WITCH: a Penning trap retardation spectrometer combination for precision studies of the weak interaction

The weak interaction trap for charged particles (WITCH) setup is being installed at the ISOLDE facility (CERN) to test the Standard Model of the electroweak interaction. This will be done by searching for scalar and tensor admixtures in nuclear $\beta$-decay. The $\beta$-neutrino angular correlation is an excellent probe to determine the possible strength of those non-Standard Model contributions. The WITCH setup combines the unique storage features of a Penning trap to produce a scattering free radioactive source with a retardation spectrometer to measure the recoil energy spectrum of the daughter nuclei after $\beta$-decay with high precision. Physics beyond the Standard Model would lead to deviations from the expected spectral shape. (7 refs).