Quarkonium
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.
) pairs, are crucial probes for studying the deconfined quark-gluon plasma created in ultra-relativistic heavy-ion collisions.
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
Due to the high mass of the top quark, direct observation of toponium (
) is exceedingly challenging as the top quark decays through the electroweak interaction before a bound state can form.
melting at higher temperatures compared to loosely bound states such as
The quantum numbers of the X(3872) particle have been measured recently by the LHCb experiment at CERN.
[3] 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).
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: In the following table, the same particle can be named with the spectroscopic notation or with its mass.
On 21 December 2011, the χb2(3P) state was the first particle discovered in the Large Hadron Collider; the discovery article was first posted on arXiv.
have been unsuccessful, The rapid decay of the top quark and the large spread in beam energy present significant experimental challenges.
[14][15] Despite this, searches continue through indirect methods, such as detecting specific decay products or anomalies indicating top quark pairs.
Studying toponium decays offers a promising approach to search for Higgs particles with masses up to around 70 GeV, while similar searches in bottomonium decays could extend this range to 160 GeV.
Additionally, studying gluon decay widths in light quarkonia can help determine the quantum chromodynamics (QCD) scale parameter.
[16] The computation of the properties of mesons in quantum chromodynamics (QCD) is a fully non-perturbative one.
As a result, the only general method available is a direct computation using lattice QCD (LQCD) techniques.
However, the speed of the charm and the bottom quarks in their respective quarkonia is sufficiently small for relativistic effects in these states to be much reduced.
NRQCD has also been quantized as a lattice gauge theory, which provides another technique for LQCD calculations to use.
Good agreement with the bottomonium masses has been found, and this provides one of the best non-perturbative tests of LQCD.
For charmonium masses the agreement is not as good, but the LQCD community is actively working on improving their techniques.
In this technique, one uses the fact that the motion of the quarks that comprise the quarkonium state is non-relativistic to assume that they move in a static potential, much like non-relativistic models of the hydrogen atom.
form is identical to the well-known Coulombic potential induced by the electromagnetic force.
, is known as the confinement part of the potential, and parameterizes the poorly understood non-perturbative effects of QCD.
Generally, when using this approach, a convenient form for the wave function of the quarks is taken, and then
are determined by fitting the results of the calculations to the masses of well-measured quarkonium states.
Relativistic and other effects can be incorporated into this approach by adding extra terms to the potential, much as is done for the model hydrogen atom in non-relativistic quantum mechanics.
[18] It is popular because it allows for accurate predictions of quarkonium parameters without a lengthy lattice computation, and provides a separation between the short-distance Coulombic effects and the long-distance confinement effects that can be useful in understanding the quark / anti-quark force generated by QCD.
Quarkonia have been suggested as a diagnostic tool of the formation of the quark–gluon plasma: Both disappearance and enhancement of their formation depending on the yield of heavy quarks in plasma can occur.
