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Charm (quantum number)

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Flavour quantum number

Flavour in particle physics
Flavourquantum numbers
Isospin:I orI₃ Charm:C Strangeness:S Topness:T Bottomness:B′
Related quantum numbers
Baryon number:B Lepton number:L Weak isospin:T orT₃ Electric charge:Q X-charge:X
Combinations
Hypercharge:Y Y = (B +S +C +B′ +T) Y = 2 (Q −I₃) Weak hypercharge:Y_W Y_W = 2 (Q −T₃) X + 2Y_W = 5 (B −L)
Flavour mixing
CKM matrix PMNS matrix Flavour complementarity
v t e

Charm (symbolC) is aflavour quantum number representing the difference between the number ofcharm quarks (c) andcharm antiquarks (c) that are present in a particle:^[1]^[2]

C=n_{\text{c}}-n_{\mathrm {\bar {c}} }\

By convention, the sign of flavour quantum numbers agree with the sign of theelectric charge carried by thequarks 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 oppositecharge (Q = −2⁄3), and flavour quantum numbers (C = −1).^[3]

As with any flavour-related quantum numbers, charm is preserved understrong andelectromagnetic interaction, but not underweak interaction (seeCKM matrix). For first-order weak decays, that is processes involving only one quark decay, charm can only vary by 1 (ΔC= ±1,0). Since first-order processes are more common than second-order processes (involving two quark decays), this can be used as an approximate "selection rule" for weak decays.^[4]