In 1979, Cornell University switched on the Cornell Electron Storage Ring, ushering in more than two decades of pre-eminence in the physics of b quarks. With the b-quark crown now passing to the B-meson factories at California’s Stanford Linear Accelerator Center and Japan’s KEK laboratory, Cornell is plotting a new course for its flagship facility.
The Cornell Laboratory for Nuclear Studies was established in 1946 by scientists returning from Los Alamos after the Second World War. Its given mission was “to investigate the particles of which atomic nuclei are composed and to discover more about the nature of the forces which hold these particles together”. Under Robert R Wilson’s guidance, a succession of electron accelerators ensued, culminating in a $10 million proposal for a 10 GeV machine to be built under the university’s sports grounds. By the time this was completed, in 1968, Wilson had moved on to become the founding director of Fermilab, but during his time at Cornell he had instilled in the laboratory a “can-do” spirit that remains to this day. Wilson believed in value for money and in getting things done by being clever, no matter how tight the budget.
By the 1970s, following pioneering work in the early 1960s at Frascati and the Stanford Linear Accelerator Center (SLAC), colliding beam machines were gaining ground. Cornell’s new director, Boyce McDaniel, decided to build an electron-positron collider at Cornell using the 10 GeV synchrotron as an injector. His first task was to convince the National Science Foundation, Cornell’s main funding agency, that a synchrotron could provide the necessary positron currents for such a machine. Happily for Cornell, accelerator expertise was on hand in the form of Maury Tigner, who had built a table-top electron storage ring in 1959 while a student of Wilson’s. Tigner had also been the first scientist to bring the idea of building an electron-positron collider to Cornell following conversations with Bjørn Wiik at Hamburg’s DESY laboratory. It was natural for McDaniel to appoint Tigner as project leader.
Building the storage ring
Tigner’s immediate challenge was to find a way of getting an intense beam of positrons into the proposed electron-positron storage ring. With the linac that Cornell used to feed the synchrotron, the process of building up a single intense bunch of positrons by accelerating individual bunches through the accelerator chain would be a long and difficult one. Tigner’s solution, which was later described as “fiendishly clever” by Karl Berkelman (the laboratory’s director through much of the Cornell Electron Storage Ring era), overcame the problem by accelerating multiple bunches and then coalescing them through a sequence of particle gymnastics in the synchrotron and the storage ring.
Under Tigner’s scheme the storage ring would be 61/60 of the circumference of the synchrotron and it would be filled with 60 bunches. In this way each bunch – starting with bunch number 2 – could be diverted into the synchrotron in turn. After n – 1 turns in the synchrotron, bunch n would be aligned with bunch 1 and could be reinjected into the storage ring, where it would coalesce.
The physics motivation for building a storage ring received a boost in November 1974 when the J/psi particle was discovered by Burton Richter at SLAC and by Sam Ting at Brookhaven. Six months later, Cornell submitted its proposal. It was then that the facility acquired its grand title. “For a while it was open season on creative names,” recalled Berkelman. “One of the wackiest I remember was suggested by Hywel White: CORNell COlliding Beams, or CORNCOB.” Eventually McDaniel ended the debate with Cornell Electron Storage Ring (CESR).
In 1979, CESR collided its first beams using Tigner’s novel coalescing scheme and the facility ran this way for several years. It was the first step in a proud tradition of Cornell particle gymnastics that would see CESR hold the world luminosity record for many years and that would be copied by other labs around the world. In the pursuit of ever-higher luminosity for CESR, Cornell physicist Rafael Littauer came up with the idea of putting more bunches in the storage ring by making the beams follow eccentric, pretzel-shaped orbits. Later on, Robert Meller’s idea of colliding the beams at a small angle, thus permitting yet more bunches, allowed the luminosity to be pushed still higher. Both of these ideas have been adopted by other labs and allow CESR to run today with a total of 45 on 45 colliding bunches in the ring.
A question of serendipity
The discovery of b anti-b quark bound states – upsilon particles – by Leon Lederman’s group at Fermilab in 1977 was “a fabulously serendipitous gift of nature that would guarantee the productivity of CESR for decades”, said Berkelman. The resonance found by Lederman’s group had a mass in the 9.4-10.4 GeV range, precisely where CESR would be looking. This was an extremely happy coincidence for Cornell, since the size of the ring, and hence its energy range, was determined by no more fundamental a parameter than the size of Cornell’s sports ground.
CESR’s detectors, CLEO and CUSB, soon resolved Lederman’s resonance into three separate peaks and McDaniel chose to announce the lab’s new facility to the world in the form of a greetings card showing these peaks. The more orthodox announcement came in the form of CLEO’s first paper, which was submitted to Physical Review Letters in February 1980. Cornell went on to play a leading role in the world of b-quark physics until 2000, when the dedicated B-factories at SLAC and KEK came into operation.
Serendipity is not the only thing that has kept Cornell in the world particle physics spotlight. “We take accelerator physics and technology very seriously as a branch of physics,” said Berkelman. It is this approach that has turned Cornell into a recognized centre of excellence in the field, with influence well beyond its Ithaca campus. Many innovations in accelerator physics, such as the concept of a linear collider and the innovation of an energy-recovery linac, have found fertile ground at Cornell over the years.
Superconducting radiofrequency is Cornell’s forte, with a tradition going back to the 1960s. Cornell was first to apply superconducting RF technology to cyclic accelerators for particle physics, installing a superconducting cavity in the 10 GeV synchrotron as early as 1975. This presented quite a challenge, since heat-load due to synchrotron radiation could easily warm the cavities to above their critical temperature. The technology developed for the synchrotron was adopted for the Continuous Electron Beam Accelerator Facility (CEBAF) – now known as the Jefferson Laboratory – in Virginia, the construction of which began in 1987 and occupied a large part of the Cornell group.
By then, Hasan Padamsee, who chose to remain at Cornell, was promoting the idea of superconducting RF for linear colliders, and in 1990 Cornell hosted the first TESLA workshop. This time it was Bjørn Wiik’s turn to take home an idea, and DESY soon became the standard bearer for the TESLA project.
Today, Cornell remains at the forefront of accelerator R&D, and the tradition that began when CEBAF adopted Cornell technology continues. As well as being used at CEBAF and TESLA, superconducting RF technology pioneered at Cornell is now finding applications in light sources, free-electron lasers, spallation neutron sources and radioactive ion-beam facilities.
