Electric charge quantisation from gauge invariance of a Lagrangian: A   catalogue of baryon number violating scalar interactions
  In gauge theories like the standard model, the electric charges of the fermions can be heavily constrained from the classical structure of the theory and from the cancellation of anomalies. There is however mounting evidence suggesting that these anomaly constraints are not as well motivated as the classical constraints. In light of this we discuss possible modifications of the minimal standard model which will give us complete electric charge quantisation from classical constraints alone. Because these modifications to the standard model involve the consideration of baryon number violating scalar interactions, we present a complete catalogue of the simplest ways to modify the standard model so as to introduce explicit baryon number violation. This has implications for proton decay searches and baryogenesis. 
J. P. Bowes, R. Foot and R. R. Volkas