Editing Meson (section)

=== Flavour quantum numbers ===
{{Main|Flavour (particle physics)#Flavour quantum numbers}}
The [[strangeness]] [[Flavour quantum numbers|quantum number]] ''S'' (not to be confused with spin) was noticed to go up and down along with particle mass. The higher the mass, the lower (more negative) the strangeness (the more s quarks). Particles could be described with isospin projections (related to charge) and strangeness (mass) (see the uds nonet figures). As other quarks were discovered, new quantum numbers were made to have similar description of udc and udb nonets. Because only the u and d mass are similar, this description of particle mass and charge in terms of isospin and flavour quantum numbers only works well for the nonets made of one u, one d and one other quark and breaks down for the other nonets (for example ucb nonet). If the quarks all had the same mass, their behaviour would be called ''symmetric'', because they would all behave in exactly the same way with respect to the strong interaction. However, as quarks do not have the same mass, they do not interact in the same way (exactly like an electron placed in an electric field will accelerate more than a proton placed in the same field because of its lighter mass), and the symmetry is said to be [[broken symmetry|broken]].

It was noted that charge (''Q'') was related to the isospin projection (''I''<sub>3</sub>), the [[baryon number]] (''B'') and flavour quantum numbers (''S'', ''C'', ''{{prime|B}}'', ''T'') by the [[Gell-Mann–Nishijima formula]]:<ref name="S.S.M Wong (1998)">{{cite book |last1=Wong |first1=S.S.M. |year=1998 |chapter=Nucleon Structure |title=Introductory Nuclear Physics |publisher=John Wiley & Sons |isbn=0-471-23973-9 |edition=2nd |location=New York |pages=21–56 }}</ref>
: <math>Q = I_3 + \frac{1}{2}(B + S + C + B^\prime + T),</math>
where ''S'', ''C'', ''{{prime|B}}'', and ''T'' represent the [[strangeness]], [[charm (quantum number)|charm]], [[bottomness]] and [[topness]] flavour quantum numbers respectively. They are related to the number of strange, charm, bottom, and top quarks and antiquark according to the relations:
: <math>\begin{align}
         S &= -(n_\text{s} - n_\bar{\text{s}}) \\
         C &= +(n_\text{c} - n_\bar{\text{c}}) \\
  B^\prime &= -(n_\text{b} - n_\bar{\text{b}}) \\
         T &= +(n_\text{t} - n_\bar{\text{t}}),
\end{align}</math>
meaning that the Gell-Mann–Nishijima formula is equivalent to the expression of charge in terms of quark content:
: <math>Q=\frac{2}{3}[(n_\text{u}-n_\bar{\text{u}})+(n_\text{c}-n_\bar{\text{c}})+(n_\text{t}-n_\bar{\text{t}})]-\frac{1}{3}[(n_\text{d}-n_\bar{\text{d}})+(n_\text{s}-n_\bar{\text{s}})+(n_\text{b}-n_\bar{\text{b}})].</math>