Hypernuclei - Bridging the gap between quarks and stars
Protons and neutrons form the tiny core of an atom. Albeit very small, atomic nuclei account for more than 99.9% of the mass of an atom and also of stars, planets as well as all living things on earth. A cube that is 2m on a side and filled with nuclear matter would have about the same weight as the water of all our earth’s oceans. In the early years of nuclear physics research the composition of an atomic nucleus in terms of protons and neutrons, its structure and basic properties were in the spotlight. Studies were focused on the nature of radioactive decays, nuclear reactions, and the synthesis of new elements and isotopes. Nowadays a nucleus is seen as a system of quarks and gluons that arrange themselves into protons and neutrons. As a consequence the scope of nuclear science has broadened and extends from the today’s fundamental particles - quarks and gluons - to the most spectacular of cosmic events like supernova explosions. Remnants of these cosmic catastrophes are neutron stars that have a core density ten times higher than normal nuclei. The properties of quarks and gluons are reasonable well understood and their mutual interaction is well described by the theory of Quantum Chromodynamics (QCD). But the appearance of neutrons and protons and other hadrons with their masses, charges, magnetisation and quark composition, together with the corresponding spatial distributions is not yet fully understood. How the nuclear force that binds protons and neutrons into stable nuclei or into neutron stars, emerges from QCD is yet another mystery and remains one of the greatest challenges for strong interaction physics.In essence nuclear physics research attempts to understand the nature of all manifestations of nuclear matter in our universe - nuclei on the small scale and dense stellar objects on the large scale. Stable nuclei and neutron stars represent important checkpoints of the QCD phase diagram of cold baryonic matter. Such investigations are complementary to studies exploring the QCD phase diagram in highly dynamical, dense and hot quark-gluon matter created in ultra-relativistic heavy ion collisions, thus mimicking the early stage of our universe. Strangeness physics is adding a new degree of freedom to our understanding of hadrons, their structure, their interactions and the cooperative effects in the many-body environment in nuclear systems. In perspective, strangeness physics might be a cornerstone for further extensions into the regions of charm and potentially even higher flavors. Hence, strangeness physics might be well considered as being the gateway into flavor physics. On the astrophysical scale the appearance of hyperons in the dense core of a neutron star has been a subject of extensive studies since the early days of neutron star researc. It seems that irrespective of the hyperon-nucleon interactions,incompressibility, and symmetry parameter used, hyperons will appear in neutron stars at densities around 2-3 times normal nuclear density and that the type of hyperons which dominates depends on the hyperon-nucleon interactions. For many of these open questions hypernuclei can give authoritative answers or serve, at least, as laboratories for explorative studies. Hypernuclei are unique in their potential of improving our knowledge on the strange particle-nucleus interaction in a many-body environment and under the controlled conditions of a cold and equilibrated host system. This, in turn, is essential to derive eventually a more general and self-consistent description of the baryon-baryon interaction.