The Beta-Neutrino Correlation in Sodium-21 and Other Nuclei

We have measured the mbox beta - nu correlation coefficient, a_beta nu, in 21Na using a laser-trapped sample. We measure the energy spectrum of the recoil nuclei by measuring their time-of-flight in coincidence with the atomic electrons shaken off in beta decay. High detection efficiency of these low-energy electrons allows good counting statistics, even with low trap density, which suppresses the photoassociation of molecular sodium, which can cause a large systematic error. Our measurement, with a 1 percent fractional uncertainty, agrees with the Standard Model prediction but disagrees with our previous measurement which was susceptible to error introduced by molecular sodium. We summarize precise measurements of a_ beta nu and their consequences for searches for Beyond Standard Model scalar and tensor current couplings