Constraining the nuclear matter equation of state from electromagnetic observables in finite nuclei

Abstract

Understanding the nature of neutron stars, where matter reaches extremely high density conditions, is one of the most fascinating questions in nuclear astrophysics. The structure, dynamics and composition of these astrophysical objects are governed by the nuclear matter equation of state, which relates the pressure supporting the star against gravitational collapse to the huge range of densities present in its interior. Around the densities reached in the atomic nucleus, the nuclear matter equation of state can be investigated in laboratory experiments targeting nuclear electromagnetic properties, such as electric dipole polarizabilities and isoscalar monopole resonances. These observables, connected to parameters entering the nuclear matter equation of state, shed light also on the collective excitations of the nucleus at low energy. Ab initio calculations, combining nuclear forces from chiral effective field theory with systematically improvable many-body methods, represent now the gold standard of nuclear structure computations in increasingly large systems. In this framework, computing electromagnetic observables involves additional challenges, as it requires the knowledge of both bound and continuum excited states of the nucleus. Such calculations have become possible in medium-mass nuclei thanks to the coupled-cluster (CC) formulation of the Lorentz integral transform (LIT) technique, known as LIT-CC method, limited thus far to doubly magic or semi-magic nuclei. This thesis extends the reach of ab initio LIT-CC calculations of electromagnetic observables beyond closed-shell nuclei and towards the dripline. We show how the LIT-CC approach can be reformulated for nuclei in the vicinity of closed shells, focusing on two-particle-attached (2PA) systems, which are characterized by having two nucleons outside a closed-shell core. For most of this thesis, we focus on the electric dipole polarizability. We first test LIT-CC predictions on new experimental data for the closed-shell 40Ca, serving as an additional benchmark for constraints on the nuclear matter equation of state from chiral forces. Next, we validate the newly developed LIT-CC method for 2PA nuclei, computing the non-energy-weighted dipole sum rule and the dipole polarizability of 16,24O in both the closed-shell and the new frameworks, finding agreement between error bars. We analyse the evolution of the dipole polarizability along the oxygen and calcium isotopic chains, providing predictions for 24O and 54,56Ca, which will serve as motivation for future experimental studies at the dripline. We then present a study of the exotic halo nucleus 8He, where we compare our predictions for the dipole polarizability with recent high-statistics data obtained at RIKEN, Japan by the SAMURAI collaboration. In the end, we move our attention to isoscalar monopole resonances, and extract a preliminary estimate of the incompressibility of symmetric nuclear matter.