Measurement of the Michel Parameter xi" in Polarized Muon Decay and   Implications on Exotic Couplings of the Leptonic Weak Interaction
  The Michel parameter xi" has been determined from a measurement of the longitudinal polarization of positrons emitted in the decay of polarized and depolarized muons. The result, xi" = 0.981 +- 0.045stat +- 0.003syst, is consistent with the Standard Model prediction of unity, and provides an order of magnitude improvement in the relative precision of this parameter. This value sets new constraints on exotic couplings beyond the dominant V-A description of the leptonic weak interaction. 
R. Prieels, O. Naviliat-Cuncic, P. Knowles, P. Van Hove, X. Morelle,   J. Egger, J. Deutsch, J. Govaerts, W. Fetscher, K. Ki...
hep-ex hep-ph nucl-ex