CLEO (particle detector)

Particle detector at Cornell University

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.

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. 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

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. The highlight of the CLEO I. V era was the observation of semi leptonic B decays to charmless final states,

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 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. 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+, Ds and τ lifetimes and D mixing. The drift chamber had better efficiency and momentum resolution.

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. 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. 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 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.

CLEO's subdetectors perform three main tasks: tracking of charged particles, calorimetry of neutral particles and electrons, and identification of charged particle type.

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.

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. 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. 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. A new drift chamber, DR2, was built to replace the original drift chamber. The new drift chamber had the same outer radius as the original one so that it could be installed before the rest of the CLEO II upgrades were ready. DR2 was a 51 layer detector, with a 000+000 axial/stereo layer arrangement. DR2 had only one layer of field wires between each layer of sense wires, allowing many more layers to fit in the allotted space. The axial sense wires had a half cell stagger to help resolve the left right ambiguity of the original drift chamber. The inner and outer field layers of the chamber were cathode strips to make measurements of the longitudinal coordinate of tracks. DR2 was also designed to make dE/dx measurements in addition to tracking measurements. The IZ chamber was replaced with a ten layer drift chamber (VD) in 1984. When the beampipe radius was reduced from 7.5  to 5.0 cm in 1986, a three layer straw chamber (IV) was built to occupy the newly available space. The IV was replaced during the CLEO II upgrade with a five layer straw tube with a 3.5 cm inner radius. The CLEO III drift chamber (DR3) was designed to have similar performance as the CLEO II/II. V drift chamber even though it would be smaller to allow space for the RICH detector. The innermost sixteen layers were axial, and the outermost 31 layers were grouped in alternating stereo four layer superlayers. The outer wall of the drift chamber was instrumented with 1 cm wide cathode pads to provide additional z measurements.

CLEO built its first silicon vertex detector for the CLEO II. V upgrade. The silicon detector was a three layer device, arranged in octants. The innermost layer was at a radius of 2.4 cm and the outermost layer was at a radius of 4.7 cm. A total of 96 silicon wafers were used, with a total of 26208 readout channels. The CLEO III upgrade included a new four layer, double sided silicon vertex detector. It was made of 447 identical 1 in × 2 in wafers with a 50 micrometre strip pitch on the r φ side and a 100 micrometre pitch on the z side. The silicon detector achieved 85% efficiency after installation, but soon began to suffer increasingly large inefficiencies. The inefficiencies were found in roughly semi circular regions on the wafers. The silicon detector was replaced for CLEO c because of its poor performance, the reduced need for vertexing capabilities, and the desire to minimize the material near the beampipe.

CLEO I had three separate calorimeters. All used layers of proportional tubes interleaved with sheets of lead. The octant shower detectors were outside the time of flight detectors in each of the octants. Each octant detector had 44 layers of proportional tubes, alternating parallel and perpendicular to the beampipe. Wires were ganged together to reduce the number of readout channels for a total of 774 gangs. The octant end shower detectors were sixteen layer devices placed at either end of the dE/dx chambers. The layers followed an azimuthal, positive stereo, azimuthal, negative stereo pattern. The stereo wires were parallel to the slanted sides of the detector. The layers were ganged in a similar fashion as the octant shower detectors. The pole tip shower detector was placed between the ends of the drift chamber and the pole tips of the magnet flux return. The pole tip shower detector had 21 layers, with seven groups of vertical, +120°, 120° layers. The shower detector on each side was built in two halves to allow access to the beampipe. The calorimetry was significantly improved during the CLEO II upgrade. The new electromagnetic calorimeter used 7784 CsI crystals doped with thallium. Each crystal was roughly 30 cm deep and had a 5 cm × 5 cm face. The central region of the calorimeter was a cylinder placed between the drift chamber and the solenoid magnet, and two endcap calorimeters were placed at either end of the drift chamber. The crystals in the endcap were oriented parallel to the beam line. The crystals in the central calorimeter faced a point displaced from the interaction point both longitudinally and transversely by a few centimeters to avoid inefficiencies from particles passing between neighboring crystals. The calorimeter primarily measured the energy of photons or electrons, however it was also used to detect antineutrons. All versions of the detector from CLEO II through CLEO c used the CsI calorimeter.

Five types of long lived, charged particles are produced at CLEO: electrons, pions, muons, kaons and protons. Proper identification of each of these types significantly improves the capabilities of the detector. Particle identification was done by both dedicated subdetectors and by the calorimeter and drift chamber. The outer portion of the CLEO detector was divided into independent octants that were primarily dedicated to charged particle identification. No clear consensus was reached on the choice of technology for particle identification, therefore two octants were equipped with dE/dx ionization chambers, two octants were equipped with high pressure gas Cerenkov detectors, and four octants were equipped with low pressure gas Cerenkov detectors. The dE/dx system demonstrated superior particle identification performance and aided in tracking, therefore in September 1981 all eight octants were equipped with dE/dx chambers. The dE/dx chambers measured the ionization of charged particles as they passed through a multiwire proportional chamber (MWPC). The CLEO I muon drift chambers were the outermost detectors. Two layers of muon detectors were outside the magnet iron on either end of CLEO. The barrel region had two additional layers of muon chambers after 15 cm and 30 cm of magnet iron. The muon detectors were between 4 and 10 radiation lengths deep and were sensitive to muons with energies of at least 1 2 GeV. The magnet yoke weighed 580 tons, and each of four movable carts at each corner of the detector weighed 240 tons, for a total of 1540 tons. CLEO II used time of flight detectors between the drift chamber and the calorimeter, one in the barrel region, the other in the endcap region. The barrel region consisted of 64 Bicron bars with light guides leading to photomultiplier tubes outside the magnetic field region. A similar system covered the endcap region. The TOF system had a timing resolution of 150 cm. The central and endcap TOF detectors combined covered 97% of the solid angle.

CLEO has published over 200 articles in Physical Review Letters and more than 180 articles in Physical Review. The reports of inclusive B physics was usually CLEO's top priority, but the collaboration has made measurements across a wide spectrum of particle physics topics.

CLEO's most cited paper reported the first measurement of the flavor changing neutral current decay b→sγ. The measurement agreed well with the Standard Model and placed significant constraints on numerous beyond the Standard Model proposals, such as charged Higgs and anomalous WWγ couplings. The analogous exclusive decay B+,0→ K*+,0 γ was also measured. CLEO and ARGUS reported nearly simultaneous measurements of inclusive charmless semileptonic B meson decays, which directly established a non zero value of the CKM matrix element |Vub|. Exclusive charmless semileptonic B meson decays were first observed by CLEO six years later in the modes B → πlν, ρlν, and were used to determine |Vub|. CLEO also discovered many of the hadronic analogs: B+,0→ K(892)+π−, φ K(*), K+π0, K0π0, π+π−, π+ρ0, π+ρ−, π+ω η K*, η′ K and K0π+, K+π−. 0→ D*0π0 B→ Λπ−, Λπ+π−, 0→ D*0π+π+π−π−, 0→ D*ρ′−, B0→ D*−pπ+, D*−p, B0→ D*+D*−, and B+→ 0 K+.

Although CLEO ran mainly near the Υ(4S) to study B mesons, it was also competitive with experiments designed to study charm hadrons. The first measurement of charm hadron properties by CLEO was the observation of the Ds. CLEO measured a mass of 1970±7 MeV, considerably lower than previous observations at 2030±60 MeV and 2020±10 MeV. CLEO discovered the DsJ(2573) and the DsJ(2463). CLEO was the first experiment to measure the doubly Cabibbo suppressed decay D0→ K+π−, and CLEO performed Dalitz analyses of D0,+ in several decay modes. CLEO studied the D*(2010)+, making the first measurement of its width and the most precise measurement of the D* D0 mass difference. CLEO c made many of the most accurate measurements of D meson branching ratios in inclusive channels, μ+νμ, semileptonic decays, and hadronic decays. These branching fractions are important inputs to B meson measurements at BaBar and Belle. CLEO first observed the purely leptonic decay D→μ+, which provided an experimental measure of the decay constant fDs. CLEO c made the most precise measurements of fD+ Other D decay modes discovered by CLEO are p, ωπ+, η ρ+, η'ρ+, φρ+, η π+, η'π+, and φ l ν. CLEO discovered many charmed baryons and discovered or improved the measurement of many charmed baryon decay modes. Before BaBar and Belle began discovering new charm baryons in 2005, CLEO had discovered thirteen of the twenty known charm baryons: Ξ, Ξ(2790), Ξ(2815), Ξ, Σ(2520), Ξ(2645), Ξ(2645), and Λ(2593). Charmed baryon decay modes discovered at CLEO are Ω→ Ω−e+e; Λ→ p0η, Ληπ+, Σ+η, Σ*+η, Λ0K+, Σ+π0, Σ+ω, Λπ+π+π−π0, Λωπ+; and Ξ→Ξ0e+ e.

Quarkonium states provide experimental input for lattice QCD and non relativistic QCD calculations. CLEO studied the Υ system until the end of the CUSB and CUSB II experiments, then returned to the Υ system with the CLEO III detector. CLEO c studied the lower mass ψ states. CLEO and CUSB published their first papers back to back, reporting observation of the first three Υ states. Earlier claims of the Υ(3S) relied on fits of one peak with three components; CLEO and CUSB's observation of three well separated peaks dispelled any remaining doubt about the existence of the Υ(3S). The Υ(4S) was discovered shortly after by CLEO and was interpreted as decaying to B mesons because of its large decay width. An excess of electrons and muons at the Υ(4S) demonstrated the existence of weak decays and confirmed the interpretation of the Υ(4S) decaying to B mesons. CLEO and CUSB later reported the existence of the Υ(5S) and Υ(6S) states. CLEO I through CLEO II had significant competition in Υ physics, primarily from the CUSB, Crystal Ball and ARGUS experiments. CLEO was able, however, to observe a number of Υ(1S) decays: τ+τ−, J/Ψ X and γ X with X = π+, π0, 2π+, π+K+, π+p, 2K+, 3π+, 2π+K+, and 2π+p. The radiative decays are sensitive to the production of glueballs. CLEO collected more data at the Υ(1 3S) resonances at the end of the CLEO III era. CLEO III discovered the Υ(1D) state, the χb1,2(2P)→ωΥ(1S) transitions, and Υ(3S)→τ+τ− decays among others. CLEO c measured many of the properties of the charmonium states. Highlights include confirmation of ηc', confirmation of Y(4260), pseudoscalar vector decays of ψ(2S), ψ(2S)→J/ψ decays, observation of thirteen new hadronic decays of ψ(2S), observation of hc(1P1), and measurement of the mass and branching fractions of η in ψ(2S)→J/ψ decay.

CLEO has studied two photon physics, where both an electron and positron radiate a photon. The two photons interact to produce either a vector meson or hadron antihadron pairs. CLEO published measurements of both the vector meson process and the hadron antihadron process. CLEO performed an energy scan for center of mass energies between 7 GeV and 10 GeV to measure the hadronic cross section ratio. CLEO made the first measurements of the π+ and K+ electromagnetic form factors above Q2 > 4 GeV2. Finally, CLEO has performed searches for Higgs and beyond SM particles: Higgs bosons, axions, magnetic monopoles, neutralinos, fractionally charged particles, bottom squarks, and familons.

Initial design of a detector for the south interaction region of CESR began in 1975. Physicists from Harvard University, Syracuse University and the University of Rochester had worked at the Cornell synchrotron, and were natural choices as collaborators with Cornell. They were joined by groups from Rutgers University and Vanderbilt University, along with collaborators from LeMoyne College and Ithaca College. Additional institutions were assigned responsibility for detector components as they joined the collaboration. Cornell appointed a physicist to oversee development of the portion of the detector inside the magnet, outside the magnet, and of the magnet itself. The structure of the collaboration was designed to avoid perceived shortcomings at SLAC, where SLAC physicists were felt to dominate operations by virtue of their access to the accelerator and detector and to computing and machine facilities. Collaborators were free to work on the analysis of their choosing, and the approval of results for publication was by collaboration wide vote. The spokesperson (later spokespeople) were also selected by collaboration wide vote, including graduate students. The other officers in the collaboration were an analysis coordinator and a run manager, then later also a software coordinator. The first CLEO paper listed 73 authors from eight institutions. Cornell University, Syracuse University and the University of Rochester have been members of CLEO for its entire history, and forty two institutions have been members of CLEO at one time. The collaboration was its largest in 1996 at 212 members, before collaborators began to move to the BaBar and Belle experiments. The largest number of authors to appear on a CLEO paper was 226. A paper published near the time CLEO stopped taking data had 123 authors.