CLEO (particle detector)

The CLEO experiment was a particle detector that operated from 1979 to 2008 at the Cornell Electron Storage Ring (CESR). The experiment was designed to study the exotic and short-lived particles produced when beams of electrons and their antimatter counterparts, positrons, were smashed together at high energies. Functioning like a giant, multi-layered digital camera, the detector recorded the subatomic debris from these collisions. This allowed physicists to study the fundamental building blocks of matter, particularly particles known as mesons and leptons containing bottom quarks, charm quarks, and tau leptons. The name CLEO is not an acronym; it is short for Cleopatra and was chosen to complement the accelerator's name, CESR (pronounced Caesar).

Over its thirty-year lifetime, the CLEO detector and the international collaboration of physicists who ran it made significant contributions to particle physics. The experiment's primary focus was the study of B mesons, particles whose behavior provides key insights into the universe's matter-antimatter imbalance and the weak force. While later, more powerful B-factories like BaBar and Belle eventually surpassed its capabilities in B physics, CLEO remained a highly productive experiment. In its later years, under the name CLEO-c, it shifted focus to high-precision measurements of particles containing charm quarks, providing crucial data for testing theories of the strong force. At the time of its decommissioning, CLEO was one of the longest-running experiments in the history of particle physics.

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 at SLAC. As late as October 1974, Cornell planned to upgrade the synchrotron to reach energies of 25 GeV and build a new synchrotron to reach 40 GeV. After the discovery of the J/Ψ in November 1974 demonstrated that interesting physics could be done with an electron-positron collider, Cornell submitted a proposal in 1975 for an electron-positron collider operating up to center-of-mass energies of 16 GeV using the existing synchrotron tunnel. An accelerator at 16 GeV would explore the energy region between that of the SPEAR accelerator and the PEP and PETRA accelerators. CESR and CLEO were approved in 1977 and mostly finished by 1979. CLEO was built in the large experimental hall at the south end of CESR; a smaller detector named CUSB (for Columbia University-Stony Brook) was built at the north interaction region. Between the proposal for and construction of CESR and CLEO, Fermilab discovered the Υ resonances and suggested that as many as three states existed. The Υ(1S) and Υ(2S) were confirmed at the DORIS accelerator. The first order of business once CESR was running was to find the Υs. CLEO and CUSB found the Υ(1S) shortly after beginning to collect data, and used the mass difference from DORIS to quickly find the Υ(2S). CESR's higher beam energies allowed CLEO and CUSB to find the more massive Υ(3S) and discover the Υ(4S). Furthermore, the presence of an excess of electrons and muons at the Υ(4S) indicated that it decayed to B mesons. CLEO proceeded to publish over sixty papers using the original CLEO I configuration of the detector.

CLEO had competition in the measurement of B mesons, particularly from the ARGUS collaboration. The CLEO collaboration was worried that the ARGUS detector at DESY would be better than CLEO, therefore it began to plan for an upgrade. The improved detector would use a new drift chamber for tracking and dE/dx measurements, a cesium iodide calorimeter inside a new solenoid magnet, time of flight counters, and new muon detectors. The new drift chamber (DR2) had the same outer radius as the original drift chamber to allow it to be installed before the other components were ready.

CLEO collected data for two years in the CLEO I.V configuration: new drift chamber, ten layer vertex detector (VD) inside the drift chamber, three layer straw tube drift chamber insert (IV) inside the VD, and a prototype CsI calorimeter replacing one of the original pole-tip shower detectors. The highlight of the CLEO I.V era was the observation of semi-leptonic B decays to charmless final states, submitted less than three weeks before a similar observation from ARGUS. The shutdown for the installation of DR2 allowed ARGUS to beat CLEO to the observation of B mixing, which was the most cited measurement of any of the symmetric B experiments.

CLEO shut down in April 1988 to begin the remainder of the CLEO II installation, and finished the upgrade in August 1989. A six layer straw chamber precision tracker (PT) replaced the IV, and the time-of-flight detectors, CsI calorimeter, solenoid magnet and iron, and muon chambers were all installed. This would be the CLEO II configuration of the detector. During the CLEO II era, the collaboration observed the flavor changing neutral current decays B^+,0→ K^*+,0 γ and b → s γ. Decays of B mesons to two charmless mesons were also discovered during CLEO II. These decays were of interest because of the possibilility to observe CP violation in decays such as K^±π⁰, although such a measurement would require large amounts of data.

Observation of time-dependent asymmetries in the production of certain flavor-symmetric final states (such as J/Ψ K⁰
_S) was an easier way to detect CP violation in B mesons, both theoretically and experimentally. An asymmetric accelerator, one in which the electrons and positrons had different energies, was necessary to measure the time difference between B⁰ and B⁰ decays. CESR and CLEO submitted a proposal to build a low energy ring in the existing tunnel and upgrade the CLEO II detector with NSF funding. SLAC also submitted a proposal to build a B factory with DOE funds. The initial designs were first reviewed in 1991, but DOE and NSF agreed that insufficient funds were available to build either facility and a decision on which one to build was postponed. The proposals were reconsidered in 1993, this time with both facilities competing for DOE money. In October 1993, it was announced that the B factory would be built at SLAC.

After losing the competition for the B factory, CESR and CLEO proceeded with a two-part plan to upgrade the accelerator and the detector. The first phase was the upgrade to the CLEO II.V configuration between May and October 1995, which included a silicon detector to replace the PT and a change of the gas mixture in the drift chamber from an argon-ethane mix to a helium-propane mix. The silicon detector provided excellent vertex resolution, allowing precise measurements of D⁰, D⁺, D_s and τ lifetimes and D mixing. The drift chamber had better efficiency and momentum resolution.