bottomonium

Bottomonium is a family of mesons formed by a bottom quark (b) and its antiquark (b̄). It is a heavy quarkonium system studied in quantum chromodynamics (QCD). The spectrum contains a set of bound states, usually labeled by spectroscopic notation nS, nP, etc. The best known members are the Upsilon (Υ) states, which are vector (spin-1) 1S, 2S, 3S, and so on, and the pseudoscalar ground state eta_b. P-wave states χ_bJ(nP) with J = 0, 1, 2 and the singlet h_b(nP) have also been observed. The lowest-lying Υ(1S) has a mass of about 9.46 GeV, with higher excitations extending upward toward 10–11 GeV.

Bottomonium is tightly bound and relatively small in size, with a small velocity of the heavy quarks, which makes nonrelativistic approximations applicable. The system is described by potential models, nonrelativistic QCD (NRQCD), and lattice QCD calculations. The leptonic decays of Υ states provide information about the wavefunction at the origin and enable precision tests of QCD.

Production occurs in high-energy hadron collisions and electron-positron annihilation. Transitions between bottomonium levels proceed via photon emission (radiative decays) or, less frequently, hadronic transitions. The states typically decay through annihilation into gluons or photons.

In heavy-ion collisions, bottomonium suppression patterns are used to probe the quark-gluon plasma and to study sequential melting according to binding energy, with higher excitations melting at lower temperatures.

Bottomonium was first observed in 1977 at Fermilab as the Upsilon family, a key example of heavy quarkonium in QCD.