Search for a tensor component in the weak interaction Hamiltonian

The search for physics beyond the standard model can, besides in high-energy experiments such as the ones at the LHC accelerator, also be carried out at lower energies. Measurements of correlation coefficients in neutron and nuclear b decay constitute a reliable and model-independent method for such efforts. The topic of this thesis is the precision measurement of the beta asymmetry parameter A. It was measured in the decay of 67Cu, which proceeds via a pure Gamow-Teller b transition, thus its A parameter is sensitive to possible tensor type currents in the weak interaction. The experiment was performed at the NICOLE setup in ISOLDE (CERN), using the technique of low temperature nuclear orientation. The b particles were observed with custom made planar high purity germanium detectors operating at around 10 K. The beta asymmetry of 68Cu was measured on-line for normalization purposes. Geant4 simulations were used to gain control over systematic effects such as electron scattering on the particle detectors. As the simulations play such a crucial role in the analysis procedure their quality and reliability was investigated in great detail. Therefore experimental spectra of different radioactive isotopes measured with germanium and silicon detectors are compared to simulated ones. Based on the results, the optimal Geant4 physics model was determined together with fine-tuning the value of several simulation parameters. The experimental result for the beta asymmetry parameter of 67Cu is A = 0.584(10), in agreement with the standard model prediction of A_SM = 0.5998(2) when taking into account recoil, radiative and Coulomb corrections. This is one of the most accurate determinations of this parameter to date. The limits obtained for the time-reversal invariant tensor coupling constants are -0.002 < (C_T + C'_T )/C_A < 0.100 (90% C.L.) and these are competitive with limits from other measurements of correlations in neutron and nuclear beta decay. Analysis of the error budget of this and previous experiments employing the low temperature nuclear orientation technique leads us to conclude that the inherent limitations of this technique have been reached. In order to remain competitive as other experiments are approaching the sub-percent precision, a new technique is being developed, i.e. precision beta spectrum shape measurements. Its sensitivity to physics beyond the standard model is via the Fierz interference term. Combining a scintillator detector with a multi-wire drift chamber a backscatter-free beta spectrum can be obtained. By again choosing a pure Gamow- Teller transition of low endpoint energy the Fierz interference term can be determined with sub percent precision. Part of the feasibility study for this new project is described here as well