These results halved the upper limit of the effective neutrino mass from a direct, model-independent laboratory measurement to 1.1 eV (90% CL).

Enrico Fermi first pointed out that the end-point energy of the daughter electrons from β-decays would fall short of the total decay energy if neutrinos are massive particles. By energy-momentum conservation, the shortfall is the neutrino mass. The KATRIN experiment, located at the Karlsruhe Institute of Technology in Germany, exploits this idea by using a high-resolution spectrometer to measure the β-electron spectrum from tritium decays near the decay energy of 18.6 keV. Figure 1 shows the inside of the spectrometer.

Figure 2. Measured spectrum (data points) and fit of electrons over a 90-eV interval from all tritium runs in the data set. Although there are 27 different measurement points (retarding potential settings), most of the measurement time was devoted to several points very close to the end-point energy.

The design, construction and commission of the various subsystems of the KATRIN experiment were truly heroic efforts by an international team of scientists and engineers from twenty institutions, including Berkeley Lab. NSD’s Alan Poon leads the local efforts in the experiment. He spent a year in Karlsruhe to assist in the design of the calibration subsystem and the simulation and analysis platform. Berkeley Lab’s Low Background Facility was used to qualify the radiopurity of the materials used in the experiment.

For this recent result, NSD postdoctoral fellow Björn Lehnert extended the software platform that was developed by Prof. Susanne Mertens (former postdoctoral fellow in Poon’s group) and her team at Technical University of Munich. The extension allows Lehnert to perform a computationally intensive Bayesian analysis of the data on National Energy Research Scientific Computing Center’s supercomputer Cori. Figure 2 shows the measured β-spectrum and the fit over a 90-eV interval near the decay energy. The fit for the parameter m2v = ( -10 ± +0.9-1.1) eV2, resulting in an upper limit of 1.1 eV at 90% confidence level.