[lnauta]

To make it more complicated, they measure mass-squared! The original KATRIN article from 2019 (which I read at the time) measured a mass-squared of m^2 = (-1.0) +0.9 -1.1 eV^2. This is obviously unphysical, as a negative mass, let alone a negative mass-squared does not mean anything. (No they are not tachyons.) To get around this, some statistical tricks like "Feldman-Cousins" and others have to be used to construct a confidence interval that can be physically interpreted. I don't know the details of FC though, but it's widely used in low-statistics experiments.