.

The timings of the scintillations are correlated, so an antiproton will produce a signal from $S_1$, then $S_2$ and finally a large signal as it decays in the final scintillator. An antineutron produces a large signal in the final detector but with no preceding signals from $S_1$ and $S_2$.

  • $\begingroup$ With respect to your great work here I beg you to give more explanations about the production of antineutrons by charge exchange and about the scintillation. I can ask in separate question if you want. $\endgroup$ Sep 6, 2014 at 6:20
  • $\begingroup$ @HolgerFiedler: I've extended my answer a bit - see if that makes things clearer. If you want to ask about charge exchange reactions this is probably a separate question. $\endgroup$ Sep 6, 2014 at 7:22
  • $\begingroup$ Really $$ p + \bar{p} \rightarrow n + \bar{n} $$ and not $$ n + \bar{p} \rightarrow p + \bar{n} $$ ? $\endgroup$ Sep 6, 2014 at 8:41
  • 1
    $\begingroup$ @HolgerFiedler The putative reaction $n+\bar p\to p+\bar n$ doesn't conserve electric charge, while $p+\bar p\to n+\bar n$ does. You would also have $\bar p\to \bar n + \pi^-$, which like the positron creation process $\gamma\to e^-+e^+$ could only occur with a nucleus nearby to absorb some of the momentum. $\endgroup$
    – rob
    Sep 6, 2014 at 13:15
  • 1
    $\begingroup$ @HolgerFiedler "Can you write done the full reaction, please. p+p¯→n+n¯ couldn't be the whole truth." It may not be the "vertex" truth, but it certainly can be the experimentally accessible observed reaction. It requires only a single Drell-Yan-like quark anti-quark even: $u + \bar{u} \to d + \bar{d}$ at low momentum transfer. $\endgroup$ Sep 6, 2014 at 16:03

I don't remember the details of the argument offhand, but a neutron-antineutron pair doesn't have the right quantum numbers to annihilate into a pair of energetic photons the way that an electron-positron pair does. An antineutron that finds a neutron will annihilate first to a "starburst" of about five pions; the neutral pions then decay into photons, while the charged pions most decay to muons and muon neutrinos.

Here's a review of low-energy nucleon-antinucleon interactions, which I haven't yet read.

  • $\begingroup$ And what is the difference in this decay relatively to a neutron-neutron colliding? Not to long bounce comments: At the end the difference is in the number of neutrinos/antineutrinos? And we are sure that they (the neutrinos) are not there own antiparticles? $\endgroup$ Sep 6, 2014 at 6:12
  • $\begingroup$ @HolgerFiedler: Two neutrons colliding at low energy will scatter elastically, but there won't be annihilation. This can be used with great advantage to probe crystal structures and the internal dynamics of solid matter with "slow" neutrons. $\endgroup$
    – CuriousOne
    Sep 6, 2014 at 6:57
  • $\begingroup$ @CuriousOne: The above described neutron-antineutron collision happens at low energy level? $\endgroup$ Sep 6, 2014 at 7:19
  • $\begingroup$ One can annihilate neutrons with anti-neutrons at very low energy, far below the production threshold of any other massive particles. $\endgroup$
    – CuriousOne
    Sep 6, 2014 at 7:28
  • $\begingroup$ @CuriousOne: Some link to a source? $\endgroup$ Sep 6, 2014 at 8:37

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