We explore the consequences of the assumption that the direct and induced weak neutral currents in an $\mathrm{SU}\mathrm{}\left(2\right)\mathrm{}\otimes \mathrm{U}\mathrm{}\left(1\right)\mathrm{}$ gauge theory conserve all quark flavors naturally, i.e., for all values of the parameters of the theory. This requires that all quarks of a given charge and helicity must have the same values of weak $T_3$ and ${\overrightarrow{T}}^2$. If all quarks have charge +2/3 or -1/3 the only acceptable theories are the "standard" and "pure vector" models, or their generalizations to six or more quarks. In addition, there are severe constraints on the couplings of Higgs bosons, which apparently cannot be satisfied in pure vector models. We also consider the possibility that neutral currents conserve strangeness but not charm. A natural seven-quark model of this sort is described. The experimental consequences of charm nonconservation in direct or induced neutral currents are found to be quite dramatic.

• Received 20 August 1976

DOI:https://doi.org/10.1103/PhysRevD.15.1958