NoMoS: Beyond the Standard Model Physics in Neutron Decay

Precision measurements in nuclear and neutron beta decay have contributed considerably to our understanding of the Standard Model of particle physics and are searching for new physics beyond. The information obtained from such experiments is complementary to direct searches for new physics at high energies with colliders. Low-energy searches with 10-4 sensitivity will probe energy scales of 1 – 100 TeV, far above the production threshold at the LHC.

To reach 10-4 sensitivity in neutron beta decay, novel experimental techniques are being developed: the proton electron radiation channel PERC and the R×B spectrometer. PERC exploits the full phase space of neutrons in a neutron guide and delivers a clean and bright beam of charged neutron decay products. The R×B spectrometer uses a novel concept for spectroscopy of these charged decay products and will be built within the scope of the NoMoS (Neutron decay prOducts MOmentum Spectrometer) project.

In the R×B spectrometer (Fig.), charged decay products are separated according to their momentum by means of a uniformly circularly curved magnetic field B. They drift perpendicular to the magnetic field curvature R and to the magnetic field direction B, known as R×B drift. The R×B spectrometer has substantial advantages compared to ‘classical’ magnetic spectrometers: It can be coupled adiabatically to a magnetic field collecting charged particles from a (long) decay region (as in PERC) and provides a strong suppression of electron backscattering and of gamma‐ray background.

The goal of the NoMoS project is electron and proton spectroscopy on the sub-10-4- respectively 10-3-level. Most of the correlation coefficients in neutron beta decay are accessible through the R×B spectrometer. These coefficients, together with the neutron lifetime, serve as observables for parameters beyond the Standard Model. In particular, we plan to find the so far not observed and in the Standard Model forbidden Fierz interference term b or to present new limits. A non-zero value for b would be an indication of the existence of scalar or tensor interactions. Scalar or tensor couplings in turn would occur if yet unknown charged Higgs bosons or leptoquarks were exchanged instead of a W boson.

This research group is financed by the ‘New Frontiers Research Groups Programme’ by the Austrian Academy of Sciences. Further Information can be found at the OAW.