Physics Colloquium - The Unbearable Lightness of Neutrinos

Ever since it was first hypothesized more than 90 years ago, the strange and ghostly particle called the neutrino has mystified and inspired particle and nuclear physicists. After decades of experimental and theoretical work, we have now firmly established that neutrinos have mass, and yet their absolute mass scale remains unknown. Now, after many years of painstaking design, construction, and commissioning work, the Karlsruhe Tritium Neutrino experiment (KATRIN) has recently improved the world's best direct neutrino-mass sensitivity by more than a factor of four, establishing at 90% confidence that the neutrino mass scale is less than 0.45 eV/c^2. I will give a tour of KATRIN's 70-m beamline, describe our spectral fits and systematic uncertainties, and show a glimpse of KATRIN's future.