Richter, Burton (physicist)

 
(1931- ) American Experimentalist, Particle Physicist

Burton Richter is an outstanding experimental particle physicist, whose career has centered on high-energy electrons and electron-positron colliding beams. He shared the 1976 Nobel Prize in physics with samuel chao chung ting for his discovery of what is now called the J/y particle, an excited state of nuclear matter exhibiting the physical attributes associated with the existence of charmed quarks, which confirmed the standard quark-lepton model of electroweak and nuclear forces.

He was born on March 22, 1931, in Brooklyn, New York City, the elder child of Abraham and Fanny Richter. He was a member of that extraordinary generation of physicists, children of European Jewish immigrants, who grew up in New York in the years of the Great Depression and World War II, who included murray gell-mann, sheldon lee glashow, leon m. leder-man, and steven weinberg. After making an excellent record for himself in the New York public schools, he was accepted in 1948 at the highly competitive Massachusetts Institute of Technology (MIT) in Cambridge. Undecided at first whether to major in physics or chemistry, he chose physics as a result of the strong influence of Francis Friedman, one of his professors, who revealed to him the inherent beauty of the subject. He was introduced to the electron-positron system, which would be important in his future career, during the summer following his junior year, when he worked with Francis Bitter in MIT’s magnet laboratory. There he assisted Martin Deutsch in his classical positronium experiments, using a large magnet. Later, Bitter agreed to direct Richter’s senior thesis on the quadratic Zeeman effect in hydrogen.

For his graduate studies, begun in 1952, Richter remained at MIT and continued working with Bitter and his group. Initially, he worked with the group on a measurement of the isotope shift and hyperfine structure of mercury isotopes. Soon, however, he became more interested in the nuclear and particle physics problems he had studied as an undergraduate. After six months at the Brookhaven National Laboratory on Long Island, New York, working at the three-giga-electron-volt proton accelerator, he knew that particle physics research was what he wanted to do. When he returned to MIT, it was to the synchrotron laboratory. He would later write, “This small machine was a magnificent training ground for students, for not only did we have to design and build the apparatus required for our experiments, but we also had to help maintain and operate the accelerator.” Working under L. S. Osborne, he completed a doctoral thesis on the photoproduction of pi mesons from hydrogen in 1956.

From 1956 to 1960, he was a research associate at the High-Energy Physics Laboratory (HEPL) at Stanford University. He was drawn to HEPL’s 700-MeV electron linear accelerator, which would allow him to pursue his interest in experimentally testing quantum electrodynamics (QED, the quantum field theory describing the interaction of electrons and electromagnetic radiation) and, specifically, to investigate the short-distance behavior of the electromagnetic interaction. His very first experimental study at HEPL, which looked at electron-positron pairs by using gamma rays, established that QED was correct to distances as small as about 10-13 cm.

In 1960, Richter became an assistant professor in the physics department at Stanford and married Laurose Becker, with whom he would have two children, Elizabeth and Matthew. For the next few years, Richter worked with G. K. O’Neill of Princeton, W. C. Barber, and B. Git-telman on the construction of the first colliding beam device, which became the prototype for all future colliding beam storage rings. Using the HEPL linear accelerator as an injector, their device allowed them to study electron-electron scattering at a center-of-mass energy 10 times larger than that used in Richter’s earlier pair experiment. In 1965, they conducted an experiment resulting in extension of the validity of QED down to less than 10-14 cm.

Richter’s next goal was to create a high-energy electron-positron colliding beam machine that would allow him to study the structure of the hadronic (strongly interacting) particles associated with the excited states of protons, neutrons, and mesons. At the Stanford Linear Accelerating Center (SLAC), which he had joined in 1963, he and his group designed the machine and embarked on the long struggle for funding. When it finally came through in 1970, they built the Stanford Positron Electron Accelerator Ring (SPEAR), including the storage ring and a large magnetic detector. Experiments began in 1973.

In November 1974, Richter and his team detected a new kind of massive hadronic particle state that had the physical properties of a meson. They gave this excited hadronic state the name y. The particle was more than twice as heavy as any comparable hadronic particle and yet a thousand times more narrow in its energy spectrum (which, according to the uncertainty principle, meant that it was a long-lived particle). They published their results in a 35-author paper (characteristic for high-energy experimental teams) in Physical Review Letters.

Meanwhile, in August 1974, at the Brookhaven Laboratory, Samuel Ting and his team had made a startling discovery: the first of a totally unpredicted new group of extremely heavy long-lived mesons. In November, after rechecking his results, Ting announced his discovery, which he called the J particle (based on the symbol for the electromagnetic current). Just before publishing his findings, Ting attended a conference at Stanford University with scientists working at SLAC, where he learned of Richter’s almost simultaneous discovery. It was at this conference that the new particle state was named the J/y particle.

The significance of Richter and Ting’s discovery was considerable. In 1961, Murray Gell-Mann and yuval ne’eman had devised the eightfold way, a system for classifying the myriad newly discovered elementary particles into eight families of elementary building blocks called quarks, of which, at that time, there were three types. The unique feature of the newly discovered J/y particle was that it did not belong to any of the families, as they were known before 1974. Its detection confirmed Sheldon Glashow’s earlier prediction of a fourth quark (i.e., a “charmed quark”), which was needed in order to make the Gell-Mann SU(3) quark theory of the strong interactions consistent with this observation. Ting and Richter’s discovery of the J/y particle changed the landscape of particle physics. Two years later, they shared the Nobel Prize for their work.

Later in the 1970s, working with physicists at CERN; Novosibirsk, Russia; and Cornell University, Richter developed the ideas that led to the creation of the SLAC Linear Collider, which he describes as “a kind of hybrid machine, with both electrons and positrons accelerated in the same linear accelerator, and with an array of magnets at the end to separate the two beams and then bring them back into head-on collisions.” Completed in 1987, it began to be used for experiments in 1990. Richter predicted,

Probably the most lasting contribution that this facility makes to particle physics will be the work of accelerator physics and beam dynamics that has been done with the machine and which forms the basis of very active R&D programs aimed at the TeV [trillion electron volt] scale linear colliders for the future.

While bringing this major project to fruition, Richter became a scientific administrator, serving as technical director of SLAC from 1982 to 1984, and then as director from 1984 until his retirement in 1999.

Richter, who has over 300 publications in high-energy physics, accelerators, and colliding beam systems, is a fellow of the American Physical Society and served as its president in 1994. In reviewing his life in physics, Richter has written:

[I] realize what a long love affair I have had with the electron. Like most love affairs, it has had its ups and its downs, but for me the joys have far outweighed the frustrations.

By experimentally verifying the need for a charmed quark, Burton Richter’s groundbreaking experiment opened the door to the further elaboration of the fully developed Standard Model of electromagnetic, weak, and nuclear forces, which currently contains six quarks: up, down, strange, charm, top, and bottom.

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