Search for four-dimensional string models

Within the framework of the spin-structure construction of fermionic string models, we describe a computerized method for generating four-dimensional heterotic models and analyzing their massless spectrum. Thus, starting with the spin structure, we easily identify the low-energy gauge group and all the massless representations, including any U(1) charges. We attempt a systematic and complete generation of all four-dimensional heterotic string models based on world-sheet periodic and antiperiodic complex fermions and compare this with the generation of models from the same category obtained by randomly generating spin structures. We thus explicitly construct over 900 four-dimensional models having N=4 supersymmetry, and over 32 000 models having N=1 supersymmetry. Some phenomenologically interesting examples are presented.