Cornell University in New York managed for a long time to keep pace with major national laboratories. Experiments at Cornell provided many important contributions to the physics of B-mesons – particles containing the fifth “b” quark. With the commissioning of new “B- factories”, Cornell’s physics is set to change direction.
The Cornell electron-positron storage ring (CESR) and the associated particle physics detector, CLEO, completed their latest very successful physics run in June. Running at collision energies on or near the Y(4S) resonance at 10.6 GeV (a bound state of the fifth “b” quark and its antiquark), the accelerator achieved its highest-ever luminosity (a measure of the particle collision rate) of 1.3×1033cm-2s-1. This figure has been surpassed by the new B-factories at SLAC, Stanford, and KEK, Japan, but for a long time CESR held the world record for electron-positron collision rate.
To accomplish this, CESR stored a total beam current of 740 mA, with each beam having nine trains of particles and five bunches per train. Crucial for obtaining these high currents was the use of superconducting radiofrequency cavities to provide power to the beams. Beginning in June 2000, CESR produced a total integrated luminosity of 13.3 fb-1for the run. At its best, the accelerator was delivering 1.5 fb-1/month – twice that of any previous run.
In anticipation of the run, the CLEO detector (now designated CLEO III) had undergone extensive modifications. A completely new 47-layer central drift chamber was installed. This chamber has endplates with a stepped “wedding cake” profile to allow for the eventual insertion of superconducting quadrupole magnets close to the interaction region and an outer radius smaller than the previous CLEO drift chamber to allow room for a particle-identification detector.
This latter detector is a ring-imaging Cherenkov counter (RICH) consisting of a solid 1 cm thick lithium fluoride radiator, followed by a 15.7 cm expansion space to allow the Cherenkov cone to enlarge, and then a thin-gap multiwire proportional chamber filled with a mixture of TEA and methane gas as the photodetector. By detecting on average 12 photoelectrons from the Cherenkov ring of each charged particle, the RICH allows the identification of pions and kaons with an efficiency of roughly 85% and a fake rate of less than 1% for a momentum of below 2.0 GeV/c rising to about 10% at 2.5 GeV/c.
A new four-layer, double-sided silicon vertex detector was installed directly around the beam pipe. Covering 93% of the solid angle, at radii of 2.5-10.2 cm from the beam, the silicon detector contains 125 000 channels of read out. Finally, the CLEO trigger and data acquisition systems were completely redesigned and rebuilt to handle the higher CESR luminosity.
Apart from some efficiency problems with the silicon detector, all of the CLEO III components, new and old, performed exceedingly well during the run, and the experiment accumulated a total integ-rated luminosity of 9.2 fb-1. Of this, 6.9 fb-1 was obtained at the Y(4S) resonance, corresponding to more than 7 million decays into B particle pairs.
While the new detector was accumulating luminosity, analyses of data collected during previous incarnations of the experiment (CLEO II and CLEO II.V) were continuing. During the year 2000 the CLEO collaboration published 30 papers from these analyses and it has already published 22 more in 2001. These papers cover a broad range of topics, including the discovery of six new charmed-baryon states, the observation of 10 new B decay modes, new limits on neutral D particle mixing and B flavour-changing neutral-current decays, a precise measurement of the Lc lifetime, and the improved measurements of the two-photon widths of several charmonium states.