Exploring the Universe through Strong Interactions

Project Information

EUSTRONG

Grant agreement ID: 101020842

DOI

10.3030/101020842

EC signature date 27 May 2021

Start date 1 October 2021

End date 30 September 2027

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 252 375,00

EU contribution

€ 2 252 375,00

Coordinated by

TECHNISCHE UNIVERSITAT DARMSTADT
Germany

Periodic Reporting for period 3 - EUSTRONG (Exploring the Universe through Strong Interactions)

Reporting period: 2024-10-01 to 2026-03-31

Summary of the context and overall objectives of the project

The strong interaction is responsible for holding neutrons and protons together in atomic nuclei and for the particle interactions in neutron stars, where matter has been crushed to extremes in density. The ERC Project EUSTRONG (“Exploring the Universe through Strong Interactions”) sheds light on strong interactions in nuclei and neutron stars, and where strong interactions are essential in dark matter direct detection and neutrino physics. Recently, great progress has been made in constraining the nuclear equation of state from nuclear physics combined with neutron star observations. At the same time, ab initio calculations of nuclei using chiral effective field theory interactions have reached nuclei with up to 100 nucleons. EUSTRONG is taking these successes to the next level.

Work Package 1 (“Strong interaction matter and neutron star observations”) explores the equation of state with the goal to provide first constraints directly on dense matter interactions from astrophysics, including from gravitational wave and NICER observations. This enables us to answer which microscopic interactions are consistent with astrophysical observations, or where there are tensions. To this end, we are developing the equation of state using Fermi liquid theory and are exploring new degrees of freedom at intermediate densities. Work Package 2 (“New frontiers for heavy neutron-rich nuclei”) is advancing the ab initio frontier to heavy neutron-rich nuclei by accelerating many-body calculations with novel innovative methods. This is realized by developing effective field theory interactions and many-body calculations, using new emulator and tensor network methods. This will take the field to a new level, with ab initio predictions of nuclear masses for r-process nucleosynthesis. Work Package 3 (“Nuclear physics of dark matter and neutrino physics”) is developing ab initio nuclear responses of heavy nuclei involved in dark matter direct detection and coherent neutrino scattering, where a reliable understanding of strong interaction effects is crucial.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Excellent progress has been made in the ERC Project EUSTRONG. In Work Package 1, we have explored the equation of state from Fermi liquid theory and investigated the phase diagram of neutron-rich matter relevant for neutron stars. This establishes the existence of a proton drip phase, which has important implications for the possibility of pasta phases in neutron stars. Moreover, we have systematically investigated the impact of strong phase transitions at intermediate densities and developed equations of state exploring new degrees of freedom in effective field theories. Combining multimessenger observations of neutron stars with our equation of state calculations, we have provided constraints for the masses and radii of neutron stars, and for the dense matter equation of state, showing how astrophysics observations provide tight constraints at intermediate densities.

In Work Package 2, we have made advances on three-nucleon interactions needed for converged ab initio calculations of heavy nuclei and developed new effective field theory interactions that yield accurate results up to heavy nuclei. A highlight has been the combination of the ab initio in-medium similarity renormalization group with the density matrix renormalization group, with superior convergence in large-space calculations and with new insights to nuclear structure using methods from quantum information. We have further explored the method of eigenvector continuation for strongly interacting systems and nuclear many-body problems. Moreover, we have established low-rank singular value decompositions for chiral effective field theory interactions.

In Work Package 3, we have explored strong interactions in nuclei that are key for neutrino physics and for new physics searches. We have predicted neutrinoless double-beta decay nuclear matrix elements based on an effective field theory for heavy nuclei. To test the momentum dependence of two-body currents, we have shown the impact of two-body currents for magnetic moments up to heavy nuclei. Finally, we have studied how ytterbium isotope shift measurements can be used to extract higher-order changes in the nuclear charge distribution and compared these to ab initio calculations of the complex structure of these heavy isotopes. The results can be used to provide limits on physics beyond the Standard Model.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We have developed novel methodologies for ab initio calculations of nuclei. In particular, we have made significant progress to include three-nucleon interactions up to heavy nuclei. These advances are also enabling the development of new effective field theory interactions with accurate results for heavy neutron-rich nuclei, including for the neutron skin from oxygen to lead isotopes. This is opening up ab initio calculations of heavy neutron-rich nuclei in the region of r-process nucleosynthesis. Going beyond the state of the art, we have combined the tensor-network-based density matrix renormalization group with nuclear ab initio methods. This hybrid scheme is enabling converged calculations in large valence spaces at two orders smaller dimensions compared to conventional diagonalization techniques. Morever, the utilization of quantum information measures, such as information entropy and mutual information, provides novel insights to nuclear structure.

We have investigated gravitational-wave and NICER observations in a Bayesian equation of state framework combined with new chiral effective field theory calculations up to nuclear densities. This provides first equation of state constraints including new NICER observations for the neutron star J0437. Our work shows how astrophysics observations provide tight constraints for the equation of state at intermediate densities. In the future, this will enable constraints directly on dense matter interactions from astrophysics, such as for the effective field theory couplings.

Moreover, we have presented ab initio calculations of magnetic dipole moments of medium-mass and heavy nuclei using chiral effective field theory currents. This work provides a first global survey of the effects of two-body currents for the description of magnetic moments of nuclei. This constitutes an important test before our applications of chiral effective field theory currents in ab initio calculations of the nuclear responses for dark matter direct detection and coherent neutrino scattering. Finally, as a spin-off and novel application, we have shown how ytterbium isotope shift measurements can be used to extract higher-order changes in the nuclear charge distribution, which provide direct information on nuclear deformation. This demonstrates the powerful role of ab initio calculations for searches beyond the Standard Model with nuclei.