To see a world in a grain of sand: what neutrons can tell us about the Universe

Abstract 

The Standard Model of Nuclear and Particle Physics, our best working attempt at a Theory of Everything, can predict experimental measurements to one part in a trillion. But there are large gaping holes in it: it cannot describe Dark Matter, it cannot explain how matter survived the Big Bang, it cannot incorporate gravity, and it cannot even be used directly to describe the everyday protons and neutrons that matter is made of. 

The neutron can be used to unlock many of these mysteries. My experiments fall in the field of low-energy, high-precision nuclear physics and Fundamental Symmetries using neutrons and neutrinos. Three of them look for a fingerprint: a new interaction leaves its mark on ordinary particles even when whatever carries it is far too heavy to produce at the world's highest-energy collider. The fourth probes the strong force inside protons and neutrons, where our exact theory of it cannot be applied directly. Being electrically neutral particles makes them particularly difficult to manipulate, requiring a broad range of technologies in an interdisciplinary manner. This includes: nuclear engineering and scintillation light detection, cryogenics down to milli-Kelvin temperatures, superconductivity and quantum liquids, polymers of ultrahigh isotopic and chemical purity, hyperpolarization and nuclear magnetic resonance, precision electromagnetic field control, and quantum sensing. 

I work in four mid-sized collaborations. Compton@HIGS measures the electric and magnetic polarizabilities of the proton and neutron by Compton scattering from light nuclei, at the High Intensity Gamma-ray Source of Triangle Universities Nuclear Lab on Duke University's campus. The neutron's are currently known only to 10% and 30%; improving them tests the bridge connecting chiral effective field theories and lattice Quantum Chromodynamics, which come from opposite ends of the energy scale. The neutron Electric Dipole Moment experiment searches for the beyond-standard-model time-reversal symmetry violation that leading explanations of the matter-antimatter asymmetry in the Universe require. Our experiment was first developed at Oak Ridge National Lab, and continues as nEDM-Superfluid, with a demonstrator at the Institut Laue-Langevin in France and ultimately the European Spallation Source in Sweden. Nab, also at Oak Ridge, measures ordinary neutron beta decay in great detail, where small deviations from the Standard Model pattern would signal a new force. And nEXO, planned for two kilometres underground at SNOLAB in Canada, searches for neutrinoless double beta decay, which would show the neutrino is its own antiparticle, a necessary ingredient for leptogenesis, another leading explanation for the same asymmetry.