Positron polarization in the decay of polarized N-12: a precision test of the Standard Model

The longitudinal polarization of positrons emitted along and opposite to th e nuclear spin direction has been measured in the decay of polarized N-12. The results are consistent with the Standard Model prediction. In manifest left-right symmetric models this measurement provides a lower bound of 310 GeV/c(2) at 90% CL on the mass of a possible right-handed gauge boson contr ibuting to the electroweak interaction. In generalized versions of this mod el our measurement provides constraints complementary to those set by high- energy experiments. (C) 2001 Elsevier Science B.V. All rights reserved.