Neutrinos are perhaps the most bizarre and elusive of the fundamental particles in the Standard Model. Although we know relatively little about these “ghost particles”, they have already provided us with perhaps the best window into what might live beyond the Standard Model (BSM). In experimental work first performed more than 20 years ago, it was shown that neutrinos have non-zero mass - thus breaking the Standard Model description of the known fundamental particles for the first time (Nobel Prize 2015). The exact value of their masses remains unknown, however we do know that they are unnaturally light - more than a million times lighter than the electron.

Extensions to the Standard Model that can account for the exceedingly light neutrino masses also predict the existence of heavy mass states that are associated with an even more elusive type of neutrino: the so-called “sterile neutrino”. Although this new particle has never been observed, there are tantalizing hints that they may exist and could be the key to unlocking the mystery of what lies beyond the Standard Model.

So, how do you measure the mass of particles that pass through your detector without ever interacting? . Our approach is to create these elusive particles in nuclear beta decay and precisely measure the other particles in the decay process using state-of-the-art “table-top-scale” quantum sensing methods. By doing this, we can infer what their mass must be based on energy and momentum conservation.

Our group currently uses superconducting quantum sensors to attack this problem by precisely measuring the decay of radioactive beryllium atoms (the BeEST experiment). We are also investigating ways to better understand the light masses using optomechanical quantum sensors that use levitating nanospheres.

Although the quarks in the Standard Model are often thought of only as the strongly interacting building blocks of protons and neutrons, they play an important role in nuclear decay and also couple to the weak interaction. The Cabibbo–Kobayashi–Maskawa (CKM) matrix is a 3x3 unitary transformation that specifies the mismatch of quantum states of the three generations of quarks when they propagate freely versus when they take part in the weak interaction (Nobel Prize 2008). If this 3x3 matrix does not turn out the be unitary in nature, then the Standard Model description of the quarks may be incomplete - possibly suggesting the existence of an additional generation beyond the 3x3 paradigm.

Since the process of nuclear beta decay involves the transformation of an up quark to a down quark (or vice versa) we can use this decay mode to probe the first element in this matrix - the up-down element or “Vud”. Since the atomic nucleus is a complicated system, in order to provide a precise probe of this quantity we require very careful case selection of our radioactive nuclei that allow for the cancellation of several complicated effects. In particular, we look for cases where the parent and daughter nuclei are nearly identical in structure which result in very specific classes of transitions known as superallowed Fermi beta decay and mirror nuclear transitions.

These transitions occur only in the most exotic rare isotopes which have half-lives of seconds or less. As a result we must perform these experiments at radioactive beam facilities - such as TRIUMF-ISAC, FRIB, or CERN-ISOLDE - which can produce and deliver the samples we need in a fraction of a second! My group makes these measurements using GRIFFIN and TITAN at TRIUMF, and SALER at FRIB. Members of my group also perform theory work on these systems with our collaborators around the world.

Some common modes of nuclear decay, such as orbital electron capture (EC) and internal conversion (IC), proceed through an interaction between the nucleus and bound electrons within the constituent atom. As a result, the probabilities of the respective decays are not only influenced by the structure of the initial and final states in the nucleus, but can also depend strongly on the atomic charge state. These effects become increasingly more significant as the atom is highly ionized by removing nearly all of its electrons. Conditions suitable for the partial or complete ionization of these rare isotopes occur naturally in hot, dense astrophysical environments, and are difficult to generate artificially in the laboratory. We lead a program at TRIUMF that uses the TITAN Electron Beam Ion Trap (EBIT) to perform various studies on highly charged radioactive ions by generating these extreme conditions to search for exotic nuclear processes.

One of the most intriguing proposed interactions is Nuclear Excitation via Electron Capture (NEEC). NEEC is the time-reversed process of IC, where a free electron is captured into an atomic vacancy simultaneously exciting the nucleus to a higher-energy state. Since NEEC is a resonant process, experimental access in the lab to study these cases requires strong atomic charge-state control over the sample, as well as careful selection and preparation of nuclear states that may be compatible with efficient electron recombination.

NEEC is not only of interest to nuclear astrophysics, but given the significant increase in energy that can be released through the input of a small kinetic energy of the electrons, it may also have applications for a high-energy-density nuclear battery. There are hints that this process may have been observed previously, however experiment an theory are in large disagreement, so it is unclear at the moment. Using the TITAN EBIT we are able to perform these studies with a high level of control and sensitivity, and are at the start of a large experimental program in this area.

The experimental evidence that neutrinos have mass (Nobel Prize 2015) requires physics beyond the Standard Model (BSM). Adding neutrino masses to our understanding of nature requires new physics, including the likely possibility that neutrinos are their own antiparticles (known as Majorana Fermions)! Trying to experimentally determine if this is true is not easy, however the is a rare nuclear decay mode that can only exist if neutrinos are indeed Majorana Fermions known as neutrinoless double-beta decay. Given that this possible decay mode, if observed, could in one stroke discover lepton number violation and elementary Majorana Fermions, it is among the highest priority experiments for the world nuclear physics community.

The difficulty in these direct neutrinoless double beta decay measurements, however, is the fact that this process is so weak that the few cases which have the possibility to decay in this way must be studied with an extremely large number of atoms. In fact, the current lower-limit on the decay half-life for this possible process is a billion-billion times longer than the age of the Universe! Thus, the detectors for these experiments are constructed from the same material that undergo this decay mode. These measurements require an unprecedented level of background control that can only be achieved deep underground where the experiments are shielded from various types of radiation.

My group and our collaborators attack this research problem using the nEXO experiment which searches for neutrinoless double beta decay in xenon. The nEXO experimental concept is based on a time projection chamber filled with five tonnes of enriched liquid xenon. The projected half-life sensitivity of the nEXO experiment after 10 years of running places it among the most competitive BSM neutrino searches of any kind planned for the next decade. The experiment is in the conceptual design phase, and is planned to be located deep underground in Canada at SNOLAB.

With the recent gravitational-wave observations of neutron-star events, efforts towards understanding how protons and neutrons arrange themselves inside the nucleus have been dramatically increased, since this information can provide insights into the nature of neutron stars themselves. The theoretical models that predict this arrangement for extreme cases are difficult to compute, and require rigorous benchmarking through comparisons with observables in nuclear systems, including the polarizabilities and susceptibilities of general nuclear matter. This arrangement of nuclear mater is fairly well constrained for symmetric systems where the number of protons and neutrons are roughly equal, but the properties for asymmetric, neutron rich matter - such as the extreme asymmetry found in neutron stars - require further investigation.

The only known experimental probe for the transition electric dipole polarizability is nuclear two-photon decay. Nuclear two-photon decay is a second-order QED process wherein the nucleus simultaneously emits two photons of continuous energy that sum to the initial excitation energy, and the two photons couple to preserve the change in angular momentum of the transition. Although two-photon decay competes with every internal dexcitation process within the nucleus (such as single gamma decay), in practice it is usually observed in very specific cases where these first-order decay modes are forbidden or highly suppressed.

My group performs these experiments in collaboration with the GRIFFIN experiment located at the TRIUMF facility in Vancouver, Canada. Our initial measurements with radioactive sources have allowed us to test and prove this concept “off-line”. Through the development of precision analysis tools and careful background rejections, the GRIFFIN array can now be used to search for this decay mode in short-lived radioactive systems and provide information on the quantum nature of each individual photon.