Natural conservation laws for neutral currents

TLDR

The study investigates the consequences of natural flavor conservation in weak neutral currents and the possibility of conserving strangeness while violating charm. The authors impose equal weak isospin for quarks of identical charge and helicity and construct a natural seven‑quark model to realize these constraints. The analysis shows that only standard or pure‑vector models (or their extensions) satisfy natural flavor conservation, Higgs couplings are severely constrained, and charm nonconservation leads to dramatic experimental signatures.

Abstract

We explore the consequences of the assumption that the direct and induced weak neutral currents in an $\mathrm{SU}(2)\ensuremath{\bigotimes}\mathrm{U}(1)$ 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 ${\stackrel{\ensuremath{\rightarrow}}{T}}^{2}$. 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.