Overview

Significant progress in nuclear many-body techniques steered by high-performance computing has enabled to employ realistic Hamiltonians in infinite matter and open-shell nuclei up to masses A≈100. Parallel advances in devising nuclear interactions and electroweak currents from effective field theory permit to reproduce nuclear saturation and the radii of medium-mass nuclei based on the underlying theory of the nuclear force. This ab initio program builds upon experimental data from radioactive ion beam facilities - such as RIBF at RIKEN and the upcoming FRIB and FAIR - to test predictions and gain insight into the structure of the most neutron-rich systems.

At the same time, reliable predictions of electroweak interactions with nuclei are required to support ground-breaking experiments aiming at resolving the neutrino-mass hierarchy, determining the phase associated with CP violation in the leptonic sector (DUNE, T2K, SuperKamiokande), and unveiling whether neutrinos are their own antiparticles (MAJORANA, KamLAND-Zen). Uncertainties in calculations of the neutrino-nucleus response at high energies and neutrinoless ββ decay matrix elements can be quantified and improved by employing consistent Hamiltonians and (electroweak) currents arising from modern theories of strong interactions (such as EFTs and LQCD).

Nevertheless, many studies still exploit simplified nuclear structure models that are not consistent with the currents being employed. In addition, performing ab initio calculations for a large pool of neutron-rich isotopes will impact astrophysics, by addressing neutrino effects in neutron stars and supernovae, and studying β decays relevant for the r-process responsible for the nucleosynthesis of heavy elements.

Goals

We aim at bridging the gap between the phenomenological nuclear models employed in neutrino oscillation and neutrinoless ββ-decay calculations and the state-of-the-art ab initio approaches commonly employed in nuclear structure studies. A central aspect of the program will be the propagation of the theoretical uncertainties associated with the chiral-EFT potentials and currents, as well as with the nuclear many-body approaches, into the neutrino-nucleus cross sections and the ββ-decay matrix elements. A crucial point of this program is to assess that consistent nuclear interactions and electroweak currents be used for studying both neutron-rich nuclei and neutron matter, so that confrontation with experimental data can constrain predictions of neutrino properties.

The program is designed to foster interactions among nuclear theorists, nuclear experimentalists and neutrino physicists, thus facilitating (much needed) inter-disciplinary collaborations and discussions

Program format

The program will run for 5 weeks. Participants will be invited to present their work during informal morning talks. We envision one or two talks in the morning and ample time for discussion sessions in the afternoon. A more intensive workshop with a larger number of participants during the second week will bring together experts on state-of-the-art nuclear many-body approaches and experimental neutrino physicists. A summary of the focus topics for each week is given below, although we plan to maintain a broad range of expertise throughout the whole program.

Week 1: Ab initio methods
Ab initio nuclear many-body approaches: strengths, limitations and further directions. Challenges and opportunities for nuclear structure and nuclear astrophysics as posed by neutrino-oscillation and 0ββ-decay experiments.

Week 2: Workshop on: Neutrino detection and interactions: challenges and opportunities for ab-initio nuclear theory
The workshop will cover the most relevant aspects of neutrino interactions with atomic nuclei and infinite matter. It is intended to foster interactions between nuclear theorists and neutrino experimental programs (oscillations, ββ-decay, supernovae...).

There will be a $40 registration fee to attend the workshop. The registration fee includes participation in the workshop, lectures, and coffee breaks.

Week 3: Nuclear interaction and currents
Development and fitting of next generation nuclear interaction (problem of saturation and prediction in neutron rich elements). Challenges in constructing and exploiting (electroweak) currents consistent with the interactions. Estimation of theoretical errors.

Week 4: Neutrino-nucleus response and oscillation experiments
How to approach neutrino energies in the few GeV region. Factorization of nuclear transition matrix elements and spectral functions. Matching relativistic current to low-energy structure. Application to neutrino response in infinite matter. Estimation of theoretical errors.

Week 5: Applications for ββ-decay.
Develop a roadmap to explore possible many-body mechanisms and many-body currents at the origin of the g_A "quenching". Relation between "quenching" in β and ββ decays. Connection between toy ββ decays in light systems and ββ decay emitters. Use of experimental data to constrain the theoretical predictions.