Hughes, Vernon Willard

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Hughes, Vernon Willard

(b. 28 May 1921 in Kankakee, Illinois; d. 25 March 2003 in New Haven, Connecticut), Yale physicist and educator who devised methods for producing polarized particle beams in accelerators; designed and performed experiments in which extremely accurate measurements of subatomic particles were made; was among the first (with his coworkers) to observe parity nonpreservation of the electron and a new experimentally useful atom, muonium; and devised experiments to confirm the validity of the standard quantum theory of matter while disclosing discrepancies.

Hughes’s father, Willard Vernon Hughes, died when Hughes was three. His mother, Jean Parr Hughes, moved with her only child to New York City near Columbia University. Jean Hughes was a librarian at Teachers College of Columbia University, and Hughes attended a private school. For one year they lived in Iowa (where Vernon’s parents were reared) but returned to Manhattan.

Vernon entered Columbia University in 1938 as a pre-law student, but after winning the Van Buren Mathematics Prize, he changed his major to physics. He earned his AB degree in three years. Desiring a change of environment, Hughes continued his studies at the California Institute of Technology, where he earned an MS in physics in 1942. Because the United States had entered World War II, Hughes chose to work on radar at the Massachusetts Institute of Technology Radiation Laboratory. His group studied the problem of obtaining extremely accurate time measurements of a reflected radar pulse, which were used to establish the distance to a target. Radar played a crucial role in the Allied victory. As a result of this work, Hughes and others edited a volume, Waveforms (1949), which became an important reference for the emerging electronic industry.

In 1946 Hughes returned to Columbia to work toward a doctorate under the Nobel laureate Isidor Isaac Rabi. For his project he built an electronic resonance apparatus to measure an intrinsic property of a nucleus (its magnetic dipole moment). He worked on the experiment with another graduate student, Lou Grabner, and they discovered the first two-photon transition in spectroscopy. Hughes worked out the theory, but he learned later that the theory had been published in 1931. Rabi suggested that Hughes use his apparatus to test whether atoms and molecules have no net electrical charge, a problem that Albert Einstein had posed. Hughes established a small upper limit for any charge the atom might have, and later at Yale University performed more sensitive experiments yielding a much smaller upper limit indicating that atoms and molecules are probably electrically neutral.

Hughes received his PhD in 1950 from Columbia, and on 18 September 1950 he married Inge Michaelson, who had fled Nazi Germany with her family in 1938. They had two sons. Hughes did postdoctoral work at Columbia, where his experiments provided evidence that the electron was a fundamental particle—in other words, that it did not consist of component particles.

The next two years (1952–1954) Hughes spent as an assistant professor at the University of Pennsylvania in Philadelphia, where Inge earned her PhD in biology. Hughes then went to Yale University, where he remained until his death. At Yale he rose from assistant professor to professor to Sterling Professor, the highest honor Yale confers on its faculty. Inge Hughes died in 1979. Hughes married Miriam Kartch on 28 November 1980; he had taken piano lessons from her at the Mannes School of Music while an undergraduate at Columbia, and she had been a family friend for many years.

Hughes devoted his scientific career to investigating fundamental electromagnetic properties of the simplest atoms. He designed apparatus, ran major accelerator experiments in collaboration with other physicists, and did the requisite theoretical calculations. Among the atoms he used were helium, positronium (a shortlived electron-positron atom), and muonium. Muonium, discovered in 1960 by Hughes and his colleagues, consists of a positively charged muon (a particle first discovered in cosmic rays) and an electron. Hughes worked with muonium for several decades. As a result of extremely precise experiments, he showed that the muon is a “heavy” electron. He also found muonium useful in testing the validity of quantum electrodynamics (QED), the theory of photon-mediated electromagnetic interactions.

Over a period of about thirty years Hughes also did extensive work with helium. One of his main objectives was to test the current QED theory of the two-electron system. Within experimental limits there was good agreement between his results and the theory.

Parity conservation is a law of classical physics positing that for any physical phenomenon its mirror image also occurs. In 1957 Chien-Shiung Wu discovered that this law was violated in beta decay. Motivated by the discovery of nonconservation of parity, Hughes and Jack Greenberg did an experiment to measure parity violation by studying polarization of beta particles. Beta particles are electrons or positrons emitted by a radioactive nucleus. These particles behave like little magnets and are said to be polarized when, on average, their magnetic poles point in the same direction. This experiment led Hughes to conclude that polarized electron sources would be important in high-energy physics. He began to study methods for producing polarized electrons. After about ten years Hughes’s pioneering work culminated in the source of polarized electrons used at the Stanford Linear Accelerator (1972.) This led to the first observation of parity nonconservation in electron scattering. The use of polarized beams in high-energy accelerators led to the possibility of a variety of new measurements of subatomic particles.

In the late 1950s Hughes began working on the design of a proton linear accelerator to produce meson (an elementary particle) and muon fluxes greater by a factor of 1000 than those then in use. The Los Alamos Meson Physics Facility was based largely on the design that Hughes and others at Yale had developed. In the mid-1960s Hughes began spending summers working at the Los Alamos facility, which began operating in 1972. Hughes continued using the facility until 1996, when it was closed as a “meson factory.”

In the mid-1980s Hughes and his Yale group were invited to join the European Muon Collaboration (EMC) at the European Center for Nuclear Research (CERN) just outside Geneva, Switzerland. The EMC was preparing to study polarized muon-nucleon scattering. (Nucleons are protons and neutrons.) The Yale group was developing a polarized proton target at that time. The result of this collaboration led to a surprise. By the mid-1970s physicists had evidence that protons and neutrons consisted of constituent elements called quarks and were bound together by gluons (analogous to photons). Quarks have a property called “spin” (intrinsic angular momentum). The surprising finding was that the quark spin accounted for only a small fraction of the proton spin. This created the “spin crisis.” Several major facilities undertook to study the problem, and at the group’s formation in 1987 Hughes became the spokesman for the Spin Muon Collaboration at CERN, consisting of about 150 physicists. Data was collected between 1992 and 1996, leading to good agreement with quantum chromo-dynamics (QCD, the theory of forces within the nucleus) but supporting the spin crisis.

Hughes was involved with the measurement of a crucial parameter useful in testing the Standard Model (a quantum-theoretical description of the atom successfully used for more than thirty years but known to be incomplete). The parameter, called the magnetic moment, is symbolized as g. According to theory, the value of g for the muon is 2. The theory does not take into account corrections necessitated by continuous emission and absorption of shortlived particles. Thus the deviation from the theoretical value, g-2, is of great interest. Hughes initiated a muon g-2 program at the Brookhaven National Laboratory. As cospokesman for the group he announced in 2002 that the newest results obtained for the g-2 value of the muon lead to a significant disagreement with the Standard Model and may lead to a new theory beyond it, perhaps incorporating supersymmetry.

Hughes was a pioneering physicist, expert in both theory and experimentation. He made many contributions to the study of fundamental particles. He was a leader of and spokesman for collaborative groups of physicists and, for six years as chair of the physics department at Yale, he greatly strengthened the faculty and graduate program. For forty years he was on the board of trustees of Associated Universities, Inc., the organization that established the Brookhaven National Laboratory in 1947 and the National Radio Astronomy Observatory in 1956.

Hughes died of medical complications after surgery for an aneurysm. He is interred at the Grove Street Cemetery in New Haven, Connecticut. His papers are at the family home in Vermont. Hughes’s son Emlyn became a physicist at the California Institute of Technology and worked on fundamental particles.

Hughes received many honors, including membership in the National Academy of Sciences; the Davisson-Germer Prize in Atomic Physics (1990) and the Tom R. Bonner Prize in Nuclear Physics (1990), both from the American Physical Society; and an honorary PhD from the University of Heidelberg (1977).

Biographical information and a partial listing of Hughes’s many publications may be found in “Various Researches in Physics” by Vernon W. Hughes, Annual Review of Nuclear & Particle Science 50 (2000), and Robert K. Adair, “Vernon Willard Hughes,” In Memory of Vernon Willard Hughes: Proceedings of the Memorial Symposium in Honor of Vernon Willard Hughes, edited by Emlyn Willard Hughes and Francesco Iachello (2004). Obituaries are in the New York Times (31 Mar. 2003), Yale Bulletin and Calendar 13, no. 4 (4 Apr. 2003), CERN Courier (July 2003), and Physics Today (Feb. 2004).

Howard Allen