Past exam of the flows, fluctuations and complexity and symmetry and relativity paper of Part B of the physics course of the University of Oxford / 2011 / 6 / 4 / Solution

Covered by the official notes: comparison with a 200 keV neutrino mass (PDF p. 12; handwritten p. 7).^[ref]

The notes stop at “no change”; the experimental implication is that this is a poorly conditioned neutrino mass measurement. In relativistic two-body decay, the corrections to the rest-frame energy and momentum enter through the square of the daughter rest mass, while the Lorentz transformation boosts both the massless value and its small correction. A measured laboratory energy or angle must therefore resolve a very small change against beam-energy spread, decay-angle uncertainty and detector resolution. No visible change at the quoted precision does not establish zero neutrino mass.