The KATRIN sensitivity to the neutrino mass and to right-handed currents in beta decay

The aim of the KArlsruhe TRItium Neutrino experiment KATRIN is the determination of the absolute neutrino mass scale down to 0.2 eV, with essentially smaller model dependence than from cosmology and neutrinoless double beta decay. For this purpose, the integral electron energy spectrum is measured close to the endpoint of molecular tritium beta decay. The endpoint, together with the neutrino mass, should be fitted from the KATRIN data as a free parameter. The right-handed couplings change the electron energy spectrum close to the endpoint, therefore they have some effect also to the precise neutrino mass determination. The statistical calculations show that, using the endpoint as a free parameter, the unaccounted right-handed couplings constrained by many beta decay experiments can change the fitted neutrino mass value, relative to the true neutrino mass, by not larger than about 5-10 %. Using, incorrectly, the endpoint as a fixed input parameter, the above change of the neutrino mass can be much larger, order of 100 %, and for some cases it can happen that for large true neutrino mass value the fitted neutrino mass squared is negative. Publications using fixed endpoint and presenting large right-handed coupling effects to the neutrino mass determination are not relevant for the KATRIN experiment. 1. Neutrino mass determination and the endpoint In the KATRIN experiment the absolute neutrino mass is determined by the measurement of the integral energy spectrum of the electrons coming from beta decay of tritium molecules. The electrons are guided from the tritium source to the detector by magnetic field. Between the source and the detector a large negative potential (-18.6 kV) is applied at the main spectrometer, with the aim that only those electrons can reach the detector that have a decay kinetic energy above the value corresponding to this potential. The transversal energy component (relative to magnetic field) of the electrons is converted into longitudinal energy by using the inverse magnetic mirror effect. Thus it is possible to measure the integral electron energy spectrum simultanously with high statistics and with high precision. For further information about the KATRIN experiment see Ref. [1]. The differential electron energy spectrum can be written (in a first approximation, close to the endpoint) as