Charm (quantum number)
Charm is a flavour quantum number representing the difference between the number of charm quarks and charm antiquarks that are present in a particle:
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 of +, therefore carries a charm of +1. The charm antiquarks have the opposite charge, and flavour quantum numbers.
As with any flavour-related quantum numbers, charm is preserved under strong and electromagnetic interaction, but not under weak interaction. For first-order weak decays, that is processes involving only one quark decay, charm can only vary by 1. Since first-order processes are more common than second-order processes, this can be used as an approximate "selection rule" for weak decays.
