A Note on Proton Stability in the Standard Model

Abstract

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