The Use of Nuclear Beta-decay as a Test of Bulk Neutrinos in Extra   Dimensions
  Theories which include neutrinos in large extra dimensions can be constrained by nuclear beta decay experiments. We examine universality of beta decay strengths of Fermi transitions. From this we find that the extra dimensional Yukawa coupling for a higher dimensional scale of 10 TeV, and two extra dimensions can be constrained to y < 1. Kinematic implications are also discussed. In particular, an extra dimensional scenario will produce a tritium decay endpoint spectrum with a different shape than that for just one massive state. 
G. C. McLaughlin and J. N. Ng