Editorial: The Future of Nuclear Structure: Challenges and Opportunities in the Microscopic Description of Nuclei
EEditorial: The Future of Nuclear Structure:Challenges and Opportunities in theMicroscopic Description of Nuclei
Luigi Coraggio , Saori Pastore , Carlo Barbieri , , Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, 80126 Napoli, Italy Physics Department and McDonnell Center for the Space Sciences at WashingtonUniversity in St. Louis, St. Louis, Missouri 63130, USA Department of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom Dipartimento di Fisica, Universit `a degli Studi di Milano, Via Celoria 16, I-20133Milano, Italy INFN, Sezione di Milano, Via Celoria 16, I-20133 Milano, Italy
Correspondence*:
The past two decades have witnessed tremendous progress in the microscopic description of atomic nuclei.Within this approach, nuclei are described in terms of nucleons interacting via realistic two- and three-bodyforces, constrained to accurately reproduce a large body of data for few nucleons systems. The goal ofthe nuclear theory community is to gain an accurate and predictable understanding of how the propertiesof many-body systems, along with their dynamics and structure, emerge from internucleon correlationsinduced by the strong interaction.Progress in the microscopic (or, ab initio ) theory has been quite notable and it has been supported by twomajor pillars: First, thanks to the advent of Effective Field Theories (EFTs), we can now systematicallydevelop nuclear Hamiltonians that are rooted in the fundamental properties and symmetries of theunderlying theory of QCD. Second, advances in computational resources and novel powerful algorithmsallow us to solve i ) the many-nucleon problem efficiently, and ii ) quantify the degree of reliability oftheoretical calculations and predictions. In many cases, microscopic computations achieve an accuracy thatis comparable or superior to the precision delivered by current EFT interactions. This sparked a renewedinterest to further broaden the focus of ab initio theory and address open problems in nuclear physics.While the status of the first pillar has been recently discussed by “The Long-Lasting Quest for NuclearInteractions: The Past, the Present and the Future” Topical Review on this Journal, here we focus on theexciting new developments in microscopic theory. At present, ab initio computations of nuclear structureinclude up to medium-mass isotopes. The heaviest systems currently reached—with different degrees ofaccuracy—have mass number A ≈ . These computational limits are constantly being pushed forward.At the same time, the community is expanding into new directions, in particular towards the study ofelectroweak observables and nuclear reactions, that nowadays require predictions with an accuracy neverreached before for similar mass ranges.In collecting the contributions for this Research Topic, we sought to gather contributions from authorswho could summarize the current state-of-the-art microscopic calculations in Nuclear Theory, favouring aselected but broad view over an attempt to cover every application. All presented contributions stem from a r X i v : . [ nu c l - t h ] J a n . Coraggio, S. Pastore, and C. Barbieri well-established methods in computational nuclear structure, and indicate recent theoretical advances andprospective outlooks, challenges and opportunities for Nuclear Theory. Most importantly, it is our hopethat this collection will confer a ‘big picture’, including references to basic material, that will be valuablefor young researches who intend to enter this exciting discipline.The richness of applications in modern ab initio nuclear theory can be appreciated in Hergert ’scontribution that provides us with a general overview of the most successful microscopic many-bodyapproaches currently in use [1]. Traditionally, the refinement and sophistication of these computationaltools has given fundamental support to advance the theories of nuclear forces. Quantum Monte Carlo(QMC) techniques allow to solve the many-body Schr¨odinger equation with high accuracy for light nucleiup to masses A ∼ Gandolfi et al. discuss the use of QMC methods (namely, Variational, Green’sFunction and Auxiliary Diffusion Monte Carlo methods) in combination with local chiral interactions incoordinate space [2]. QMC methods are used in lattice effective field theory, where the EFT Lagrangianis implemented in momentum space with nucleons and pions placed on a lattice.
Lee discusses the basicfeatures of this approach and its high potential for understanding clustering phenomena [3].For heavier isotopes, ab initio theories can be pushed to masses A ∼
140 provided that one retains onlythe relevant nuclear excitations, as it is done through all-orders resummations. Among these methods, theself-consistent Green’s function (SCGF) theory gives direct access to the spectral information probed bya wide range of experiments as reviewed in detail by
Som`a ’s contribution [4]. Once in the region of thenuclear chart that corresponds to medium masses, open shell isotopes become the next challenge to beaddressed by the theory. In fact, resolving the degeneracy in uncorrelated systems requires large scaleconfiguration mixing.
Coraggio and Itaco demonstrate how this can be handled by projecting the correlatedmany-body states into a shell model Hamiltonian, using the so-called “Q-box” formalism [5]. A similarstrategy is shared by other computational frameworks, such as coupled cluster and in-medium SRG, thatare touched upon in the contribution by
Hergert [1]. A less conventional approach to open shells is to breakSU(1) symmetry (in short, allowing for breaking particle number conservation). This is discussed by
Som`a within SCGFs [4] and by
Tichai et al. in the framework of many-body perturbation theory [6].The remainder of this topical review focuses on selected open challenges in Nuclear Theory that requirean ab initio approach. Two contributions show different aspect of studying infinite nucleon systems andthe implications for astrophysical scenarios.
Tews covers QMC calculations of the equation of state (EoS)of dense matter in neutron stars [7]. With the recent observation of star mergers and the birth of multi-messenger astronomy, it has become of prime importance to understand the finite temperature properties ofthe EoS.
Rios discusses this topic and how the structure of neutron matter depends on temperature, usingSCGF theory [8].In the quest for physics beyond the Standard Model, Nuclear Theory, and in particular accuratecalculations of neutrino-nucleus interactions at all energy scaler, plays a crucial role. This is carefullyanalyzed by
Rocco ’s contribution that address this challenge with emphasis on impacts to neutrinooscillations experimental programs [9]. The last contribution of this Topical Review addresses one of thehardest open challenges in the interpretation of experimental data: the lack of a truly first-principles theorythat can describe consistently both structure and reaction processes.
Rotureau highlights recent steps inderiving an ab inito optical potential using the coupled cluster method (that, together with SCGF, is one ofthe two possible approaches to this problem) [10].We are really grateful to all the scientists participating in this project and hope that the reader will enjoythis Topical Review. . Coraggio, S. Pastore, and C. Barbieri REFERENCES [
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