bbbar

bbbar refers to a bound state of a bottom quark (b) and its antiquark (b̄). As a heavy quark–antiquark system, bb̄ is a central object in the study of quantum chromodynamics (QCD) in the nonrelativistic regime. The bottomonium family forms when the two heavy quarks remain bound as a color-singlet state, and it serves as a clean laboratory for testing QCD-inspired approaches such as potential models, lattice QCD, and nonrelativistic QCD (NRQCD).

The ground state of bottomonium is the Upsilon(1S), with a mass around 9.460 GeV. Excited states include Upsilon(2S) near 10.023 GeV and Upsilon(3S) near 10.355 GeV, along with various P-wave states labeled χbJ(nP). Transitions between bottomonium levels often occur via photon emission, and the states can decay leptonically (for example to electron-positron pairs), providing clear experimental signatures.

In high-energy collisions, bb̄ pairs are produced predominantly by gluon fusion and quark-antiquark annihilation. These pairs may form bound bottomonium states or hadronize into open-bottom hadrons, such as B mesons and bottom baryons. Studying bb̄ production rates, decay patterns, and spectroscopy across different energies helps refine models of quark confinement, calibrate lattice calculations, and improve determinations of fundamental parameters such as the bottom-quark mass and the strong coupling constant.

Overall, bbbar systems, especially bottomonium, occupy a key place in hadron physics by bridging experimental measurements and theoretical approaches to the strong interaction.