Neutron beta decay is described by two parameters in the Standard Model of particle physics, the ratio of axial vector and vector coupling constant and the quark mixing matrix element for the first generation.

However, in neutron decay many different observables can be defined, for example beta-, neutrino-, or proton asymmetry or the precise shapes of the electron or the proton spectrum. These observables are related to the parameters of the theory. Measuring them with high precision does not only allow calculating these parameters but also checking the consistency of the theory. In this way, exotic couplings like right-handed, scalar- or tensor-like can be searched for.

Presently, this research is limited experimentally at the 10-3 precision level, by statistics and systematic uncertainty. PERC [Dubbers et al. 2008] is a new instrument to overcome these limitations. It will solve the issue of insufficient statistics by extracting charged neutron decay products from a long neutron guide, using a strong magnetic field. All systematic effects can be controlled on the 10-4 level.

The design of its main component, a 12m long superconducting magnet system, is described in [Wang et al. 2019].