Heavy and light scalar leptoquarks in proton decay

We list scalar leptoquarks which mediate proton decay via renormalizable couplings to the standard model fermions. We employ a general basis of baryon number violating operators to parametrize contributions of each leptoquark towards proton decay. This then sets the stage for investigation of bounds on the leptoquark couplings to fermions with respect to the most current Super Kamiokande results on proton stability. We quantify if, and when, it is necessary to have leptoquark masses close to a scale of grand unification in the realistic SU(5) and flipped SU(5) frameworks. The most and the least conservative lower bounds on the leptoquark masses are then presented. We furthermore single out a leptoquark without phenomenologically dangerous tree-level exchanges which might explain discrepancy of the forward-backward asymmetries in tt production observed at Tevatron, if relatively light. The same state could also play a significant role in explaining muon anomalous magnetic moment. We identify contributions of this leptoquark to dimension-six operators, mediated through a box diagram, and tree-level dimension-nine operators, which would destabilize the proton if sizable leptoquark and diquark couplings were to be simultaneously present.