NA5-THEIA: Strange Hadrons and the Equation-of-State of Compact Stars

In a neutron star, the occurrence of hyperons emerges rather naturally at nuclear densities larger than about two times nuclear density. Usually, the nuclear equation-of-state (EOS) is expected to soften by this additional degree of freedom. The recent observation of massive neutron stars with about twice the solar mass posed, therefore, a serious challenge for our understanding of nuclear physics at high density. The difficult reconciliation of these observations with the presence of hyperons in neutron stars is referred to as the hyperon puzzle.

Several hyperonic EOS, based on phenomenological approaches recently developed are sufficiently stiff to stabilize two solar mass neutron stars. However, this is not the case for microscopic approaches based on realistic interactions for which this is still an open problem. Our incomplete understanding of the underlying baryon-baryon and of even more subtle multi-body interactions in baryonic systems seems to be the most natural reason for the hyperon puzzle.

The cooperation of world-leading experimentalists and theoreticians in the field of strangeness nuclear physics with experts of the neutron star community in astrophysics within the networking activity THEIA will allow to critically assess the status of our present understanding, to determine the impact of terrestrial observations for the hadronic EOS, and to identify possible new avenues to follow. Calculations and theoretical models that will be dramatically improved by the participants of THEIA will allow us to better predict and better understand results that will emerge from present research activities. It will enable us to shape, boost and coordinate the next experimental activities at the major experimental infrastructures in Europe (e.g. LHC, DAFNE, MAMI, FAIR, NICA) as well as in the US (TJNAF) or Asia (J-PARC). Coordinating cutting edge studies employing new methods and challenging techniques will bolster the efficient use of European research infrastructures and will thus guarantee to maintain the excellence in the proposed studies. Although the hadronic EOS is also related to many other branches in nuclear or hadronic physics, the focus on the strangeness aspect guarantees specific, very effective and fruitful interactions among all participating groups.