In the last decade, ab initio approaches have shown remarkable advances in the description of nuclear matter as well as of nuclear structure, reaching medium-, and in specific cases, even heavy-mass nuclei. The main goal of this WP is to refine approaches such as Self-consistent Green's functions (SCGF), Quantum Monte Carlo (QMC) and Shell Model (SM), employing effective interactions derived within the chiral EFT including 3-nucleon (3N) terms. New computational schemes will be devised, including Configuration Interaction Monte Carlo (CIMC), the automatic computation of Feynman diagrams and the use of quantum computing and neural-network quantum states (see WP4), to extend the domain of applicability of these approaches, in particular concerning nuclear and hyperonic matter, exotic and deformed nuclei. We also plan to perform consistent calculations of nuclear reactions, deriving accurate optical nucleon- and nucleus-nucleus potentials within the SCGF approach at low energy and improving the applications of multiple scattering at higher energies.

WP2: Advanced theoretical studies of nuclear phenomena: addressing the experimental challenges

The MONSTRE project is characterized by a strong connection with the experimental activities carried out worldwide, focusing on evidences which are not yet fully understood, and concerning both nuclear structure and reactions. Theoretical studies, based on the methods discussed in WP1 as well as in the framework of Density Functional Theory (DFT) will be performed. Efforts are underway to connect Energy Density Functional (EDF) models with ab initio approaches. Atomic parity-violating experiment are providing data that challenge current EDF models. New phenomenological functionals incorporating this kind of information will be developed. Attempts to include short-range-correlations (SRCs) within the EDF approach will be made, to describe recent evidences from nucleon knock-out experiments of high- energy electrons.

Exotic nuclei pose several challenges for theory, such as the evolution of the shell structure, where the 3N force plays a crucial role, or the appearance of low-lying structures in the energy spectrum, like the Pygmy Dipole Resonance (PDR), whose properties are still to be fully clarified. Phenomenological nucleus-nucleus optical potentials (OPs) for exotic nuclei will be derived and used to extract spectroscopic information via breakup and transfer reactions. Attention will be devoted to direct reactions populating weakly bound systems and/or unbound low-energy resonances.

Collective modes represent a very important source of information on the Equation of State (EoS), and an increasing number of experiments focus on deformed nuclei. A quantitative understanding of the properties of collective modes requires advanced models, possibly overcoming the mean-field approximation by including many-body correlations and/or restoring broken symmetries. We plan to perform extensive calculations for different multipolarities, re-examining the relationship between giant resonance properties and bulk nuclear matter parameters.

Single and double-beta decay represent also an important challenge. The comparison among different models (Quasi-particle Random Phase Approximation (QRPA), Second RPA, particle-vibration coupling (PVC), SM) can shed light on open issues, such as the quenching problem and the large uncertainties on the nuclear matrix elements of the neutrino-less double-beta decay. Single and double-charge reactions might also provide useful information in this regard.

The quantitative microscopic understanding of clustering represents a major open problem in nuclear structure. The role of molecular orbitals and antisymmetrization thereof is a key aspect to be studied before embarking in systematic studies, and reactions involving cluster nuclei will be studied by merging symmetry methods with molecular models. Alpha-like correlations induced by the proton-neutron pairing force are crucial importance in medium-heavy N=Z nuclei. We have developed an approach to describe these nuclei in a formalism of quartets which will be applied to understand anomalies observed in elastic alpha-scattering and alpha-decay.

WP3) Nuclear matter under extreme conditions: from nuclear dynamics to        compact objects

The goal of this WP is the study of the properties of nuclear matter, focusing on the investigation of the EoS far from stability, which is crucial for the physics of compact stars. Heavy ion collisions (HICS) at Fermi/intermediate energies represent a powerful tool to this purpose. DWBA, Coupled-Channel (CC) calculations, semi-classical and transport models will be used to study the collision mechanism. An important international effort is currently underway, to compare different transport codes, to minimize the model-dependence and improve the robustness of the extracted EoS constraints, making use of advanced statistical analyses to combine astrophysical and terrestrial information.

The study of exotic nuclei will help understanding how the in- medium nucleon-nucleon interaction depends on isospin asymmetry and how this impacts the EoS. The formation of clustering structures at sub-saturation densities is crucial for the modelization of the EoS as well as for nuclear structure and reaction properties. These clustering phenomena will be studied within extended EDF approaches incorporating SRCs and the Constrained Molecular Dynamics Model (CoMD). 

WP4) Emerging  computational technologies: quantum information and machine learning techniques

Quantum Computing (QC) and Quantum Information are rapidly growing fields. The application of QC to nuclear physics poses great challenges related to the complex features of the nuclear interaction, but their payoff is remarkable. On the one hand, simulations on quantum devices could represent efficient means to access physical observables, including inclusive and semi-exclusive scattering cross sections, and to deal with non-equilibrium phenomena, that are difficult to treat within ab initio approaches on classical computers; on the other hand, key concepts developed in quantum information like entanglement can serve as fertile ground to complement our understanding of low-energy effective theories of quantum chromodynamics and many- body correlations acting in nuclear systems.

Artificial neural networks are able to represent compactly quantum many-body states in systems characterized by non- perturbative interactions. Recent progress includes the development of variational Monte Carlo methods based on neural network quantum states that solve the nuclear Schrödinger equation in a systematically improvable manner with a cost scaling polynomially with the number of nucleons. In addition, they are well-suited for next- generation computers, as they are designed to leverage the power of GPUs.