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.Berkelman (2004) pp. 136-146 The name CLEO is not an acronym; it is short for Cleopatra and was chosen to complement the accelerator's name, CESR (pronounced Caesar).Berkelman (2004) p. 24
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,Berkelman (2004) p. 116 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.Berkelman (2004) p. 96
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 at SLAC.Berkelman (2004) p. 13 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.AIP Study p. 104 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.Berkelman (2004) p. 19 CESR and CLEO were approved in 1977Berkelman (2004) p. 26 and mostly finished by 1979.Berkelman (2004) p. 28 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.Berkelman (2004) pp. 134-146
CLEO had competition in the measurement of B mesons, particularly from the ARGUS collaboration.Berkelman (2004) p. 56 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.Berkelman (2004) p. 57 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 II CLEO shut down in April 1988 to begin the remainder of the CLEO II installation, and finished the upgrade in August 1989.Berkelman (2004) p. 66 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 K) 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 0 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.Berkelman (2004) p. 82-85
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.Berkelman (2004) p. 93 The silicon detector provided excellent vertex resolution, allowing precise measurements of D0, D+, Ds and τ lifetimes and D mixing. The drift chamber had better efficiency and momentum resolution.
CLEO III The second phase of the upgrade included new superconducting quadrupoles near the detector. The VD and DR2 detectors would need to be replaced to make room for the quadrupole magnets. A new silicon detector and particle identification chamber would also be included in the CLEO-III configuration.
The CLEO III upgrade replaced the drift chamber and silicon detector and added a ring-imaging Cherenkov (RICH) detector for enhanced particle identification. The CLEO III drift chamber (DR3) achieved the same momentum resolution as the CLEO II.V drift chamber, despite having a shorter lever arm to accommodate the RICH detector. The mass of the CLEO III endplates was also reduced to allow better resolution in the endcap calorimeters.Yellow Book p. 155
CLEO II.V had stopped collecting data in February 1999. The RICH detector was installed beginning in June 1999, and DR3 was installed immediately afterwards. The silicon detector was to be installed next, but it was still being built. An engineering run was taken until the silicon detector was ready for installation in February 2000.Berkelman (2004) p. 100 CLEO III collected 6 fb−1 of data at the Υ(4S) and another 2 fb−1 below the Υ(4S).
With the advent of the high luminosity BaBar and Belle experiments, CLEO could no longer make competitive measurements of most of the properties of the B mesons. CLEO decided to study the various bottom and charm quarkonia states and charm mesons. The program began by revisiting the Υ states below the B meson threshold and the last data collected with the CLEO-III detector was at the Υ(1-3S) resonances.
CLEO-c CLEO-c was the final version of the detector, and it was optimized for taking data at the reduced beam energies needed for studies of the charm quark. It replaced the CLEO III silicon detector, which suffered from lower-than-expected efficiency, with a six layer, all stereo drift chamber (ZD). CLEO-c also operated with the solenoid magnet at a reduced magnetic field of 1 T to improve the detection of low momentum charged particles. The low particle multiplicities at these energies allowed efficient reconstruction of D mesons. CLEO-c measured properties of the D mesons that served as inputs to the measurements made by the B factories. It also measured many of the quarkonia states that helped verify lattice QCD calculations.
Detector CLEO's subdetectors perform three main tasks: tracking of charged particles, calorimetry of neutral particles and electrons, and identification of charged particle type.
Tracking CLEO has always used a solenoid magnet to allow the measurement of charged particles. The original CLEO design called for a superconducting solenoid, but it was clear that one could not be built in time. A conventional 0.42 T solenoid was installed first, then replaced by the superconducting magnet in September 1981. The superconducting coil was designed to operate at 1.2 T, but it was never operated above 1.0 T. A new magnet was built for the CLEO II upgrade and was placed between the calorimeter and the muon detector. It operated at 1.5 T until CLEO-c, when the magnetic field was reduced to 1.0 T.
Wire chambers The original CLEO detector used three separate tracking chambers. The innermost chamber (IZ) was a three layer proportional wire chamber that occupied the region between a radius of 9 cm and 17 cm. Each layer had 240 anode wires to measure track azimuth and 144 cathode strip hoops 5 mm wide inside and outside the anode wires (864 cathode strips total) to measure track z.CLEO I NIM p. 53
The CLEO I drift chamber (DR) was immediately outside the IZ and occupied the region between a radius of 17.3 cm and 95 cm. It consisted of seventeen layers of 11.3 mm × 10.0 mm cells with 42.5 mm between the layers, for a total of 5304 cells. There were two layers of field wires for every layer of sense wires. The odd-numbered layers were axial layers, and the even-numbered layers were alternating stereo layers.CLEO I NIM p. 51
The last CLEO I dedicated tracking chamber was the planar outer Z drift chamber (OZ) between the solenoid magnet and the dE/dx chambers. It consisted of three layers separated radially by 2.5 cm. The innermost layer was perpendicular to the beamline, and the outer two layers were at ±10° relative to the innermost chamber to provide some azimuthal tracking information. Each octant was equipped with an OZ chamber.CLEO I NIM p. 67