(eng)
We have set limits on contributions of scalar interactions to nuclear beta
decay. A magneto-optical trap (MOT) provides a localized source of atoms
suspended in space, so the low-energy recoiling nuclei can freely escape and be
detected in coincidence with the beta. This allows reconstruction of the
neutrino momentum, and the measurement of the beta-neutrino correlation, in a
more direct fashion than previously possible. The beta-neutrino correlation
parameter of the 0+ to 0+ pure Fermi decay of 38mK is
$\tilde{a}$=0.9981+-0.0030+-0.0037, consistent with the standard model
prediction a=1.
Comment: 4 pages, 3 figures, accepted by Phys. Rev. Lett., centroid changed
