The original form of the Gell-Mann–Nishijima formula is:
This equation was originally based on empirical experiments. It is now understood as a result of the quark model. In particular, the electric chargeQ of a quark or hadron particle is related to its isospinI3 and its hyperchargeY via the relation:
Since the discovery of charm, top, and bottom quark flavors, this formula has been generalized. It now takes the form:
Expressed in terms of quark content, these would become:
By convention, the flavor quantum numbers (strangeness, charm, bottomness, and topness) carry the same sign as the electric charge of the particle. So, since the strange and bottom quarks have a negative charge, they have flavor quantum numbers equal to −1. And since the charm and top quarks have positive electric charge, their flavor quantum numbers are +1.
From a quantum chromodynamics point of view, the Gell-Mann–Nishijima formula and its generalized version can be derived using an approximate SU(3)flavour symmetry because the charges can be defined using the corresponding conserved Noether currents.
In 1961 Glashow proposed a relation similar formula would also apply to the weak interaction:[4][5]: 152
Here the charge is related to the projection of weak isospin and the hypercharge.
^Greiner, Walter; Müller, Berndt; Greiner, Walter (1996). Gauge theory of weak interactions (2 ed.). Berlin Heidelberg New York Barcelona Budapest Hong Kong London Milan Paris Santa Clara Singapore Tokyo: Springer. ISBN978-3-540-60227-9.