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, theneutron decay productsmo___mentumspectrometer, presents a novel method of momentum spectroscopy: it utilizes theR×Bdrift 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 coefficientaand the Fierz interference termbwith an ultimate precision of Δa/a< 0.3% and Δb< 10−3respectively. In this paper, we present the measurement principles, discuss measurement uncertainties and systematics, and give a status update.
Highlights
Neutron beta decay in the standard modelThe Standard Model of particle physics (SM) is the basis of our current understanding of elementary particles and their fundamental interactions
Precision measurements of a and b are planned for tests of the SM and searches for new physics beyond
The majority of systematic effects is already included in the transport function, which enables a direct fit of the detected spectra
Summary
The Standard Model of particle physics (SM) is the basis of our current understanding of elementary particles and their fundamental interactions. For high precision experiments, it can be coupled adiabatically to a magnetic field collecting the charged decay products from a long decay volume and afterwards filtering their incident angle using the magnetic mirror effect, as in the new facility PERC [36,37,38] or later at a PERC-like instrument at the proposed pulsed cold neutron beam facility ANNI [39] at the ESS. Beam monitor: In the standalone case, an electron detector will be installed at the upstream end of the experimental interface to detect those electrons emitted towards the upstream end, reflected at the small magnetic field gradient in the decay volume or from the magnetic filter, and those backscatteredoff of the aperture This detector will monitor the time stability of the particle beam and cross-check the angular selection (for details see the section). At PERC or later ANNI, the time stability is monitored by a small parasitic monitor (out of sight of the aperture)
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