These are exciting times for the collaboration Guido Drexlin The KATRIN analysis improves on these experimental results, with systematic uncertainties reduced by a factor of six and statistical uncertainties reduced by a factor of two. The combination of these methods culminated in the previous best limits of 2.3 eV at 95% confidence in 2005, and 2.05 eV at 95% confidence in 2011, by physicists working in Mainz, Germany and Troitsk, Russia, respectively. It builds on the invention of gaseous molecular tritium sources and spectrometers based on the principle of magnetic adiabatic collimation with electrostatic filtering. KATRIN is now the most precise experiment of this kind. The direct mass measurement method hinges on precisely measuring the energy spectrum of beta-decay electrons, and is considered model independent as the extracted neutrino mass depends only on the kinematics of the decay. Physicists have pursued direct mass measurements since Reines and Cowan observed electron antineutrinos in inverse beta decays in 1956. Despite their success in constraining neutrino mixing, such experiments are sensitive only to squared mass differences between the eigenstates, and not to the neutrino masses themselves.
Their three known mass eigenstates do not match up with the better-known flavour eigenstates, but mix according to the PMNS matrix, resulting in the flavour transmutations seen by neutrino-oscillation experiments. Neutrinos are among the least well understood particles in the Standard Model. At a colloquium today, the collaboration reported an upper limit of 1.1 eV at 90% confidence, almost halving the previous bound.
Credit: KATRINīased on just four weeks of running, researchers at the Karlsruhe Tritium Neutrino (KATRIN) experiment in Germany have set a new model-independent bound on the mass of the neutrino. Inside the KATRIN spectrometer at the Karlsruhe Institute of Technology.
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