B − L
In particle physics, B − L (pronounced "bee minus ell") is a quantum number which is the difference between the baryon number (B) and the lepton number (L) of a quantum system. == Details == This quantum number is the charge of a global/gauge U(1) symmetry in some Grand Unified Theory models, called U(1)B−L. Unlike baryon number alone or lepton number alone, this hypothetical symmetry would not be broken by chiral anomalies or gravitational anomalies, as long as this symmetry is global, which is why this symmetry is often invoked.