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In particle physics, quarkonium (from quark + onium, pl. quarkonia) designates a flavorless meson whose constituents are a quark and its own antiquark. Examples of quarkonia are the J/ψ meson (an example of charmonium, ) and the (bottomonium, ). Because of the high mass of the top quark, toponium does not exist, since the top quark decays through the electroweak interaction before a bound state can form. Usually quarkonium refers only to charmonium and bottomonium, and not to any of the lighter quark–antiquark states. This usage is because the lighter quarks (up, down, and strange) are much less massive than the heavier quarks, and so the physical states actually seen in experiments (η, η′, and π0 mesons) are quantum mechanical mixtures of the light quark states. The much larger mass differences between the charm and bottom quarks and the lighter quarks results in states that are well defined in terms of a quark–antiquark pair of a given flavor. ==Charmonium states== In the following table, the same particle can be named with the spectroscopic notation or with its mass. In some cases excitation series are used: Ψ' is the first excitation of Ψ (for historical reasons, this one is called ''J/ψ'' particle); Ψ" is a second excitation, and so on. That is, names in the same cell are synonymous. Some of the states are predicted, but have not been identified; others are unconfirmed. The quantum numbers of the X(3872) particle have been measured recently by the LHCb experiment at CERN〔 〕 . This measurement shed some light on its identity, excluding the third option among the three envised, which are : * a candidate for the 11D2 state; * a charmonium hybrid state; * a molecule. In 2005, the BaBar experiment announced the discovery of a new state: Y(4260).〔 〕〔 〕 CLEO and Belle have since corroborated these observations. At first, Y(4260) was thought to be a charmonium state, but the evidence suggests more exotic explanations, such as a D "molecule", a 4-quark construct, or a hybrid meson. Notes: : * Needs confirmation. :† Predicted, but not yet identified. :† Interpretation as a 1−− charmonium state not favored. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Quarkonium」の詳細全文を読む スポンサード リンク
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