CLEO was a general purpose particle detector at the Cornell Electron Storage Ring (CESR), and the name of the collaboration of physicists who operated the detector. The name CLEO is not an acronym; it is short for Cleopatra and was chosen to go with CESR (pronounced Caesar). CESR was a particle accelerator designed to collide electrons and positrons at a center-of-mass energy of approximately 10 GeV. The energy of the accelerator was chosen before the first three bottom quark Upsilon resonances were discovered between 9.4 GeV and 10.4 GeV in 1977. The fourth Υ resonance, the Υ(4S), was slightly above the threshold for, and therefore ideal for the study of, B meson production. CLEO was a hermetic detector that in all of its versions consisted of a tracking system inside a solenoid magnet, a calorimeter, particle identification systems, and a muon detector. The detector underwent five major upgrades over the course of its thirty-year lifetime, both to upgrade the capabilities of the detector and to optimize it for the study of B mesons. The CLEO I detector began collecting data in October 1979, and CLEO-c finished collecting data on March 3, 2008. CLEO initially measured the properties of the Υ(1–3S) resonances below the threshold for producing B mesons. Increasing amounts of accelerator time were spent at the Υ(4S) as the collaboration became more interested in the study of B mesons. Once the CUSB experiment was discontinued in the late 1980s, CLEO then spent most of its time at the Υ(4S) and measured many important properties of the B mesons. While CLEO was studying the B mesons, it was also able to measure the properties of D mesons and tau leptons, and discover many new charm hadrons. When the BaBar and Belle B factories began to collect large amounts of data in the early 2000s, CLEO was no longer able to make competitive measurements of B mesons. CLEO revisited the Υ(1-3S) resonances, then underwent its last upgrade to CLEO-c. CESR ran at lower energies and CLEO measured many properties of the ψ resonances and D mesons. CLEO was the longest running experiment in the history of particle physics. History Proposal and construction Cornell University had built a series of synchrotrons since the 1940s. The 10 GeV synchrotron in operation during the 1970s had conducted a number of experiments, but it ran at much lower energy than the 20 GeV linear accelerator