Physics:Charm (quantum number)
Flavour in particle physics 

Flavour quantum numbers 

Related quantum numbers 

Combinations 

Flavour mixing 
Charm (symbol C) is a flavour quantum number representing the difference between the number of charm quarks (_{}c_{}) and charm antiquarks (_{}c_{}) that are present in a particle:
 [math]\displaystyle{ C = n_\text{c}  n_{\mathrm{\bar{c}}}\ }[/math]
By convention, the sign of flavour quantum numbers agree with the sign of the electric charge carried by the quarks of corresponding flavour. The charm quark, which carries an electric charge (Q) of +^{2}⁄_{3}, therefore carries a charm of +1. The charm antiquarks have the opposite charge (Q = −^{2}⁄_{3}), and flavour quantum numbers (C = −1).
As with any flavourrelated quantum numbers, charm is preserved under strong and electromagnetic interaction, but not under weak interaction (see CKM matrix). For firstorder weak decays, that is processes involving only one quark decay, charm can only vary by 1 (ΔC= ±1,0). Since firstorder processes are more common than secondorder processes (involving two quark decays), this can be used as an approximate "selection rule" for weak decays.
See also
References
Further reading
 Lessons in Particle Physics Luis Anchordoqui and Francis Halzen, University of Wisconsin, 18th Dec. 2009
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Original source: https://en.wikipedia.org/wiki/Charm (quantum number).
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