Positron-neutrino correlation measurements in the beta decay of magneto-optically trapped 38mK atoms

This thesis describes the measurement of the angular correlation between the positron and the neutrino emitted in the beta decay of the isomer 38mK. This is a superallowed transition between nuclear states of the same spin and parity (0+), which is known to result primarily from the vector component of the weak interaction. The angular correlation involves two parameters. In the Standard Model of the weak interaction these have the values a=1 and b=0. Any meaningful deviation from this result can be interpreted as evidence for the existence of a scalar component in the weak interaction. The fundamentally new method that was used involved selectively confining neutral atoms of the isomer in a magneto-optical trap located between two detectors, one to measure the energy and direction of the positron and the other to detect the 38Ar nuclei that recoil with a momentum pR=-(pe+pv). The 38mK atoms were produced using the TRIUMF/ISAC facility. The trap provided a pure, cold, compact source essential to avoid distortion of the recoil momenta. For those events in which the positron was detected, the recoil momenta were deduced by measuring the time of flight from the trap to the recoil detector. About 500,000 positron-recoil coincident events were recorded. When the analysis, based on detailed Monte Carlo simulations, was restricted to positrons with kinetic energy >2.5 MeV, it showed that the angular correlation could be characterized by a "reduced" correlation parameter ã=0.9988±0.0028(stat)±0.0034(syst) (68% CL) where ã=a/(1+0.1503b). This measurement is consistent with the Standard Model and is 33% more restrictive than the only comparable previous measurement for such a transition. In the most general form, the strength of a possible scalar interaction can be specified in terms of two complex numbers, L and R, which define, respectively, the coupling to left- and right-handed neutrinos. This experiment did not usefully restrict the value of Re(L) (or b). Other experiments do provide rather strict limits on Re(L). If these are combined with the result of the present experiment one obtains the most restrictive direct limits available on Re(R), Im(R) and Im(L).