NoMos: an R*B drift momentum spectrometer for beta decay studies

The beta decay of the free neutron provides several probes to test the Standard Model of particle physics as well as to search for extensions thereof. Hence, multiple experiments investigating the decay have already been performed, are under way or are being prepared. These measure the mean lifetime, angular correlation coefficients or various spectra of the charged decay products (proton and electron). NoMoS, the neutron decay products momentum spectrometer, presents a novel method of momentum spectroscopy: it utilizes the R * B (where R is the curvature radius and B the magnetic field) drift effect to disperse charged particles dependent on their momentum in an uniformly curved magnetic field. This spectrometer is designed to precisely measure momentum spectra and angular correlation coefficients in free neutron beta decay to test the Standard Model and to search for new physics beyond. With NoMoS, we aim to measure, among others, the electron-antineutrino correlation coefficient a and the Fierz interference term b with an ultimate precision of Δa/a < 0.3% and Δb < 10^-3 respectively. In this paper, we present the measurement principles, discuss measurement uncertainties and systematics, and give a status update. (authors)