Heavy particles and the rho parameter in the standard model

A new formalism is presented for the calculation of the contribution deltarho to rho = Msub(w)2/Msub(z)2cos2thetasub(w) from heavy particles transforming according to arbitrary representations of SU(2) x U(1). A conjecture for the necessary and sufficient conditions that deltarho >= 0 for all values of masses and mixing angles within a particular multiplet is formulated. A number of examples are given (all consistent with the conjecture) and the significance of improved knowledge of rho vis a vis the possible existence of undiscovered heavy particles discussed. (orig.)