High Energy Physics Seminars

The study of generalized symmetries has led to new insights across high energy, condensed matter, and mathematical physics, but we are still in the early days of applications to particle physics. In this talk, I’ll briefly survey some of the generalized symmetries relevant for particle physics and present two examples where broken higher-form symmetries shed new light on phenomenology: the quality problem for extra-dimensional axions, and the quantum violation of baryon number in classically baryon-conserving unified theories. Both examples have novel astrophysical and cosmological implications. No prior familiarity with generalized symmetries is assumed or required.

Neutrinos, the only neutral elementary fermions, have provided many surprises. Flavor oscillations reveal the non-conservation of the lepton flavor number and demonstrate that neutrino masses are finite; yet they are surprisingly much smaller than those of other fermions (by at least six orders of magnitude!) It is then natural to ask if the mechanism providing the mass to neutrinos is the same that gives masses to the other (charged) elementary fermions and if neutrinos are described by 4-component Dirac wavefunctions or, as is possible for neutral particles, by 2-component Majorana ones. The hypothetical phenomenon of neutrino-less double-beta decay can probe the Majorana nature of neutrinos and the conservation of the total lepton number. It may also help elucidating the origins of mass in the neutrino sector. This is the Frontier of neutrino physics. Following the well-known principle that there is no free lunch in life, interesting half-lives for neutrino-less double-beta decay exceed 10^{25} years (or ~10^{15} times the age of the Universe!) making experiments rather challenging. I will describe nEXO, a 5-tonne, enriched Xenon experiment with a sensitivity reaching beyond 10^{28} years, or >100 times the current state of the art. The nEXO detector derives directly from EXO-200, a very successful, rogue detector built by a collaboration with a heavy SLAC-Stanford participation.

TBA