Professor Susan Gardner was awarded the University of Kentucky’s Albert and Elizabeth Kirwan Memorial Prize. The prize is bestowed each year to a faculty member in recognition of their outstanding contributions to original research or scholarship, with an emphasis on work produced four years prior to the award. Below is Professor Gardner’s description of her recent research. 

I thank my faculty colleagues for recognizing my research accomplishments with the 2022 Albert and Elizabeth Kirwan Memorial Prize. My research concerns the theoretical investigation of "fundamental symmetries,'' and their violation, to the end of identifying and interpreting new physical phenomena. In the last years, my colleagues - including graduate students and postdocs in our department - and I have applied these ideas broadly, to systems ranging from sub-femtometer to kiloparsec (kpc) length scales. At subatomic scales, our work has focused on discrete symmetries, such as parity P, charge-conjugation C (particle-antiparticle), or CP (matter-antimatter) symmetries, as well as those of baryon-number B and lepton-number L. These symmetries are all broken in the Standard Model of particle physics, but often only in inaccessibly small ways, so that the measurement of a significantly non-zero symmetry-breaking observable can signal physics beyond the Standard Model. These symmetry-breaking effects have been argued, variously, to play key roles in the dynamics of the early-Universe that gives a cosmic excess of baryons over antibaryons - and detecting them should give a keen discriminant of the possibilities.

At kpc scales, we have probed the expected symmetries of the Milky Way using precision astrometric observations of stars made with the Gaia space telescope. We note that terrestrial experiments that seek to discover particle dark matter assume the local matter is in a steady state, and we have developed (using Noether's theorem, which links continuous symmetries to constants of motion) how a pattern of spatial-symmetry-breaking in the stellar number counts, left and right of the ray that connects the sun to the Galactic center of our Milky Way galaxy and north and south of the midplane of the Galactic stellar disk, probes the possibility of non-steady-state effects. We have shown that the left-right symmetry breaking in our stellar sample of 11.7 million stars points to a small, non-zero torque exerted by the Large Magellanic Cloud, and that north-south symmetry breaking effects are much larger still – these results, given a theoretical framework of the collisionless Boltzmann and Poisson equations for the stellar fluid, show that the local stars are not in steady state, possibly as a result of ancient collisions with the galactic disk.

Turning to subatomic scales, we have shown that spontaneous baryon number violation can give rise to a new, weak force between quarks, permitting neutron stars to be heavier than expected, as recent LIGO discoveries suggest. We have shown that spontaneous lepton number violation can modify the anomalous magnetic moments of charged leptons, which are the subject of intense experimental investigation, both in table-top (electron) and accelerator (muon) experiments, and that parity-violating electron-electron scattering experiments can also severely limit such possibilities. We have shown that explicit baryon number violation can be probed through precision studies of energy loss in binary pulsar systems - i.e. that the difference between the observed energy loss and the computed energy loss from gravitational wave radiation limits the possibility of dark decays of the neutron, suggested by the neutron lifetime anomaly, to exquisitely sensitive levels, well beyond the reach of terrestrial experiments. Finally, we have also shown that any observed patterns of mirror symmetry breaking in the momentum distribution of the decay of the eta meson into three distinct pions (under the exchange of the momenta of the two charged pions) probes new sources of C and CP symmetry breaking in new ways. Taken altogether, we hope to identify the dynamical ingredients that drive the emergence of dark matter and the cosmic excess of baryons.