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 Upsilon(4S) is produced in electron-positron collisions at center-of-mass energies near 10.58 GeV, a regime exploited
In summary, Upsilon(4S) is the fourth S-wave bottomonium state, uniquely positioned above the BB̄ threshold with