Nicola Cabibbo was born in Rome on 10 April 1935 and died there on 16 August 2010. He was one of the most brilliant theoretical physicists of the post–World War II period and was universally known for his theory of weak semileptonic decays.
With Bruno Touschek as his adviser, Cabibbo graduated from the University of Rome I (“La Sapienza”) in 1958 with a thesis on weak interactions. He then went to the Frascati National Laboratories, where he worked with Raoul Gatto on a comprehensive study of electron–positron interactions; that work had a decisive role in shaping research with colliding-beam accelerators. Cabibbo became a full professor in 1965 at the University of L’Aquila, then moved to La Sapienza, where he taught from 1966 to 1981; to the University of Rome II (“Tor Vergata”), where he was a professor from 1981 to 1993; and then back to La Sapienza, where he taught until 2010. He spent several years abroad, mainly in the US, Switzerland, and France. Unfortunately, he did not live long enough to collect his two last prestigious awards, the 2010 Dirac Medal from the Abdus Salam International Center for Theoretical Physics and the 2011 Benjamin Franklin Medal in Physics from the Franklin Institute.
During the past 30 years, Cabibbo led several institutions—for example, as president of the Italian National Institute for Nuclear Physics (INFN) in 1983–92 and of the Italian National Institute for Alternative Energies in 1993–98. As president of the Pontifical Academy of Sciences from 1993 to 2010, Cabibbo always defended the scientific correctness of Darwinian evolution against the attack of creationism, which he considered to be a nonscientific viewpoint. As a leader at those institutions, he was highly appreciated for his balanced mind and objective judgments. Édouard Brézin, a former president of the French Academy of Sciences, has succinctly written that Cabibbo “was a great physicist and a real gentleman. I liked his style, made of restraint and wise remarks in a few words.” Cabibbo’s kindness was proverbial and touched many students.
The first years of Cabibbo’s career were mainly devoted to obtaining confirmations, beyond mass formulae, of the theory of broken SU(3) symmetry between strange and nonstrange hadrons. In 1961 he and Gatto derived the SU(3) predictions for the electromagnetic decay of the η meson and for the matrix elements of the electromagnetic current between baryons. Cabibbo used a similar approach in his fundamental 1963 paper, in which he argued that the hadronic current of weak semileptonic decays was uniquely parameterized by an angle θ (universally known as the Cabibbo angle, which characterized the strength of the strangeness-changing weak interactions). With that approach, Cabibbo was able to explain the results of several experiments. The paper had an immense resonance; the verification of its predictions kept experimentalists busy for many years.
Cabibbo’s theory was based on symmetry arguments, and it could be readily formulated in terms of quarks. In 1970 Sheldon Glashow, John Iliopoulos, and Luciano Maiani showed that the rarity of double-strangeness-changing events could be explained in the framework of Cabibbo’s theory by the existence of a fourth (charmed) quark. In 1973 Makoto Kobayashi and Toshihide Maskawa showed how the extension to a six-quark model could explain the presence of CP violations in weak interactions; the two received the Nobel Prize in Physics in 2008 for their contribution (see PHYSICS TODAY, December 2008, page 16).
In later years Cabibbo continued to study electromagnetic and weak forces, mainly in a field-theory framework, trying to compare the theories with such experiments as those for semileptonic decays of charmed particles. Among his seminal contributions are a detailed study of the neutrino oscillations and the prediction of the existence of a quark plasma phase, which is being observed today at the Large Hadron Collider.
Cabibbo’s many institutional positions did not interrupt his scientific research, which included publishing more than 200 papers. While he was president of the INFN, he started the APE project to design, construct, and exploit a supercomputer dedicated to lattice quantum chromodynamics. For a short time it was the fastest computer in the world, and its descendants are still active. I remember the Saturday morning meetings in Cabibbo’s large, sunny office, where in front of a blackboard he would assess the situation, analyze the difficulties we were encountering, and often suggest the best solutions.
His scientific influence was overwhelming. Despite the renaissance of Italian experimental physics after World War II, Italian theoretical physics was slow to start. Thanks to his international achievements, Cabibbo became an exemplar whom both younger physicists and those his age wanted to imitate. He showed that it was possible to have an excellent school of theoretical physics in Italy, as demonstrated by the accomplishments of his students and younger collaborators, including Maiani, Roberto Petronzio, Guido Martinelli, and me.
Cabibbo liked making objects by hand—for example, the mirror of an amateur reflecting telescope—not only for their possible use but also for the pleasure of crafting something well. He had always been a talented photographer, and in the digital era he would spend long afternoons improving the quality of a single photo. When I visited Cabibbo at home last July during his illness, I noticed on the walls large abstract pictures with strong, round spots of pure colors. I expressed my appreciation and asked who the artist was, and he said happily, “I made them.” He explained to me, with plenty of technical details, how he developed a technique to transform his digital photos and create such impressive images of sheer beauty.
Cabibbo had an infectious enthusiasm for physics. He was a born problem solver; to him, physics was a kind of play, like putting together the pieces of a puzzle to form a meaningful pattern from an incoherent data set. I will always remember him saying, “Why should we study this problem if we do not amuse ourselves in solving it?”
