Also found in: Dictionary, Thesaurus, Medical.
A family of elementary particles whose first three members were discovered in 1977. The upsilon mesons, &Ugr;, are the heaviest known vector mesons, with masses greater than 10 times that of the proton. They are bound states of a heavy quark and its antiquark. The quarks which bind to form the upsilons carry a new quantum number called beauty or bottomness, and they are called b-quarks or b. The mass of the b-quark is around 5 GeV. The anti-b-quark or b carries antibeauty, and therefore the upsilons carry no beauty and are often called hidden beauty states. Direct proof of the existence of the b-quark was obtained by observing the existence of B mesons which consist of a b-quark bound to a lighter quark. Thirteen bb-mesons have also been observed thus far. See Elementary particle, Meson, Quarks