The charm quark, often written as the c quark, is one of the six fundamental quark flavors in the Standard Model of particle physics. It is a spin-1/2 fermion that carries color charge and participates in the strong, weak and electromagnetic interactions. As an elementary particle it is not known to have any internal structure; see elementary particles for background. The charm quark has electric charge +2/3, the same sign and magnitude as the up quark, and is heavier than the up, down and strange quarks while lighter than the bottom and top quarks.
Basic properties
Like all quarks, the charm quark cannot be observed in isolation because of color confinement; it appears only bound inside hadrons. Its intrinsic spin is 1/2 (it is a fermion), and it carries fractional electric charge (+2/3) as discussed in sources about charge. In strong interactions charm is assigned a conserved quantum number called charm (C), which is conserved by the strong and electromagnetic forces but not necessarily by the weak interaction.
Charmed hadrons and examples
Charm quarks form two broad classes of hadrons: hidden-charm (charmonium) and open-charm states. Examples include:
- Charmonium: bound c c̄ states such as the J/ψ and ψ′ which behave somewhat like a heavy quark analogue of positronium.
- Open-charm mesons: structures made of one charm quark and one lighter antiquark, notably the D and Ds mesons.
- Charmed baryons: three-quark states containing a charm quark, for example the Λc.
These particles are routinely created in high-energy collisions and identified through their decay products. Experimental searches often exploit the finite lifetime of charmed hadrons by reconstructing displaced decay vertices in detectors.
History and discovery
The existence of a fourth quark flavor was anticipated on theoretical grounds—most prominently by the Glashow–Iliopoulos–Maiani (GIM) mechanism—to explain the suppression of certain weak processes. The experimental confirmation came in 1974, when two independent experiments discovered the narrow J/ψ resonance, an event sometimes called the "November Revolution." The simultaneous observations by groups led by Burton Richter and by Samuel C. C. Ting provided strong evidence for a new quark and rapidly reshaped particle physics.
Production, detection and experimental role
Charm quarks are produced in high-energy collisions of particles such as protons or electrons; modern particle accelerators and colliders create copious charmed hadrons. They can also appear in interactions of high-energy cosmic rays with atomic nuclei in the atmosphere or targets made from atoms. Detection relies on tracking charged decay products, vertex separation and invariant mass reconstruction of the parent hadron. Observables from charm decays provide precision tests of the strong force (QCD) and the weak interaction.
Scientific importance and distinctions
Charm physics occupies an intermediate energy scale that is especially useful for studying both perturbative and non-perturbative aspects of Quantum Chromodynamics. Measurements of charm production rates, lifetimes and rare decay modes inform determinations of the elements of the Cabibbo–Kobayashi–Maskawa matrix and studies of CP violation. Distinctive features of the charm quark include its role in forming bound charmonium states and its heavier mass compared with strange and lighter quarks, which affects decay patterns and lifetimes. Many contemporary experiments continue to exploit charm decays to search for physics beyond the Standard Model.
For additional foundations and related topics see entries on particles, experimental methods in particle accelerators, the nature of atoms, and technical descriptions of spin and quantum numbers such as spin.