Neutrino mass sensitivity by MAC-E-Filter based time-of-flight spectroscopy with the example of KATRIN

Abstract

The KArlsruhe TRItium Neutrino (KATRIN) experiment aims at a measurement of the neutrino mass with a 90% confidence limit (CL) sensitivity of 0.2 eV/ c ^2 by measuring the endpoint region of the tritium β decay spectrum from a windowless gaseous molecular tritium source using an integrating spectrometer of the magnetic adiabatic collimation with an electrostatic filter (MAC-E-Filter) type. We discuss the idea of using the MAC-E-Filter in a time-of-flight mode (MAC-E-TOF) in which the neutrino mass is determined by a measurement of the electron TOF spectrum that depends on the neutrino mass. MAC-E-TOF spectroscopy here is a very sensitive method since the β -electrons are slowed down to distinguishable velocities by the MAC-E-Filter. Their velocity depends strongly on their surplus energy above the electric retarding potential. Using MAC-E-TOF, a statistical sensitivity gain is expected. Because a small number of retarding-potential settings is sufficient for a complete measurement, in contrast to about 40 different retarding potentials used in the standard MAC-E-Filter mode, there is a gain in measurement time and hence statistical power. The improvement of the statistical uncertainty of the squared neutrino mass has been determined by Monte Carlo simulation to be a factor 5 for an ideal case neglecting background and timing uncertainty. Additionally, two scenarios to determine the TOF of the β -electrons are discussed, which use the KATRIN detector for creating the stop signal and different methods for obtaining a start signal. These comprise the hypothetical case of an ‘electron tagger’ which detects passing electrons with minimal interference and the more realistic case of ‘gated filtering’, where the electron flux is periodically cut off by pulsing the pre-spectrometer potential.