# NoMoS: An $R \times B$ Drift Momentum Spectrometer for Beta Decay Studies

## TL;DR

NoMoS is a novel $R imes B$ drift spectrometer designed to precisely measure momentum spectra and angular correlations in neutron beta decay, aiding tests of the Standard Model and searches for new physics.

## Contribution

It introduces a new momentum spectroscopy method using the $R imes B$ drift effect in a curved magnetic field for neutron decay studies.

## Key findings

- Conceptual design and measurement principles outlined
- Discussion of uncertainties and systematic effects
- Status update on development and implementation

## Abstract

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 $R \times B$ 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 inter alia the electron-antineutrino correlation coefficient $a$ and the Fierz interference term $b$ with an ultimate precision of $\Delta a/a < 0.3\%$ and $\Delta b < 10^{-3}$ respectively. In this paper, we present the measurement principles, discuss measurement uncertainties and systematics, and give a status update.