Quarkonium

In particle physics, quarkonium (from quark and -onium, pl. quarkonia) is a flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle. The name "quarkonium" is analogous to positronium, the bound state of electron and anti-electron. The particles are short-lived due to matter-antimatter annihilation. Vector meson Light 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. Examples of quarkonia are the J/ψ meson (the ground state of charmonium, _Charm quark_Charm antiquark) and the _Upsilon meson (bottomonium, _Bottom quark_Bottom antiquark). Because of the high mass of the top quark, toponium (θ meson) does not exist, since the top quark decays through the electroweak interaction before a bound state can form (a rare example of a weak process proceeding more quickly than a strong process). Usually, the word "quarkonium" refers only to charmonium and bottomonium, and not to any of the lighter quark–antiquark states. J/ψ meson 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 (which, for historical reasons, is called _J/psi 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 envisioned, which are: a charmonium hybrid state a _D0 _AntiD*0 molecule a candidate for the 11D2 state In 2005, the BaBar experiment announced the discovery of a new state: Y(4260).