Beta-neutrino correlations from the beta decay of optically trapped $\rm^{38m}$K atoms

The $\beta\!-\!\nu$ correlation parameter, $a$, has been measured in the $0^+\!\rightarrow 0^+$ beta decay of trapped $\rm^{38m}$K (lifetime 0.924s) atoms to place limits on the possible contribution of a scalar interaction to nuclear beta decay [A.Gorelov et al., PRL, {\bf 94}, 142501(2005)]. A magneto-optical trap provides an isomerically selected and backing-free source of atoms, localized in a volume less than 1mm in diameter, so the low-energy recoiling nuclei can freely escape and be detected in coincidence with betas in back-to-back geometry. The $\beta\!-\!\nu$ correlation is measured by observing the positron in a $\Delta E-E$ telescope and the time of flight of the recoiling Ar nucleus in a micro-channel plate (MCP). The application of a uniform electric field along the detection axis toward the MCP allows separation in time of the $\rm Ar^0,Ar^+$ and higher charge states of Ar ions as well as increasing both their collection and detection efficiencies. Analysis of about 160,000 events with the positron energy above $2.5\,$MeV (1/2 the $Q-$value) resulted in the $\beta\!-\!\nu$ correlation parameter $\tilde a=0.9981\pm 0.0030^{+0.0032}_{-0.0037}$, consistent with the Standard Model prediction $\tilde a=1$. Future modifications of the experimental apparatus and data analysis may give us a possibility to reduce systematic errors and extend the search to lower beta energies.