Upsilon1S

Upsilon(1S), also written Υ(1S), is the ground state of bottomonium, a heavy quarkonium system composed of a bottom quark and its antiquark (b b̄). It is a vector meson with quantum numbers J^PC = 1^−−. Its mass is about 9.460 GeV/c^2 and it has a very narrow total width, on the order of tens of keV, corresponding to a lifetime around 10^−20 seconds. Because its mass lies below the threshold for decays into open-bottom meson pairs, its decays proceed mainly through strong annihilation into lighter hadrons and, with smaller rates, through electromagnetic channels to lepton pairs.

Upsilon(1S) was discovered in 1977 by the E288 experiment at Fermilab, observed as a sharp resonance in muon pairs near 9.46 GeV in proton–antiproton collisions. It is the lowest member of the Υ(nS) family and serves as a benchmark state for testing quantum chromodynamics (QCD) in the non-relativistic regime. Its properties, such as mass, width, and transition rates, provide information about the bottom quark mass and the interquark potential used in quarkonium models.

Production and decays: Υ(1S) is produced in high-energy electron–positron collisions and in hadron colliders, and it appears in decay cascades from higher bottomonium states. Although below the open-bottom threshold, it participates in studies of QCD through radiative and hadronic transitions to excited states and via cascades that feed down from Υ(2S) and Υ(3S). Its relatively clean experimental signature makes it a key system for calibrating detectors and testing QCD predictions.