For nearly 25 years, the CLEO experiment at the Cornell Electron Storage Ring (CESR) has investigated the production and decay of the b quark, as well as properties of charm quarks, tau leptons and many other aspects of the Standard Model of elementary particles and their interactions. CLEO's research results have been reported in over 350 publications in refereed journals, and have been the thesis projects of nearly 200 Ph.D. students.

There have been many milestones in the CLEO program (see the left-hand figure below, click to enlarge), including the first observation of B decay, the discovery of charmless B decays and the discovery of "radiative penguin" B decays. A large fraction of what we know about B decays, and much of detailed picture of charm and tau decays, have come from CLEO. CLEO's success has resulted from the extraordinary performance and continual improvement of the CESR accelerator, the key advances for which are highlighted in the figure. Also critical have been the outstanding capabilities of the CLEO detectors. CLEO II was the first detector to combine excellent charged-particle tracking with a state-of-the-art crystal calorimeter for detection of photons. CLEO III took this versatility another step forward by providing improved charged-particle identification with a RICH (ring-imaging Cerenkov) detector. With modest modifications designed to optimize performance at lower energy, CLEO-c (see the right-hand figure below, click to enlarge) is now the most powerful detector ever to investigate the decays of charmed particles near threshold.

The CLEO collaboration consists of roughly 125 physicists from 20 institutions. The University of Minnesota has been a part of the collaboration since 1988. Our group members have held many positions of leadership within the collaboration, including Spokesperson, Analysis Coordinator and Software Coordinator. Ten Minnesota graduate students have earned Ph.D.'s on CLEO.

Analysis of CLEO II and CLEO III data collected at the Upsilon(4S) resonance is approaching completion. CLEO results on B-meson decays continue to be competitive with those of the spectacularly successful B-factories at SLAC and KEK, largely because the experience of the collaboration and the detailed understanding of the response of the CLEO detectors allow measurements of CKM parameters and other quantities with small and well-understood systematic uncertainties.

CLEO ended running at the Upsilon(4S) resonance in the summer of 2001 and then spent approximately a year and a half collecting data in the bottomonium region away from the Upsilon(4S). The 4 inverse-femtobarns collected in this running represent more than an order-of-magnitude increase in the world's samples and results are now emerging.

With B mesons becoming the province of the B-factories, the CLEO collaboration and the Cornell Laboratory of Elementary Particle Physics developed a plan to operate CESR in the energy range between 3 and 5 GeV. The principal objective of this CLEO-c program is to study charmed particles near their production threshold, allowing measurements of unprecedented precision of branching fractions, decay constants and other parameters. A primary benefit of these studies is reduction in the uncertainties in B-decay measurements, which in many cases rely on poorly known properties of the charmed particles into which B's almost always decay. These charm measurements also provide a rich opportunity for testing the theoretical framework of heavy-flavor decays, allowing refinement of our procedures for determining the CKM matrix and testing the Standard Model. Other objectives of the low-energy running include detailed studies of QCD, with numerous precision tests of lattice calculations and searches for glueballs and exotic hadronic states. The CLEO-c project description is available here.