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A hydrogen nucleus consists of a single proton. A 2-hydrogen (deuterium) nucleus consists of a proton and a neutron. A tritium nucleus consists of a proton and two neutrons.

This makes me wonder how an atomic nucleus made of a proton and a "minus one neutron" would look like, and the closest thing to a "minus one neutron" I can imagine is an antineutron.

What happens if we combine a proton and an antineutron? Are things like this even possible?

If such a thing is an atomic nucleus, can we add an electron and get an atom?

Edit: in the comments below, I also asked this more specific question (I suppose it's useful to also mention it here in order not to create a complete chaos):

If the proton and antineutron annihilate, is it still possible that the thing they annihilate to remains somehow stable enough to behave like an atomic nucleus?

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    $\begingroup$ At low energies proton annihilates with antineutron $\endgroup$
    – voix
    Dec 27, 2011 at 4:53
  • $\begingroup$ Ok, but this raises two more questions: (1) what about at other energies or some other circumstances? (2) If it annihilates, is it still possible that the thing it annihilates to remains somehow stable enough to behave like an atomic nucleus? $\endgroup$
    – Dejan Govc
    Dec 27, 2011 at 11:22
  • $\begingroup$ Since an antineutron is made of quarks -u, -d, -d and a proton is made of quarks u, u, d, I can imagine what remains is a particle made of two quarks: u and -d. According to en.wikipedia.org/wiki/List_of_mesons that could be a pion or a charged rho meson ... Could it behave like an atomic nucleus? $\endgroup$
    – Dejan Govc
    Dec 27, 2011 at 11:29
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    $\begingroup$ @DejanGovc Don't imagine that the annihilation works neatly every time. There are virtual quark pairs that contribute to the nucleon (that why I write "valence content"), and this is an energetic event and there is plenty of energy around for making light hadrons and leptons. $\endgroup$ Dec 27, 2011 at 16:36
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    $\begingroup$ @DejanGovc, kinetic energy of the resulting pions is too high to form a nucleus. See also pionium. It can be created by low-energy pions but it has a short lifetime. $\endgroup$
    – voix
    Dec 27, 2011 at 18:43

2 Answers 2

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Given that the valence quark content of a proton is $(uud)$ and that of a anti-neutron in $(\overline{udd})$ the answer is that sooner later some of the constituent quarks will annihilate and you get a spray assorted particles.

The lifetime of such a nucleus will depend on it's orbital angular momentum, with s-states being very short lived and high angular momentum states lasting a little longer.

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In addition to dmckee's answer this link summarizes the experiments of antiprotons catching protons and neutrons, creating temporary nuclei.

It is the symmetric state to the one in the question : an anti-proton-neutron nucleus that lasts for a bit (fig 5.7). Anti-protons can be made in abundance and controlled experimentally because they are charged, anti-neutrons do not give easy handles.

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    $\begingroup$ Oohhh...data. Just what I wanted for Christmas! $\endgroup$ Dec 27, 2011 at 16:39
  • $\begingroup$ sadly, links don't last forever... :( $\endgroup$
    – Michael
    Aug 26, 2022 at 15:43
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    $\begingroup$ @Michael since 2011 there is a rule here to quote numbers , in case the link is broken. This is by the same author I think, who has learned to model the data arxiv.org/abs/1912.07385 $\endgroup$
    – anna v
    Aug 26, 2022 at 17:38

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