Upsilon4S

Upsilon(4S) is a bottomonium state, a bound system of a bottom quark and its antiquark (b b̄) in a radially excited S-wave configuration. It is a vector meson with quantum numbers J^P C = 1^−− and is the fourth S-state (n = 4) in the Upsilon family. Its mass is about 10.58 GeV/c^2, placing it just above the threshold for producing pairs of B mesons. Its natural width is relatively narrow, on the order of a few tens of MeV.

Because its mass lies above the open-bottom threshold, the Upsilon(4S) decays predominantly into B meson pairs. The dominant decay mode is into BB̄, including B0 B̄0 and B+ B− final states, often with one or both B mesons in an excited B* state that subsequently decays to a ground-state B by emitting a photon. Other hadronic and radiative decays to lighter quarkonia or multi-particle final states are rare in comparison.

The Upsilon(4S) is produced in electron-positron collisions at center-of-mass energies near 10.58 GeV, a regime exploited by B-factory experiments such as BaBar and Belle (and their successors, including Belle II) to study B meson properties, CP violation, and the CKM matrix. Its clean BB̄ production environment provides abundant B-meson samples and well-controlled backgrounds, enabling precision measurements in heavy-flavor physics.

In summary, Upsilon(4S) is the fourth S-wave bottomonium state, uniquely positioned above the BB̄ threshold with a key role in contemporary B-physics experiments.