Cornell Laboratory for Elementary Particle Physics
CORNELL LABORATORY FOR ELEMENTARY PARTICLE PHYSICS
The Cornell Laboratory for Elementary Particle Physics (formerly called the Cornell Laboratory of Nuclear Studies) was established in 1946 by the Trustees of the University as a research unit within the Physics Department. Its mission then 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." Over the next two decades under the leadership of Robert R. Wilson the Laboratory designed, built, and operated a series of electron synchrotrons of successively higher energies.
In a synchrotron a beam of electrons runs around in a ring-shaped vacuum chamber, guided by electromagnets and accelerated on each turn by microwave cavities. The 10-GeV synchrotron was completed in 1968, when Wilson left Cornell to become director of Fermilab. It was housed in a ring tunnel one-half mile in circumference and 12 meters under the Cornell campus. Experimenters from Cornell and collaborating universities used the electron and photon beams extracted tangentially from the synchrotrons to map the internal structure of the proton and neutron, study the production of pi and K mesons, and test the theory of quantum electro-dynamics.
The Cornell Electron Storage Ring
By the late 1970s it became clear that it was more economical to reach higher energies by colliding oppositely circulating electron and positron beams head on than by bombarding targets with electrons. Positrons are the positively charged antiparticles of the negatively charged electrons. Electron-positron pairs can be created by running a high-energy electron beam into a target. When an electron and positron annihilate in a colliding beam ring, all of their energy can go into the production of new states of matter, and none of it has to be wasted in conserving the net forward momentum of a beam-target collision.
So, under the leadership of Boyce McDaniel, the next laboratory director after Wilson, and Maury Tigner, the chief designer and project manager, and with the support of the National Science Foundation, the Laboratory constructed the Cornell Electron Storage Ring (CESR) in the same tunnel alongside the existing synchrotron. After electrons or positrons reach their maximum energy in a fraction of a second in the synchrotron, a pulsed electromagnet kicks them into a transfer channel that takes them into the storage ring where they can coast around and around for an hour or more. A storage ring is similar to a synchrotron except that the beam energy is held fixed; that is, the microwave cavities provide just enough push to make up for the energy that is lost by radiation. In CESR the two beams orbit at energies up to six billion electron volts per particle, that is, at velocities that are within 1.1 m/s of the 299,792,458 m/s speed of light.
The CLEO Experiment
The beams in CESR are configured so that the electrons and positrons collide at one point in the ring. Surrounding that point is a large, sophisticated, multipurpose apparatus designed to detect and identify all the particles that are produced in the electron-positron annihilation. It is built and operated by a collaboration of about 200 faculty, staff, and graduate students from about twenty universities. This collaboration, called CLEO, has been studying the products of the high-energy electron-positron collisions since CESR began operating in 1979. The CLEO detection apparatus records the dozen or so particles that are produced in each of the millions of electron-positron collisions.
Present understanding of the basic constituents of matter and the laws that govern their strong, electromagnetic, and weak interactions is embodied in the Standard Model. It explains how quarks make up the protons and neutrons in atomic nuclei and how the nuclei and the electrons (the lightest leptons) combine to make atoms, molecules, and bulk matter. It explains radioactivity, radio waves, chemistry—all sorts of phenomena familiar and unfamiliar. Although the model has had many successes in correlating the properties and behavior of the fundamental quarks, leptons, and bosons that make up the universe, it is incomplete. Basic questions are unanswered. Why are there six quarks and six leptons? Why do they have the masses and coupling strengths that they do? How did the symmetry between particles and antiparticles get broken to give the very asymmetric abundances in the present universe? The mission of the Cornell Laboratory for Elementary Particle Physics has over the years evolved from an original concern with nuclei to a quest for understanding the basis of the Standard Model of quarks and leptons.
An electron-positron collision in CESR is like a miniature version of the Big Bang and is an ideal way to create the more exotic heavier quarks and leptons that have decayed away since the Big Bang. CESR is particularly well adapted to the production of the charm and bottom quarks and the tau lepton. Since the discovery of the B meson (a bottom quark and a light antiquark) and the first measurement of its mass at CESR in 1980, CLEO has measured rates for over a hundred different decay modes of the B
meson, including many that are sensitive to violation of particle-antiparticle symmetry. The radiative decay of the bottom quark to the strange quark is particularly sensitive to the top quark and to the possible existence of hypothetical particles too massive to be directly produced in any present-day accelerator, and the CLEO measurement of the decay rate has been used to constrain numerous theoretical speculations on possible extensions of the Standard Model. CLEO has also been the leader in mapping out the spectroscopy of the particle states formed by the charm quark. Most of the known charmed baryon states (a charm quark bound to two other quarks) were discovered by CLEO.
Research in Accelerator Physics and Technology at Cornell
In spite of the advantages of electron-positron collisions for the study of heavy quarks, there is an important drawback; the annihilation probability is very small; the electrons and positrons usually pass by each other without interacting. Progress in particle research is therefore limited by the achievable beam-beam luminosity, a measure of the rate of collisions. This motivated efforts to raise the beam currents and focus them more tightly where they intersect. Instead of circulating a single bunch of electrons and a single bunch of positrons as in the original design, CESR now has forty-five bunches in each beam, with electric fields separating the orbits so that the bunches can collide at only one point. Thanks to these and other innovations, CESR held through the 1990s the world's luminosity record for colliding beam machines. These tricks have been copied in the design of later storage rings, surpassing CESR's record. CESR physicists have pioneered in the application of superconductivity to microwave particle acceleration cavities.
The circulating electron and positron beams in CESR emit X rays tangentially to the beam orbit. This by-product radiation is thousands of times brighter than normal laboratory X-ray sources. Nine experimental stations are administered by a separate National Science Foundation (NSF) supported organization, called CHESS (for Cornell High Energy Synchrotron Source). Hundreds of X-ray experiments have been carried out in material science, molecular biology, medicine, and other fields by scientists from Cornell, other universities, government laboratories, and industry.
The theory group at the Laboratory of Nuclear Studies has a distinguished history, marked by Nobel Prizes for Hans Bethe in 1967 and for Kenneth Wilson in 1982. Current work covers a wide range, from the physics of supernovae, through quantum electrodynamics and lattice quantum chromodynamics, to superstring theory and relativistic astrophysics.
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