Don ' $t$ stop thinking about leptoquarks: Constructing new models

We discuss the general framework for the construction of new models containing a single fermion number zero scalar leptoquark of mass {approx_equal}200{endash}220thinspGeV which can both satisfy the D0 and CDF search constraints as well as low-energy data, and can lead to both neutral and charged currentlike final states at DESY HERA. The class of models of this kind necessarily contain new vectorlike fermions with masses at the TeV scale which mix with those of the standard model after symmetry breaking. In this paper we classify all models of this type and examine their phenomenological implications as well as their potential embedding into supersymmetric (SUSY) and non-SUSY grand unified theory scenarios. The general coupling parameter space allowed by low-energy as well as collider data for these models is described and requires no fine-tuning of the parameters. {copyright} {ital 1998} {ital The American Physical Society}