Determination of the coupling constants in muon decay from experiments

From experiments on the muon decay μ → e+ν+ν, five different parameters (functions of the coupling constants) are known. The measured values of these parameters are in accord with the predictions of the V-A theory, so that the theoretical V-A values can be used to obtain various relationships among the coupling constants. When all the available information is used, the S, P and T terms vanish for the particular ordering chosen in the interaction Hamiltonian. However, only the sum of the absolute values of the two remaining coupling constants, i.e., |gV|2+|gA|2, can be determined. Future experiments which can test the validity of the universal V-A interaction in the above decay are discussed. Further, the effect of the present-day experimental uncertainties on the coupling constants is briefly examined.