From b to c
Now that the b-quark crown has passed to SLAC and KEK, CESR has mapped out new realms to conquer. True to the Wilson spirit, the laboratory has defined a programme that will provide important contributions to the interpretation of the new B-factory data and play an important role in the future of lattice QCD.
For the first year of the new era, CESR will drop down in energy to the narrow upsilon resonances to complete unfinished b-quark business. Then, if the CESR-c/CLEO-c proposal, which was submitted in October 2000, is approved (CERN Courier November 2001), CESR will be adapted to deliver high luminosity throughout the 3-10 GeV energy range. The lab will then focus on charm quark physics. Where it will differ from previous tau charm factory proposals is that CESR-c is not about tau and not entirely about charm either. The new proposal is just as much about B physics.
Charm-quark-containing D mesons provide a natural laboratory for learning about QCD. The number and precision of the measurements possible with CLEO-c will allow theorists to test and establish the range of validity of calculational tools for addressing QCD - such as lattice techniques. This will then determine the ultimate precision of B physics, since QCD uncertainties will be a major contribution to the errors in B-factory results and the same calculational methods are essential there. Pinning down the QCD parameters at Cornell will provide an important input to B-factory analyses and keep Cornell on the B-physics map.
CESR-c's contribution to lattice QCD came again through serendipity - via a coincidence of meetings at Cornell in January 2000. When the experimenters were discussing ways of putting meat on the CESR-c/CLEO-c proposal, about a dozen lattice theorists from around the world were meeting at Cornell to discuss exciting plans to incorporate quark loops into their calculations. This, they believe, will allow lattice calculations to produce predictions at the few-percent level for the first time. It turned out that the parameters that CLEO-c will be measuring are those that the theorists will be calculating. So the CESR-c/CLEO-c proposal became a combination of experiment and theory, and the lattice theorists now find themselves in the unusual position of racing an experiment to get predictions out before the measurements. The CESR-c/CLEO-c programme, if approved, will keep CESR in business for another four to five years.
Developments through CHESS
It is not only in particle physics that CESR has an enviable track record. Even before CESR was operational, Neil Ashcroft and Bob Batterman proposed building the Cornell High-Energy Synchrotron Source (CHESS), which would be parasitic on CESR's beams. CHESS has notched up an impressive number of firsts, such as the structure of the common cold virus, nanosecond diffraction and work on X-ray standing waves. Developments at CHESS catalysed much of the protein crystallographic revolution that is now under way at synchrotron X-ray sources, including CCD detectors and the cryoloop freezing method. Although small, CHESS has contributed to almost a fifth of the most significant protein structures that have been solved.
For the future, CHESS director Sol Gruner is pinning his hopes on dedicating CESR to X-ray use and on a new kind of accelerator - an Energy Recovery Linac (ERL) - using TESLA-like cavities. The ERL is a next-generation light source that will produce brighter beams and shorter pulses than available at even the best synchrotrons, thereby opening up new science directions. An advantage of an ERL source is that it will also serve almost all existing X-ray synchrotron source applications, and ERLs would complement Free-Electron Laser (FEL) sources, which are also being developed as next-generation sources. By contrast, FEL sources would provide peak X-ray intensities sufficiently high that new experimental techniques will have to be developed.
Whereas synchrotrons recycle electrons, an ERL will recycle energy. The idea is to accelerate a beam in a linac, bend it round to the start of the linac and then decelerate it to extract energy that will be used to accelerate the next bunch. Synchrotron radiation will be available as the electrons are returned to the linac. The advantage over synchrotrons is that the intensity of the bunch is exploited to the full, since it makes only one pass and does not settle to an equilibrium state. This means that the beam characteristics are limited by the injector, not the lattice. An ERL would also bring timing flexibility, because bunches could be delivered with any time structure required, and that would allow the investigation of the time development of processes. Cornell is working with Jefferson Laboratory, where a small ERL has been successfully built as a proof-of-principle machine.
Cornell is a small laboratory. CESR and CLEO share a common control room, with one rack of electronics between them, and CHESS is not far off. For Maury Tigner, now the laboratory's director, this intimacy is part of Cornell's strength. "We treasure our association with the X-ray science," he said. "It is a living example of the unity of science." Tigner also draws attention to the benefits for students of having a front-line research atmosphere on campus. Cornell typically has some 50 undergraduates involved with the CESR programme each year, as well as graduate students. "One thing we see as a very important contribution is the training of students in accelerator physics and technology," said Tigner. The presence of on-campus accelerator facilities allows students in accelerator physics the same kind of hands-on training that students in other branches of experimental physics can receive.
The Cornell trustees who defined the lab's mission back in the 1940s would have good reason to be pleased with the way their lab has evolved. Cornell has maintained a position at the cutting edge of particle physics research for more than half a century - a unique achievement for a university lab.
Further reading
D Andrews et al. 1980 Phys. Rev. Lett. 44 1108 (CLEO's
first physics paper).
H Padamsee 2001 The science and technology of superconducting cavities for
accelerators Supercond. Sci. Technol. 14 R28-R51.
The Cornell Laboratory Web site is at
http://www.lns.cornell.edu/.