Stability of Free Neutron, Antineutron
One of my students asked me: "Why is a free anti-neutron
stable but a free neutron decays fairly rapidly?"
I think your student was mistaken.
The little theory I know, and the quick look-around that I just did,
indicate that the antineutron would be just as unstable as the neutron.
There are principles of "parity" in particle physics,
and charge is one of them.
This means each anti-particle will act the same as it's normal counterpart,
other than being deflected ion opposite directions by EM forces.
The anti-quarks making up each anti-neutron would be bound
to act the same as the normal quarks making up a normal neutron.
A scientist at Fermilab wrote this answer:
and mentioned nothing about differing lifetime,
though I think the fact would beg to be mentioned.
Violations of the parities do occasionally happen, and it's a pretty advanced topic.
As I understand it, there is no known strong reason for
the dominant polarity of matter we have in this universe.
If neutron and anti-neutron had differing half-lives,
it would be a key clue to investigating that question.
It would have been a hot topic, and I think would have heard of it before.
I can only guess that maybe your student saw "neutron" and "antineutrino"
in the same paper and mis-recognized the latter.
Hope that doesn't disrespect him.
Antineutrinos are considered nominally stable, like normal neutrinos.
(Though oscillation between the three flavors of normal neutrino is a "recent" discovery.)
I do not actually know how much anti-neutrons have been played with before.
If it is a new thing to be able to measure their lifetime,
then perhaps this exotic news could happen.
Actually the term is "symmetry" not "parity". My mistake.
The symmetries are "CPT": Charge, Parity, and Time.
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Update: June 2012