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Flavour in particle physics
Flavour quantum numbers:

Related quantum numbers:


Flavour mixing

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Flavour in particle physics
Flavour quantum numbers:

Related quantum numbers:


Flavour mixing

In particle physics, the hypercharge Y of a particle is related to the strong interaction, and is distinct from the similarly named weak hypercharge, which has an analogous role in the electroweak interaction. The concept of hypercharge combines and unifies isospin and flavour into a single charge.


Hypercharge in particle physics is a quantum number relating the strong interactions of the SU(3) model. Isospin is defined in the SU(2) model while the SU(3) model defines hypercharge.

SU(3) weight diagrams (see below) are 2-dimensional with the coordinates referring to two quantum numbers, Iz, which is the z-component of isospin and Y, which is the hypercharge (the sum of strangeness (S), charm (C), bottomness (B′), topness (T), and baryon number (B)). Mathematically, hypercharge is

Y =  S+C+B^\prime+T+B

and conservation of hypercharge implies a conservation of flavour. Strong interactions conserve hypercharge, but weak interactions do not.

Relation with Electric charge and Isospin[edit]

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

 Q = I_3 + \frac{1}{2}Y,

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:

 Y = 2 \bar Q.

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

SU(3) model in relation to hypercharge[edit]

The SU(2) model has multiplets characterized by a quantum number J, which is the total angular momentum. Each multiplet consists of 2J + 1 substates with equally spaced values of Jz, forming a symmetric arrangement seen in atomic spectra and isospin. This formalises the observation that certain strong baryon decays were not observed, leading to the prediction of the mass, strangeness and charge of the Ω baryon.

The SU(3) has supermultiplets containing SU(2) multiplets. SU(3) now needs 2 numbers to specify all its sub-states which are denoted by λ1 and λ2.

(λ1 + 1) specifies the number of points in the topmost side of the hexagon while (λ2 + 1) specifies the number of points on the bottom side.

SU(3) weight diagram, where Y is Hypercharge and I3 is the third component of isospin
SU(3) weight diagram
Note similarity with both charts on the right.
Mesons of spin 0 form a nonet, K = kaon, π = pion, η = eta meson
The octet of light spin-1/2 baryons described in SU(3). n = neutron, p = proton, Λ = Lambda baryon, Σ = Sigma baryon, Ξ = Xi baryon
Note similarity with chart on the right.
A combination of three u, d or s-quarks with a total spin of 3/2 form the so-called baryon decuplet. The lower six are hyperons. S = strangeness, Q = electric charge


Practical obsolescence[edit]

Hypercharge was a concept developed in the 1960s, to organize groups of particles in the "particle zoo" and to develop ad hoc conservation laws based on their observed transformations. With the advent of the quark model, it is now obvious that (if one only includes the up, down and strange quarks out of the total 6 quarks in the Standard Model), hypercharge Y is the following combination of the numbers of up (nu), down (nd), and strange quarks(ns):

 Y = {1 \over 3} (n_u + n_d - 2 n_s).

In modern descriptions of hadron interaction, it has become more obvious to draw Feynman diagrams that trace through individual quarks composing the interacting baryons and mesons, rather than counting hypercharge quantum numbers. Weak hypercharge, however, remains of practical use in various theories of the electroweak interaction.