Hypernucleus

A hypernucleus is a nucleus which contains at least one hyperon (a baryon carrying the strangeness quantum number) in addition to nucleons. The first was discovered by Marian Danysz and Jerzy Pniewski in 1952 using the nuclear emulsion technique.

The strangeness quantum number is conserved by the strong and electromagnetic interactions, a variety of reactions give access to depositing one or more units of strangeness in a nucleus. Hypernuclei containing the lightest hyperon, the Lambda, live long enough to have sharp nuclear energy levels. Therefore, they offer opportunities for nuclear spectroscopy, as well as reaction mechanism study and other types of nuclear physics (hypernuclear physics).

Hypernuclear physics differs from that of normal nuclei because a hyperon, having a non-zero strangeness quantum number, can share space and momentum coordinates with the usual four nucleon states that can differ from each other in spin and isospin. That is, they are not restricted by the Pauli Exclusion Principle from any single-particle state in the nucleus. The ground state of helium-5-Lambda, for example, must resemble helium-4 more than it does helium-5 or lithium-5 and must be stable, apart from the eventual weak decay of the Lambda. Sigma hypernuclei have been sought, as have doubly-strange nuclei containing Cascade baryons.

Hypernuclei can be made by a nucleus capturing a Lambda or K meson and boiling off neutrons in a compound nuclear reaction, or, perhaps most easily, by the direct strangeness exchange reaction.

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