Hypernucleus

A hypernucleus is a nucleus which contains at least one hyperon (a baryon carrying the strangeness quantum number) in addition to the normal protons and neutrons. 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,[1] 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.


K
 + nucleus →
π
 + hypernucleus

A generalized mass formula developed for both the non-strange normal nuclei and strange hypernuclei can estimate masses of hypernuclei containing Lambda, Lambda-Lambda, Sigma, Cascade and Theta+ hyperon(s).[2][3] The neutron and proton driplines for hypernuclei are predicted and existence of some exotic hypernuclei beyond the normal neutron and proton driplines are suggested.[4] This generalized mass formula was named as "Samanta Formula" by Botvina and Pochodzalla and used to predict relative yields of hypernuclei in multifragmentation of nuclear spectator matter.[5]

Hypernuclei were first observed by their energetic but delayed decay, but have also been studied by measuring the momenta of the K and pi mesons in the direct strangeness exchange reactions.

References

  1. ↑ M. May (1994). "Recent results and directions in hypernuclear and kaon physics". In A. Pascolini. PAN XIII: Particles and Nuclei (PDF). World Scientific. ISBN 978-981-02-1799-0. OSTI 10107402.
  2. ↑ C. Samanta (2006). "Mass formula from normal to hypernuclei". In S. Stoica; L. Trache; R.E. Tribble. Proceedings of the Carpathian Summer School of Physics 2005. World Scientific. p. 29. ISBN 978-981-270-007-0.
  3. ↑ C. Samanta, P. Roy Chowdhury, D.N.Basu (2006). "Generalized mass formula for non-strange and hyper nuclei with SU(6) symmetry breaking". Journal of Physics G. 32 (3): 363–373. arXiv:nucl-th/0504085. Bibcode:2006JPhG...32..363S. doi:10.1088/0954-3899/32/3/010.
  4. ↑ C. Samanta, P. Roy Chowdhury and D.N.Basu (2008). "Lambda hyperonic effect on the normal driplines". Journal of Physics G. 35 (6): 065101–065110. arXiv:0802.3172. Bibcode:2008JPhG...35f5101S. doi:10.1088/0954-3899/35/6/065101.
  5. ↑ A.S. Botvina; J. Pochodzalla (2007). "Production of hypernuclei in multifragmentation of nuclear spectator matter". Physical Review C. 76 (2): 024909–024912. arXiv:0705.2968. Bibcode:2007PhRvC..76b4909B. doi:10.1103/PhysRevC.76.024909.
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