Nicola Cabibbo (1935–2010) CERN to Italy, Where He Taught Theoretical Pioneer in Understanding the Weak Force of Particle Physics
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NEWS & VIEWS NATURE|Vol 467|16 September 2010 OBITUARY talent as a teacher and conference speaker made Cabibbo an internationally known and influential figure. In 1966, he returned from Nicola Cabibbo (1935–2010) CERN to Italy, where he taught theoretical Pioneer in understanding the weak force of particle physics. physics at the universities of L’Aquila and Sapienza, moving in 1981 to the University of Rome Tor Vergata and returning to In his 1961 book The Theory of Fundamental Sapienza in 1993. In Rome, he created Processes, Richard Feynman vividly a school of outstanding young particle described the satisfaction that he and Murray physicists. Gell-Mann felt at finding an explanation Inspired by Cabibbo’s intuition for physics, for the close similarity between the Fermi mathematical skill and personal charisma, CONTRASTO/EYEVINE constants describing the β decays of neutrons the Rome school contributed significantly and muons. Feynman and Gell-Mann to establishing what is today known as the had discovered the ‘universality’ of standard theory of particle physics — an weak interactions. This is similar to the elaborate framework describing the weak, the universality of electromagnetic interactions strong and the electromagnetic interactions. and thus provided a tantalizing hint of a Among Cabibbo’s many achievements are common origin for the two forces. But the describing electron–positron annihilation book also recorded Feynman’s disquiet that into hadrons in terms of point-like the Fermi constant of ‘strange’ particles entities termed partons; calculation (nuclear particles such as the lambda, of the contribution of the combined comprising an up, a down and a strange electromagnetic and weak interactions to the quark) turned out to be smaller by a factor of anomalous magnetic moment of the muon; 4 to 5. It was Nicola Cabibbo who reconciled and the prediction of a phase transition these strange-particle decays with the of nuclear matter to a plasma of quarks universality of weak interactions. In doing and gluons in quantum chromodynamics so, he paved the way to the modern unified (QCD) — the particle-physics theory that theory of the electromagnetic and weak Cabibbo later reformulated the weak- today describes strong interactions. With forces that is central to the standard model interaction universality in terms of quarks — Giorgio Parisi, Cabibbo proposed and built of particle physics. the elementary particles that constitute a parallel supercomputer, termed the array Cabibbo died in Rome on 16 August hadronic particles such as protons, neutrons processor experiment (APE), for calculations following a severe respiratory illness. The and lambdas. Only three types of quark in QCD. APE and its subsequent evolutions son of a Sicilian lawyer, he was born in the (up, down and strange) were then known, have proved useful in elucidating basic QCD same city on 10 April 1935. He graduated and Cabibbo showed that β decays could be phenomena in a regime where the usual in theoretical physics at the Sapienza explained by a coupling between the types mathematical approach of perturbation University of Rome under the supervision of quark encoded in the Cabibbo angle. (Put theory cannot be applied. of Bruno Touschek, whom he would always more technically, the weak interaction couples From 1983 to 1992, Cabibbo served consider his mentor. In the early 1960s, in the up quark to an orthogonal combination as president of the National Institute of Frascati just outside Rome, Touschek and his of the down and strange quarks, determined Nuclear Physics in Frascati, making the collaborators were building the first collider by the Cabibbo angle.) This concept of the transition (unexpected to most) to science of electrons and their antiparticles, positrons. mixing of the different types of quark had administration. He was president of the With Raoul Gatto, who was also to become major implications for particle physics, and Italian energy agency ENEA from 1993 to a leading figure in particle theory, the young in 1970, Sheldon Glashow, John Iliopoulos 1998, and from 1993 was president of the Cabibbo wrote an exploratory paper on the and I extended it to include a fourth type of Pontifical Academy of Sciences, based in the physics that could be studied using electron– quark, charm, whose existence was confirmed Vatican. He brought to these positions vision, positron collisions, a paper that soon became experimentally in 1974. managerial skill and a universally appreciated a standard reference in the field. Cabibbo universality and the charm integrity. In 1991, he became one of the first In 1962, while working at CERN, quark made it possible to extend the recipients of the European Physical Society’s Europe’s particle-physics laboratory near unified electroweak theory, which had been prize in high-energy and particle physics. He Geneva, Switzerland, Cabibbo found the proposed by Steve Weinberg and Abdus was awarded the American Physical Society’s solution to the puzzle of the weak decays of Salam, to the interactions of hadrons, ironing Sakurai prize in 1989 and the Dirac medal strange particles. Formulating what came out the difficulties that had plagued the in 2010. to be known as Cabibbo universality, he three-quark scheme. But the phenomenon of Nicola liked teaching, and was still doing assumed that all β decays, whether they CP violation, observed in the decay of certain so until the last months of his life. Like all involved strange particles or not, had to be elementary particles, also had to be included. great thinkers, he could find simple ways described in terms of a weak current that To account for it, Makoto Kobayashi and to explain the most difficult concepts. His was determined by a single parameter — Toshihide Maskawa extended the theory students were captivated by his simplicity, the Cabibbo angle. Using this angle, now further, introducing two more quarks, the gentleness and sense of humour. So were determined to be about 12°, and exploiting bottom and top. In this six-quark model, all of us who had the privilege to be his the particle symmetries that had been Cabibbo’s quark-mixing scheme is embedded collaborators and friends. discovered by Gell-Mann and Yuval Ne’eman, in what is known as the Cabibbo–Kobayashi– Luciano Maiani Cabibbo could not only explain the Maskawa matrix. Luciano Maiani is in the Department of Physics, β decays of strange particles, but could also The success of Cabibbo’s theory — now Sapienza University of Rome, and president of the account for the small discrepancy between supported by decades of data, including the Italian National Research Council, Rome 00185, the Fermi constants of neutron and muon most recent data from CERN and Fermilab Italy. β decays. in Batavia, Illinois — and his exceptional e-mail: [email protected] 284 © 2010 Macmillan Publishers Limited. All rights reserved.