Charm (quantum number)

{{Flavour_quantum_numbers}}

Charm (symbol C) is a flavour quantum number representing the difference between the number of charm quarks ({{SubatomicParticle|charm quark}}) and charm antiquarks ({{SubatomicParticle|Charm antiquark}}) that are present in a particle:{{Cite journal |last1=Appelquist |first1=T. |last2=Barnett |first2=R. M. |last3=Lane |first3=K. |date=December 1978 |title=Charm and Beyond |url=https://s3.cern.ch/inspire-prod-files-8/8a52d73eae423ea1562ffcaf5fa0ae15 |journal=Annual Review of Nuclear and Particle Science |language=en |volume=28 |issue=1 |pages=387–499 |bibcode=1978ARNPS..28..387A |doi=10.1146/annurev.ns.28.120178.002131 |issn=0066-4243}}{{Cite journal |last=Ball |first=Philip |author-link=Philip Ball |date=2021-12-04 |title=The search for charmed states of matter |url=https://academic.oup.com/nsr/article/doi/10.1093/nsr/nwab008/6105227 |journal=National Science Review |language=en |volume=8 |issue=11 |pages=nwab008 |doi=10.1093/nsr/nwab008 |issn=2095-5138 |pmc=8645036 |pmid=34876990}}

:$C\; =\; n\_\backslash text\{c\}\; -\; n\_\{\backslash mathrm\{\backslash bar\{c\}\}\}\backslash$

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 +{{Frac|2|3}}, therefore carries a charm of +1. The charm antiquarks have the opposite charge ({{Nowrap|Q {{=}} −{{Frac|2|3}}}}), and flavour quantum numbers ({{Nowrap|C {{=}} −1}}).PDG (2024), [https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-model.pdf The Quark Model]. "The convention is that the quark flavor ... has

the same sign as its charge Q."

As with any flavour-related quantum numbers, charm is preserved under strong and electromagnetic interaction, but not under weak interaction (see CKM matrix). For first-order weak decays, that is processes involving only one quark decay, charm can only vary by 1 ({{Nowrap|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.Alexander Belyaev [http://www.personal.soton.ac.uk/ab1u06/teaching/phys3002/course/17_weak.pdf Chapter 17 Weak Interactions], PHYS3002: Nuclei and Particles, p. 129