Hypercharge

In particle physics, the hypercharge (a portmanteau of hyperonic and charge) Y of a particle is a quantum number conserved under the strong interaction. The concept of hypercharge provides a single charge operator that accounts for properties of isospin, electric charge, and flavour. The hypercharge is useful to classify hadrons; the similarly named weak hypercharge has an analogous role in the electroweak interaction.

Hypercharge is one of two quantum numbers of the SU(3) model of hadrons, alongside isospin I₃. The isospin alone was sufficient for two quark flavours — namely u and d — whereas presently 6 flavours of quarks are known.

SU(3) weight diagrams (see below) are 2 dimensional, with the coordinates referring to two quantum numbers: I₃ (also known as I_z), which is the z component of isospin, and Y, which is the hypercharge (defined by strangeness S, charm C, bottomness B′, topness T′, and baryon number B). Mathematically, hypercharge is

Strong interactions conserve hypercharge (and weak hypercharge), but weak interactions do not.

The Gell-Mann–Nishijima formula relates isospin and electric charge

where I₃ is the third component of isospin and Q is the particle's charge.

Isospin creates multiplets of particles whose average charge is related to the hypercharge by:

since the hypercharge is the same for all members of a multiplet, and the average of the I₃ values is 0.