Computational and Theoretical Advances for Exotic Isotopes in the Medium Mass Region

Overview

A key challenge for ab-initio theory is to describe and predict properties of medium mass nuclei from the valley of stability towards the driplines, especially in relation to the wealth of new experimental data now coming from radioactive beam facilities. The nuclear many-body problem is a difficult undertaking from both the computational and theoretical points of view. Techniques such as Green's function Monte Carlo (GFMC) and no-core shell model (NCSM) allow essentially exact calculations, but are limited to light nuclei. For mid-mass isotopes above A=16, the challenge posed by the numerical scaling demands innovative many-body theory techniques and computational approaches. This is especially true for the extensions to nuclei with an open-shell character. Techniques such as self-consistent Green's function (SCGF), coupled cluster (CC), in-medium similarity renormalization group (IMSRG) or importance truncated no-core shell model (IT-NCSM) are promising methods due to relatively soft scaling with particle number. Current implementations of these methods are able to describe nuclei in the immediate vicinity of (sub-)closed shells up to the Nickel's mass regions. The current frontier consists of extending such calculations to higher masses and developing extensions to truly open-shell nuclei, and describing structure and reaction properties in a consistent ab-initio framework starting from modern 2N and 3N interactions. Continued progress in these areas will open the possibility of theoretical predictions for a wealth of processes that were previously inaccessible to microscopic methods. In this sense, we are about to witness a major turning point in low-energy nuclear physics.

The program will provide an opportunity for colleagues from different subfields to gather and discuss recent breakthroughs associated with the above challenges and to foster discussions and future collaborations. We aim at bringing together four sub-communities within nuclear theory that can play a fundamental role: experts in effective field theory and renormalization group methods, experts in reaction theory, many-body theorists engaged in the development of ab-initio techniques, and those involved in the development of energy density functional (EDF) approaches. The program will provide opportunities to strengthen and expand the links between these communities, and will help guide future research directions in nuclear structure and reaction theory. A strong outcome is expected in terms of providing theoretical support to the experimental efforts.

Key questions to be addressed

The program will focus on theoretical and computational advances in ab-initio many-body theory needed to develop a unified description of structure and reaction properties in nuclei far from stability. We will concentrate on the following questions and challenges that confront the field:

How can we extend current ab-initio methods to describe open-shell and deformed nuclei?
How can we include the effects of three-nucleon forces (3NF) in a computationally efficient manner?
How can we describe the onset of pairing in nuclei within various ab-initio frameworks?
Benchmarking and accuracy: can we develop reliable theoretical error estimates?
How can we bridge structure and reactions in a consistent fashion?
How can we generate reliable predictions for the drip-lines?
How can we use ab-initio theory to constrain EDF methods and derive microscopically-informed parameterizations of the associated energy density functional kernels?
How can the effect of 3NFs be embedded in nuclear EDF parameterizations?
Which extensions to the functional are required in order to achieve sufficient accuracy for astrophysical applications?

Advances on these fronts are pivotal to the analysis of next generation experiments, to be carried out at upcoming radioactive beam facilities such as NSCL/FRIB, RIKEN, SPIRAL2 and FAIR. New experiments will generate many new data encoding invaluable information about the structure of exotic nuclei that eventually have decisive implications for several astrophysical processes, provided that there is high-quality theoretical support to give reliable predictions of such exotic systems, and direct experimental efforts towards unexplored territories of the nuclear chart.. The main goal of the program is to foster new developments in ab-initio theory that can eventually give response to such needs.

Organization

We plan to start the program with a one-week overview where the main practitioners will have ample time to come together and discuss. The following weeks will consist of daily lectures intermixed with presentations of new results, with ample time for collaborations and discussions. Each of these three weeks will be devoted to a specific topic, namely ab-initio theories for open-shells, microscopic description of scattering phenomena and open quantum systems, computational challenges in the mid-mass region, and linking ab-initio theory to energy density functionals. At each time, we will aim at having the participation of the experts of that week's specific focus plus some involved in the other subjects with the intent of maximizing the exchange of ideas and cross fertilization. Ideally, a mix of junior (students and postdocs) and senior participants is desired at all times.