A Note on Proton Stability in the Standard Model

In this short note, we describe the symmetry responsible for absolute, nonperturbative proton stability in the Standard Model. The SM with N-c colors and N-g generations has an exact, anomaly-free, generation-independent, global symmetry group U(1)(B - NcL) x Z(Ng)(L), which contains a subgroup of baryon plus lepton number of order 2N(c)N(g). This disallows proton decay for N-g > 1. Many well-studied models beyond the SM explicitly break this global symmetry, and the alternative deserves further attention.