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Strong interactions for precision nuclear physics

Periodic Reporting for period 2 - PrecisionNuclei (Strong interactions for precision nuclear physics)

Reporting period: 2019-08-01 to 2021-01-31

Nuclear physics is a cornerstone in our scientific endeavour to understand the universe. Indeed, atomic nuclei bring us closer to study both the stellar explosions in the macrocosmos, where the elements are formed, and the fundamental symmetries of the microcosmos. Having access to a a precise description of the interactions between protons and neutrons would provide a key to new knowledge across 20 orders of magnitude; from neutrinos to neutron stars. Despite a century of the finest efforts, a systematic description of strongly interacting matter at low energies is still lacking. Successful theoretical approaches, such as mean-field and shell models, rely on uncontrolled approximations that severely limit their predictive power in regions where the model has not been adjusted.

In this project I will develop a novel methodology to use experimental information from heavy atomic nuclei in the construction of nuclear interactions from chiral effective field theory. I expect this approach to enable me and my team to make precise ab initio predictions of various nuclear observables in a wide mass-range from hydrogen to lead as well as infinite nuclear matter. I will apply Bayesian regression and methods from machine learning to quantify the statistical and systematic uncertainties of the theoretical predictions. The novelty and challenge in this project lies in synthesising (i) the design of nuclear interactions, (ii) ab initio calculations of nuclei, and (iii) statistical inference in the confrontation between theory and experimental data. This alignment of methods, harboured within the same project, will create a clear scientific advantage and allow me to tackle the following big research questions: How can atomic nuclei be described in chiral effective field theories of quantum chromo dynamics? What is the probability for neutrinoless double-beta decay in atomic nuclei? What are the nuclear uncertainties in muonic atoms relevant for the proton radius puzzle?
This project is making progress on the development of novel methodologies for using experimental information from heavy atomic nuclei in the construction of nuclear interactions from effective field theory. Significant progress is made on all three fundamental questions that drive this project forward ( (i) the design of nuclear interactions, (ii) ab initio calculations of nuclei, and (iii) statistical inference in the confrontation between theory and experimental data.)

This far into the project, the single most important achievement is the development of a novel technology called Subspace-Projected Coupled Cluster (SPCC) ( published in Andreas Ekström and Gaute Hagen Phys. Rev. Lett. 123, 252501 – Published 20 December 2019). The SPCC method is a game changer for theoretical analyses of atomic nuclei. In particular for performing Bayesian inference analyses of complex atomic nuclei and reveal unknown patterns of the underlying nuclear interaction and to correlate experimental information across the nuclear chart. We expect this exciting technology to significantly leverage additional breakthroughs and spawn new research frontiers during and beyond the lifetime of the present action.

In addition, an array of dedicated efforts has been, and is, carried out within the project. We are currently working on breakthrough statistical applications of chiral effective field theory for analyzing medium-mass and heavy nuclei, including infinite nuclear matter. The bulk part of the project is invested in developing novel methods, and implementing necessary computational technology, for applying Bayesian inference methods to analyze and predict experimentally relevant nuclei.
We are making progress on developing beyond the state of the art methods for emulating quantum systems. We expect to couple novel emulation methods with numerical methods for evaluating relevant Bayesian posterior probability distributions, or at least non-implausible parameter regions, for low energy constants in effective field theory. The suite of methods developed in this project will enable experimental design studies and statistical inference analyses to generate new knowledge about the interaction between nucleons inside atomic nuclei and infinite nuclear matter. The latter has a direct bearing on improving our understanding of neutron stars.