Nicola Cabibbo

Nicola Cabibbo, born in Rome on 10 April 1935, was President of the Pontifical Academy of Sciences for 17 years till his death on 16 August 2010. He was initially appointed as a Pontifical Academician on 9 June 1986 by Pope John Paul II, who also appointed him on 30 March 1993 as President of the Academy. He was, undoubtedly, one of the most important theoretical physicists of our time, and yet he was one of the most humble persons – a perfect gentleman, as well as a highly esteemed President of the Academy. Nicola Cabibbo was one of the fundamental pioneers in the development of high-energy physics. His contributions were in the field of weak interactions; and he did a large part of this work right here in Rome. It was here that the great Enrico Fermi taught physics at the University of Rome, La Sapienza, which became one of the leading Centres of theoretical physics. The work of Italian physicists in the development of high-energy physics has been excellent, and a part of it was awarded the Nobel Prize. There was, of course, the great work of Enrico Fermi on the theory of Beta Decay. One of Fermi’s colleagues in Rome was Emilio Segrè, who also won a Nobel Prize for the discovery of the anti-proton. Amongst Fermi’s collaborators was the young Ettore Majorana, who was highly gifted in mathematics and whose work on neutrino masses, the famous Majorana Equation and the Majorana Neutrino, is so well known. Several other famous Italian physicists, including Nicola Cabibbo, contributed importantly to this well-established Italian school of physics. The Cabibbo angle appears to be a simple parameter, but it was fundamental to the development of the theory of high-energy physics, particularly of what is called today ‘The Standard Model’. We can also recall the experimental work, 60 years ago, which was characterized by what was then referred to as the ‘theta-tau’ puzzle, raising the question of how the same strange particle could, on one occasion, go into two pions or, otherwise, into three pions. On this basis, Richard Feynman and Murray Gell-Mann proposed in 1958 their famous current x current form of weak interaction theory, which extended Fermi’s theory and made it applicable to all forms of weak decays. In a late recognition of Nicola Cabibbo’s important contributions to this field of research, the International Council of Theoretical Physics (ICTP) awarded the 2010 Dirac Medal to Nicola Cabibbo and E.C. George Sudarshan on 9 August 2010, just one week before Cabibbo passed away. The underlying theory was a beautiful generalization of the Fermi theory and it also included the axial vector current, in addition to the original vector current of Fermi, in the form of the V-A interaction of Sudarshan and Marshak, thus incorporating the maximal parity violation discovered by T.D. Lee, C.N. Yang and C.S. Wu in 1956. An important feature of the Fermi/Gell-Mann theory was the idea that weak interactions are universal. Cabibbo succeeded in reformulating the hypothesis of universality in such a way that the discrepancy noted in the comparison of the decay rates of the strange particles with those of the non-strange particles could be explained beautifully. This was done in his seminal paper of 1963. By 1963, SU(3) symmetry of Gell-Mann and Ne’eman which was at that time called Unitary Symmetry, was already becoming a part of high energy physics. Following Gell-Mann, Cabibbo took the strangeness-conserving and strangeness-violating weak hadronic currents as members of the same octet representation under SU(3). This allowed him a more precise formulation of the weak interaction. He proposed instead his form of universality that came to be known as Cabibbo Universality. All this was done within the framework of SU(3) and hence the agreement with experimental results helped to establish the usefulness of SU(3) not only in the classification of hadrons, but also in correlating their weak decays. Cabibbo’s paper refers to the Gell-Mann-Levy paper, but we must give substantial credit to Cabibbo as the author of Cabibbo Universality; the GellMann-Levy remark remained as a footnote for almost three years and it was Cabibbo who took it seriously and developed it into a full-fledged theory of the leptonic decays of hadrons with predictions that were verified experimentally. Cabibbo himself in his original paper was rather modest; he wrote, ‘I will restrict himself to a weaker form of universality’. On this scientific basis other scientists have successfully further developed this field of research. It is Nicola Cabibbo’s seminal work that laid the foundation for our modern understanding of the weak interactions among the quarks. Theorists who boldly followed his idea of universality of weak interactions to its logical completion not only successfully predicted the existence of the charmed quark, but also the top and bottom quarks which were necessary for CP violation. Finally, the universality that was formulated by Cabibbo got enshrined in the Standard Model of High-Energy Physics, in the form of the equality of gauge coupling to all the particles. It became a cornerstone of the Standard Model. It can thus be seen that Nicola Cabibbo is one of the several Italian distinguished scientists who laid the building blocks of the particle physics Nicola Cabibbo’s interests and concerns were not limited to theoretical physics but they extended to all fields of scientific investigations and to the application of scientific knowledge. This is obvious to all members of the Pontifical Academy of Sciences present in this commemorative session and who very highly appreciate and recognize his guidance as our President for the last 17 years.

M.G.K. Menon and Werner Arber