Evidence of rare hypernucleus

A view of one of the three events found by FINUDA: a schematic frontal view of the apparatus is shown, and the two blue lines represent the two 'pi' mesons moving along opposite bent trajectories in the magnetic field of the apparatus. Image credit: FINUDA collaboration

(PhysOrg.com) -- Physicists in Italy have discovered the first evidence of a rare nucleus that doesn’t exist in nature and lives for just 10^-10 seconds before decaying. It’s a type of hypernucleus that, like all nuclei, contains an assortment of neutrons and protons. But unlike ordinary nuclei, hypernuclei also contain at least one hyperon, a particle that consists of three quarks, including at least one strange quark. Hypernuclei are thought to form the core of strange matter that may exist in distant parts of the universe, and could also allow physicists to probe the inside of the nucleus.

The particular hypernucleus investigated here, called "hydrogen six Lambda" (⁶_ΛH).

A hypernucleus is a nucleus which contains at least one hyperon in addition to nucleons. The first was discovered by Marian Danysz and Jerzy Pniewski in 1952.

Since the strangeness quantum number is conserved by the strong and electromagnetic interactions, at least 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). Their physics is different from that of normal nuclei because a hyperon, having a different value of the strangeness quantum number, can share space and momentum coordinates with the usual four nucleons that can differ from each other in spin and isospin. 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, except for the weak decay of the Lambda. Sigma hypernuclei have been sought with apparent success.^[1]

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.

In particle physics, a hyperon is any baryon containing one or more strange quarks, but no charm quarks or bottom quarks.

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