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The strong interaction at neutron-rich extremes

Final Report Summary - STRONGINT (The strong interaction at neutron-rich extremes)

The ERC Project STRONGINT ("The strong interaction at neutron-rich extremes") investigates the structure of matter at the neutron-rich frontier in the laboratory and in the cosmos based on chiral effective field theory (EFT) interactions. We have shown that neutron-rich systems become increasingly sensitive to three-nucleon forces, with pioneering calculations of exotic nuclei and of neutron-rich matter and the impact on neutron stars. Moreover, our research has advanced chiral EFT to neutrino physics and astroparticle physics.

Chiral EFT opens up a systematic path for exploring many-body forces and their impact on neutron-rich systems, because all three- and four-neutron forces are predicted to high order (N3LO). We have achieved the milestone of a first complete N3LO calculation of neutron matter, including all two-, three- and four-nucleon interactions. We have also carried out the first asymmetric (neutron-rich) matter calculations including two- and three-nucleon interactions. Our results provide tight constraints for the nuclear equation of state and for the properties of neutron stars, in particular for the radius, which is a key science driver for x-ray astronomy and for the observation of gravitational waves from neutron-star mergers. Our results also make predictions for the symmetry energy and the neutron skin thickness of heavy nuclei.

Quantum Monte Carlo methods present one of the most accurate methods for strongly interacting system. However, their application with chiral EFT interactions was not possible before the ERC Grant. We succeeded in performing the first Quantum Monte Carlo calculations for neutron matter, light nuclei, neutron-alpha scattering, few-neutron resonances, and most recently with the heaviest nucleus calculated with three-nucleon interactions in Quantum Monte Carlo.

Three-nucleon forces are a frontier in strong interaction theory and for the physics of neutron-rich nuclei studied at rare isotope beam facilities worldwide. We have carried out pioneering calculations of medium-mass nuclei, in particular for the calcium isotopes, with highlights for the masses, shell structure and radii of calcium isotopes, and most recently up to tin-100. To this end, we developed an efficient method to calculate three-nucleon matrix elements, which is critical for three-nucleon matrix elements at N3LO. To access open-shell nuclei, we have developed the valence-space in-medium similarity renormalization group, which enables a nonperturbative derivation of valence-shell interactions. This has enabled us to perform ab initio calculations for all nuclei in the sd shell (mass number A=16-40), in the lower pf shell (A=40 to ~60) and selected heavier isotopes.

The in-medium similarity renormalization group also opens up consistent calculations of nuclear matrix elements for fundamental symmetries based on chiral EFT. We have developed all leading two-body currents at finite momentum transfer with a goal of applications to nuclear matrix elements and studied the importance of the consistency of interactions and currents. As a new development, we have calculated all spin-independent WIMP-nucleus cross sections based on chiral EFT and developed general analysis strategies. Our work provides limits for a range of dark matter models from direct detection experiments and LHC searchers.

We have presented first calculations of neutrino-matter interactions in supernova matter based on chiral EFT interactions over a broad range of densities, temperatures, and electron fractions. For neutrino interactions involving two nucleons, we have shown that neutrino rates are enhanced in mixtures of neutrons and protons at subnuclear densities due to the large scattering lengths. Finally, we have demonstrated, using EFT arguments and explicit Quantum Monte Carlo calculations, that the short range correlation scaling factor, extracted from high-energy reactions, is an observable quantity and can be obtained successfully using chiral EFT.