PhDThesis -- Abstract -RPM 08.02.06 - Mention what rho and delta represent. - New measurements by TWIST, using a blind analysis. - Give the new measurements; they're consistent with SM. Factor of two better than the measurements previously published by TWIST, and five times more precise than prior measurements. - Combined with other muon decay data, an indirect limit on PmuXi is obtained... - These results set new model-independent constraints on the possible weak interactions of right-handed particles. Specific implications for left-right symmetric models are discussed.