charm

(particle physics) A quantum number which has been proposed to account for an apparent lack of symmetry in the behavior of hadrons relative to that of leptons, to explain why certain reactions of elementary particles do not occur, and to account for the longevity of the J-1 and J-2 particles.

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A term used to describe a class of elementary particles. Ordinary atoms of matter consist of a nucleus composed of neutrons and protons and surrounded by electrons. Over the years, however, a host of other particles with unexpected properties have been found, associated with both electrons (leptons) and protons (hadrons). The hadrons number in the hundreds, and can be explained as composites of more fundamental constituents, called quarks. The originally simple situation of having an up quark (u) and a down quark (d) has evolved as several more varieties or flavors have had to be added. These are the strange quark (s) with the additional property or quantum number of strangeness to account for the unexpected characteristics of a family of strange particles; the charm quark (c) possessing charm and no strangeness, to explain the discovery of the J/ψ particles, massive states three times heavier than the proton; and a fifth quark (b) to explain the existence of the even more massive upsilon (γ) particles. See also Hadron; Quarks.

The members of the family of particles associated with charm fall into two classes: those with hidden charm, where the states are a combination of charm and anticharm quarks (cc), charmonium; and those where the charm property is clearly evident, such as the D⁺ (cd) meson and Λ_c⁺ (cud) baryon. Although reasonable progress has been made in the study of charmed states, much work remains to be done. See also Elementary particle.