Measurement of the beta-neutrino angular correlation in allowed transitions
Nuclear beta decay processes are described phenomenologically, in the framework of the Standard Model (SM), in terms of vector and axial interactions (V-A theory) coupling hadronic and leptonic currents. The contributions due to other Lorentz invariants, for example of scalar and tensor type, which can be formally introduced in the Hamiltonian describing these processes, are excluded from the theory. The measurement of the beta-neutrino angular correlation in carefully chosen nuclear beta decays allows to probe the presence of such exotic couplings. Indeed, in the case of a pure Gamow-Teller (G-T) or Fermi (F) transition, the correlation is sensitive to the presence of specific currents, of axial and tensor type or vector and scalar respectively. Any deviation of the correlation parameters from the values predicted by the SM would be the signature of a physics beyond this model. Measuring the beta-neutrino angular correlation in a mirror transition (G-T and F mixing) allows to deduce the GT/F mixing ratio of the transition. This parameter is essential in a test of the conservation of the vector current in the nuclear medium (CVC hypothesis), carried out from an exhaustive study of mirror transitions. The LPCTrap device, built and used by our group, is unique among the existing devices in the world for this type of measurements. The angular correlation parameter is deduced from the measurement of the time of flight between the recoil ions and the beta particles detected in coincidence. The low energy of the ions requires the use of a material-free ion confinement system. The central system of the device is a transparent Paul trap with an open geometry for injection and extraction of ions and for detection of decay particles. A transport and handling line for the ions (cooling and bundling) has been built by the LPC CAEN. The device was installed in 2004 on the LIRAT low energy line of the SPIRAL facility at GANIL, where it was used until 2013. The final analysis of the data acquired with 6He1+ (pure G-T), 35Ar1+ and 19Ne1+ ions (mirror transitions), which requires very advanced realistic simulations of the experiments, is still in progress.