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 to 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.

Many of the most useful cases involve short-lived rare isotopes. 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! Our work has used GRIFFIN and TITAN at TRIUMF, and we are extending this research with 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. Our work with the TITAN Electron Beam Ion Trap (EBIT) at TRIUMF has used highly charged radioactive ions to study the influence of the atomic environment on nuclear decay and 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 and theory are in large disagreement, so it is unclear at the moment. These studies require precise control of both the ionic charge state and nuclear state, providing a way to investigate the coupling between atomic and nuclear degrees of freedom.

Neutrinoless double-beta decay is one of the most sensitive ways to investigate the fundamental nature of the neutrino. In this hypothetical process, a nucleus emits two electrons without accompanying antineutrinos. Observing it would establish lepton-number violation and demonstrate that neutrinos have a Majorana component to their mass, revealing physics beyond the Standard Model.

XLZD is a proposed underground liquid-xenon observatory with a major search for this decay in xenon-136. Its planned 60–80 tonne target contains several tonnes of xenon-136 at natural isotopic abundance, providing a large source of candidate decays within the detector itself. The experimental challenge is to identify an exceptionally rare signal while suppressing backgrounds from detector materials, environmental radioactivity, and other interactions.

Published sensitivity studies show that XLZD could probe decay half-lives of order 10²⁸ years with a decade of data, depending on detector size and background performance. This makes neutrinoless double-beta decay a central part of XLZD’s physics program and a complementary approach to our precision studies of nuclear weak decay.

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 matter 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 de-excitation 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.