Investigating the weak interaction’s structure and symmetries at low energy

Nathal Severijns research is dealing with precision measurements at the border of nuclear and particle physics, to investigate the properties of the weak interaction at low energy. The focus is on studying the symmetries and structure of the weak interaction in the beta decay of radioisotopes or using ultracold neutrons. Experiments and projects are always performed in collaboration with partners from international universities/institutes and make use of facilities at the Cyclotron Research Center (Louvain-la-Neuve, BE), ISOLDE-CERN (Geneva, CH), GANIL (Caen, FR) and the Paul Scherrer Institute (PSI, Villigen, CH). Each time new experimental setups and/or detection methods are developed, often in parallel with new theoretical work.

Initial studies dealt with precise tests of parity violation, searching for so-called right-handed weak currents in innovative measurements of the longitudinal polarization of positrons emitted in the beta decay of nuclei that were polarized using different polarization methods. Later, measurements of both the beta-asymmetry parameter (using low temperature nuclear orientation) and the beta-neutrino correlation (using e.g. ion trapping and precision spectroscopy techniques) were and are still pursued, to search for possible scalar and tensor contributions to the weak interaction. Currently a beta-neutrino correlation project is ongoing at ISOLDE/CERN using the beta-delayed proton decay of the short-lived isotope 32Ar (WISArD), while beta-spectrum shape measurements are performed on 114In. These use scintillation and semiconductor detectors, as well as wire-chamber based detection technology. In the case of 114In one focuses also on the small effect of the strong interaction (called weak magnetism) in weak interaction processes.

Measurements of the time-reversal violating so-called R-triple correlation in polarized neutron decay provided stringent limits on T-violating scalar weak currents. Since 2007 his team is a member of the neutron electric dipole moment collaboration at PSI, the results of which could shed new light on the observed large difference between matter and antimatter in the universe. The team contributes unique expertise in precision magnetometry for which a dedicated development laboratory has been constructed at KU Leuven.

N. Severijns has written several review papers outlining the state-of-the-art and indicating future directions for precision weak interaction studies in neutron and nuclear beta decay.