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Structure of the weak interaction by angular correlation measurements
The vector and axial-vector structure of weak interactions, discovered 50 years ago in nuclear beta decay, has been imbedded in the Standard Model (SM) where the interaction between leptons and quarks is described by the exchange of charged weak vector bosons. Several extensions to the SM introduce new exchange bosons, which would be manifested by the presence of scalar and tensor interactions in nuclear beta decay. A robust observable for the structure of the weak interaction is the angular correlation between the emitted electron and recoiling nucleus (or the positron and the neutrino). This correlation probes the presence of exotic couplings without assumptions on their discrete space and time transformation properties. Direct and indirect techniques have been implemented to measure the nuclear recoils following beta decay. In particular, experiments using atom traps have recently achieved an unprecedented level of precision, of few parts in 10-3 on the correlation coefficient, to search for scalar couplings. The availability of very high intensity radioactive beams enables consideration of new techniques beyond atom and ion traps, in which the angular correlation is determined from the measurement of decays in flight from a very low energy beam with small transverse size. Such techniques require dedicated beam preparation in order to achieve a number of decays of about 1000 per second over the beam volume seen by the detectors.
