Charm (symbol ''C'') is a

flavour quantum number In particle physics, flavour or flavor refers to the ''species'' of an elementary particle. The Standard Model counts six flavours of quarks and six flavours of leptons. They are conventionally parameterized with ''flavour quantum numbers'' th ...

representing the difference between the number of

charm quark The charm quark, charmed quark or c quark (from its symbol, c) is the third-most massive of all quarks, a type of elementary particle. Charm quarks are found in hadrons, which are subatomic particles made of quarks. Examples of hadrons containin ...

s () and

charm antiquark The charm quark, charmed quark or c quark (from its symbol, c) is the third-most massive of all quarks, a type of elementary particle. Charm quarks are found in hadrons, which are subatomic particles made of quarks. Examples of hadrons containin ...

s () that are present in a particle: :

C = n_\text - n_\

By convention, the sign of flavour quantum numbers agree with the sign of the

electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...

carried by the quarks of corresponding flavour. The charm quark, which carries an electric charge (''Q'') of +, therefore carries a charm of +1. The charm antiquarks have the opposite

charge Charge or charged may refer to: Arts, entertainment, and media Films * '' Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * ''Charge'' (David Ford album) * ''Charge'' (Machel Montano album) * ''Charge!!'', an album by The Aqu ...

(), and flavour quantum numbers (). As with any flavour-related quantum numbers, charm is preserved under strong and

electromagnetic interaction In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...

, but not under

weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction ...

(see CKM matrix). 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 (involving two quark decays), this can be used as an approximate "

selection rule In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electromagnetic transitions in molecules, in atoms, in ...

" for weak decays.

See also

References

Further reading