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.

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.

Quarkonia, bound state of charmonium (${\displaystyle c{\bar {c}}}$) and bottomonium (${\displaystyle b{\bar {b}}}$) pairs, are crucial probes for studying the deconfined quark-gluon plasma created in ultra-relativistic heavy-ion collisions. The ${\displaystyle \psi }$ and ${\displaystyle \Upsilon }$ families provide direct evidence of the quark structure of hadrons, support the quark-gluon picture of perturbative quantum chromodynamics (QCO), and help determine the QCD scale parameter ${\displaystyle \Lambda }$. The dissociation temperature of quarkonium states depends on their binding energy, with strongly bound states like ${\displaystyle J/\psi }$ and ${\displaystyle \Upsilon (1S)}$ melting at higher temperatures compared to loosely bound states such as ${\displaystyle \psi (2S)}$, ${\displaystyle \chi _{c}}$ for the charmonium family, and ${\displaystyle \Upsilon (2S)}$, ${\displaystyle \Upsilon (3S)}$ for bottomonia. This sequential dissociation process enables the use of quarkonium dissociation probabilities to estimate the medium temperature, assuming quarkonium dissociation is the primary mechanism involved.

Due to the high mass top quarks decay through the electroweak interaction before a bound state can form. However, near the pair production threshold, a pseudo-bound state emerges, leading to an enhancement that resembles a resonance peak. This pseudo-bound state is sometimes interpreted as toponium.

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/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^[when?] by the LHCb experiment at CERN. This measurement shed some light on its identity, excluding the third option among the three envisioned, which are:

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: