Variants of the standard model with electroweak-singlet quarks

The successful description of current data provided by the standard model includes fundamental fermions that are color-singlets and electroweak- nonsinglets, but no fermions that are electroweak-singlets and color-nonsinglets. In an effort to understand the absence of such fermions, we construct and study gedanken models that do contain electroweak-singlet chiral quark fields. These models exhibit several distinctive properties, including the absence of any neutral lepton and the fact that both the (uud) and (ddu) nucleons are electrically charged. We also explore how such models could arise as low-energy limits of grand unified theories and, in this more restrictive context, we show that they exhibit further exotic properties.