- 01 TRIUMF Theory Department
- 02 Theory Science Highlights
- 03 Nuclear Theory at TRIUMF
- 04 Particle Theory at TRIUMF

### 01 TRIUMF Theory Department

TRIUMF’s Theory Department is unique in Canada as a theoretical team embedded in a world-leading rare isotope laboratory. This context provides a synergistic interface between theorists whose original work is informed by leading-edge experimental technologies and results, and in turn whose independent research guides and inspires experimental approaches.

The Theory Department specializes in two areas: nuclear theory and particle theory.

TRIUMF’s nuclear theorists are leaders in building a first-principles understanding of nuclear structure and reactions using computational modeling with the world’s largest supercomputers. This involves developing and applying first-principle, or *ab initio*, methods to explain how the interaction of quarks and gluons produce the observed structure and properties of atomic nuclei. These *ab initio* methods are at the scientific forefront in using computational mathematics to model complex systems, including rare isotopes. In the past decade, TRIUMF nuclear theorists have contributed to extending *ab initio* techniques to larger, more complex nuclei up to an atomic mass 100.

TRIUMF’s particle theorists are pushing the scientific frontiers in the search for new beyond-Standard Model (SM) physics and proposing ways to test beyond-SM theories in existing or future experimental facilities. This includes direct and indirect searches for dark matter (DM), searches for new particles in high energy colliders, and new tests of neutrino characteristics. With big-data particle physics experiments the volume of data itself can be a limiting factor to discovery and TRIUMF’s particle theorists are providing direction to narrow in on the most potentially fruitful avenues for research.

The Theory Department’s five core staff lead a diverse research program that involves, at any one time, more than a dozen additional team members from across Canada and around the world. In 2018, this included five postdoctoral researchers from the United States, Italy, Australia and Israel; four graduate students from TRIUMF member universities, and four undergraduate co-op students.

The Theory Department is also a hub for the Canadian and international subatomic physics theory communities. This includes hosting annual workshops on “Progress in *Ab Initio* Techniques in Nuclear Physics” and new topics in particle physics. Similarly, the Theory Department contributes to linking TRIUMF, through partnerships and collaborations, to a range of major national and international institutions, including the Institute for Particle Physics and the Perimeter Institute in Canada, CERN in Europe, U.S. Department of Energy national laboratories including Lawrence Livermore and Oak Ridge, and the Japan Proton Accelerator Research Complex (J-PARC).

### 02 Theory Science Highlights

### 03 Nuclear Theory at TRIUMF

TRIUMF nuclear theorists use first principle, or *ab initio* models, to tackle a core question in nuclear theory: how overall nuclear structure emerges from quantum chromodynamics, the SM theory that describes the complex interactions of quarks and gluons. A key strength of TRIUMF’s nuclear theory group lies in its ability to apply a broad toolbox of different computational methods grounded in Chiral Effective Field Theory, which describes nucleon interactions using quantum chromodynamics.

TRIUMF theorists are extending *ab initio* techniques to increasingly complex and rare nuclei from exotic light nuclei, to complex medium-sized nuclei. These *ab initio *insights are helping constrain and guide experiments in extreme nuclear structure at TRIUMF, and beyond, including for astrophysical nuclear reactions, the evolution of shell structure far from stability and for a range of leading-edge physics experiments, including fusion energy.

**Nuclear Theory and Rare Isotopes **

Short-lived, extreme nuclei are the scientific frontier for the discovery of new aspects of nucleon interactions, including in neutron-rich nuclei with halo neutrons, unbound, or unstable, nuclei, and in excited and deformed nuclei.

The weakly bound and unbound exotic nuclei produced in TRIUMF experiments cannot be understood using traditional nuclear bound-state techniques. As a result, TRIUMF theorists are leaders in developing an *ab initio* many-body approach, no-core shell model with continuum (NCSMC), which provides a unified description of both bound and unbound nucleon states. This enables theorists to simultaneously investigate both nuclear structure and reactions. The method combines a state-of-the-art technique to incorporate short- and medium-range many-nucleon correlations with a continuous process to incorporate long-range correlations between clusters of nucleons.

With NCSMC, TRIUMF theorists predict the ground- and excited-state energies of light nuclei, their electromagnetic moments and transitions, resonances and cross-sections. These calculations provide both guidance for experimentalists and a theoretical test and framework for interpreting experimental results. As well, understanding and predicting the formation of shell structure in exotic nuclei is a central challenge for nuclear theory. The Theory Department is refining models and collaborating with TRIUMF experimentalists, including in measurements performed at TRIUMF’s TITAN revealing nuclear binding energies, and the structure of exotic halo nuclei such as ^{6}He and ^{11}Be.

To better understand extreme nuclei, the Theory Department is pioneering the use of the in-medium similarity renormalization group (IMSRG) approach and coupled-cluster theory that uses only nuclear forces to calculate the energy of nuclei and other observables such as radii and electric dipole polarizability. This has provided the first predictions for the limits of nuclear existence and the evolution of nuclear shell structure in exotic medium-mass isotopes and the first ab initio computation of the neutron-skin thickness in ^{48}Ca, results which will guide experiments at TRIUMF and world-wide.

The Theory Department is also extending these theoretical tools to address some of the most fundamental questions in nuclear weak physics, including the nature of neutrino masses and mixing angles (through calculations of neutrino scattering and neutrino-less double-beta decay) and the nature of DM, through calculations of DM particle-nucleus scattering cross sections–how likely it is that a DM particle will interact with a particular nucleus.

**High-Performance Computing **

Nuclear structure and reaction modeling are among the most computationally intense research and thus continually pushing the edge of scientific computing.

Along with a TRIUMF-based computational cluster for initial calculations, TRIUMF nuclear theorists use some of the world’s largest and most advanced supercomputers, providing scientists and students access to world-class computing resources. These high-performance computing facilities include: Oak Ridge National Laboratory’s TITAN supercomputer and Compute Canada’s Cedar supercomputer at Simon Fraser University, Canada’s most powerful research supercomputer.

*Ab initio* atomic and nuclear modeling is also one of the key areas driving the development of quantum computing. TRIUMF nuclear theorists are establishing collaborations with Vancouver’s D-Wave to explore the application of quantum computing to nuclear modeling to solve currently intractable large nuclei physics problems.

**Nuclear Theory and ****Muonic**** Atoms**

The discovery of the proton-radius puzzle and the subsequent deuteron-radius puzzle catalyzed an on-going discussion about explanations for the difference in the observed radii obtained from muonic atoms and electron-nucleus systems. Atomic nuclei have an intricate internal structure that must be taken into account when analyzing experimental results. TRIUMF theorists took a leading position in the study of corrections to the structure and the polarizability of nuclei in light muonic atoms. Such results are widely known, and the contribution to the study of the general problem is universally recognized.

### 04 Particle Theory at TRIUMF

The global particle physics community is in a dynamic period of work to open the frontier of beyond-SM physics. TRIUMF’s Particle theorists are developing mathematical models of beyond-SM particles and forces and exploring how high-energy physics experimental facilities, from high-energy colliders, ultra-sensitive detectors and TRIUMF’s rare isotope facilities, can be leveraged for innovative beyond-SM searches. The beyond-SM theories address DM, the origin of neutrino masses, sources of the matter-antimatter asymmetry, and solutions to the electroweak hierarchy problem.

**Electroweak Symmetry Breaking and the Higgs **

In the SM, electroweak symmetry breaking, the process through which force carriers achieve mass, is induced by the Higgs field, but still largely unexplored. The Theory Department has extensive expertise in Higgs physics and the ability to predict the rates of Higgs boson production and simulate the signals these processes would generate in collider detectors for both the Higgs of the SM and beyond-SM extensions. These calculations help to guide precise measurements of the Higgs boson, and searches for new Higgs decay channels, at CERN’s Large Hadron Collider (LHC).

Similarly, the strong sensitivity of the SM electroweak sector to quantum corrections suggests that there are undiscovered particles and forces with masses just above the weak scale, an energy range now being explored by the LHC. The Theory Department has contributed significantly to the development of theories to elucidate the nature of these possible particles and forces, and also applies Monte Carlo simulation tools to model how they would appear in high-energy colliders.

**Dark Matter **

TRIUMF theorists are developing a wide range of innovative theoretical frameworks for both possible dark matter (DM) candidate particles, and experimental approaches for the direct and indirect detection of them.

The design of DM experiments depends critically on the projected DM particle’s mass and the nature of its predicted interaction with electrons, protons and neutrons. To guide this, TRIUMF theorists are exploring DM theories that produce DM particles over a wide range of masses and have proposed novel DM candidates with a variety of interaction mechanisms with electrons and nucleons.

For example, TRIUMF theorists are collaborating with both SNOLAB’s DEAP-3600 (targeted at heavier DM candidate particles) and SuperCDMS (focused on lighter DM candidates) collaborations. The Theory Department is providing calculations of the expected signal rates of DM candidates in deep underground direct-search experiments, cosmic-ray telescopes, astrophysical systems and at particle colliders. This is essential for testing whether a DM candidate is consistent with current experimental data and it helps to guide future searches for that candidate.

TRIUMF theorists are also exploring new ways to explore for DM using experimental facilities designed for other purposes. This includes searches for very light DM particles that could be created in TRIUMF’s new ARIEL high-intensity electron beam, or in the collisions used to produce neutrinos at the T2K experiment.

**Neutrinos **

The Theory Department is developing models to explain one of the central questions in neutrino and beyond-SM physics: the origin of neutrino masses.

Similarly, the Theory Department is exploring the experimental implications of these neutrino mass mechanisms. This includes neutrino mass models that could be tested in leading neutrino experiments, such as the T2K collaboration, of which TRIUMF is a key participant, and possible connections between neutrino masses and the top quark that could lead to new signals at the LHC.

**Origin of Matter Asymmetry **

The observable universe contains much more matter than antimatter, something not explained by the SM. TRIUMF theorists are studying mechanisms through which this matter asymmetry could have emerged, including identifying experimental tests that could reveal mechanisms for the matter asymmetry. These potential tests include modified Higgs boson production and decay rates that could be observed at the LHC, neutron-antineutron oscillations that could be found in precision neutron measurements, and possible proton decay signals that could detected by the T2K-related Super-Kamiokande detector.