(1934- ) Italian Experimentalist, Particle Physicist
Carlo Rubbia designed and headed the synchrotron experiments at the Center for European Nuclear Research (CERN) that led to the discovery of the intermediate vector boson particles W and Z. At the deepest level of matter these extremely heavy particles act as carriers of the weak interaction between quarks and lep-tons, thereby generating radioactive decay. Their existence was theoretically predicted by the electroweak quantum field theory developed by sheldon lee glashow, abdus salam, and steven weinberg, which unified the electromagnetic force associated with quantum electrodynamics (QED) and the weak force associated with radioactive decay into a single theory. Rubbia shared the 1984 Nobel Prize in physics with his collaborator Simon Van der Meer for this work.
Rubbia was born on March 31, 1934, in the small town of Gorizia, near Trieste, in Italy. His father worked as an electrical engineer for the local telephone company, and his mother was an elementary school teacher. When, at the end of World War II, the province of Gorizia was taken over by Yugoslavia, the Rubbia family fled, first to Venice and then to Udine. As a boy, Rubbia early evinced the qualities that would lead him to experimental physics: he read everything he could find on the electrical and mechanical ideas that so fascinated him and was drawn to “the hardware and construction aspects” rather than to theoretical concerns.
His formal education had been badly disrupted by the war, however, and he failed his entrance examinations to the Scuola Normale in Pisa, where he had hoped to study physics. He had resigned himself to studying engineering in Milan when the withdrawal of one of the winning contestants allowed him to enter the Scuola Normale after all. Overjoyed by this happy accident, he moved to Pisa, where he struggled to make up for the shortcomings in his preparation. Under his thesis adviser Marcello Conversi, he participated in the construction of new instruments such as the first pulsed gas particle detector and earned his undergraduate degree with a thesis on cosmic ray experiments.
Eager to learn about giant particle accelerators, Rubbia, in 1958, went to the United States; there he worked at the Nevis Cyclotron Laboratory at Columbia University. With W. Baker, he performed the first of a series of experiments on weak interactions, which would be his primary focus. The two men measured the angular symmetry associated with the capture of polarized muons, thereby showing that parity violation occurs in this basic process.
Rubbia was attracted back to Europe two years later by the establishment of CERN in Geneva, Switzerland. Using a cyclotron superior to that at the Nevis Laboratory, Rubbia and his colleagues carried out a number of important experiments on the structure of weak interactions, notably including the discovery of the radioactive beta decay process of the positive pion and the first observation of the muon capture by free hydrogen.
In the early 1960s, Rubbia began working on the newly constructed proton synchrotron at CERN, where he determined the parity violation in the beta decay of the lambda hyperon (an excited state of the nucleus). After val logsdon fitch announced the discovery of charge-parity symmetry violation (known as CP or T violation) in some particle processes, Rubbia began a long series of CP symmetry— related observations associated with the K0 particle decay and on the KL-KS particle mass difference.
A few years later, in 1973, together with David Cline and Alfred Mann, Rubbia proposed a major neutrino experiment at the Fermi Laboratory in Batavia, Illinois. After more than a year of hard work they were able to observe cleanly the presence of the all-muon events in neutrino interactions needed to confirm experimentally new theoretical predictions made at CERN about the existence of a “charmed quark” and the y particle resonance.
By 1976, it was already clear that the unified electroweak theory of the symmetry type SU(2)x(1) had a good chance of predicting the existence and masses of the triplet of extremely massive intermediate vector bosons W and the neutral Z particle singlet that were required to carry the electroweak interaction. The problem was finding a practical way to discover them. To achieve high enough energies to create these bosons (roughly 100 times heavier than protons), experimentalists needed a radically new approach.
At CERN, under the leadership of Victor Weisskopf, a new type of colliding beam machine had been built with intersecting storage rings in which counterrotating beams of protons collide with one another. Rubbia and his collaborators transformed this high-energy accelerator into a colliding beam device in which a beam of protons and antiprotons coun-terrotate and collide head-on. For this purpose, they had to develop techniques for creating antiprotons, confining them in a concentrated beam, and colliding them with an intense proton beam. Rubbia did this with Van der Meer, Guido Petrucci, and Jacques Gareyte at CERN, where the first collisions were observed in 1981.
Hundreds of scientists were involved in these experiments in teams throughout the world. In 1983, Rubbia and collaborating teams of scientists announced the discovery of the W and Z particles on the basis of signals from detectors specially designed for this purpose. When the 1984 Nobel Prize was awarded for this work, Rubbia was cited for developing the idea; Van der Meer, the equipment.
Their work was the culmination of 50 years of research into the weak interaction, begun in 1934 with enrico fermi’s discovery that beta decay radiates into a final state involving an electron and a neutrino pair. Fermi assumed that the electron and the neutrino pair were created directly, when neutrons were transformed into protons. Rubbia’s work showed that pair creation is a two-step process, in which the particles W and Z are emitted in the first intermediate step and then convert into an electron and a neutrino pair in the second and final step. (This process has an analogy in QED, in which two electron bodies interact with each other by exchanging photons.)
For many years Rubbia divided his time between Cambridge, Massachusetts, where he taught for one semester a year at Harvard University, where he had been appointed professor in 1960, and at Geneva, where he conducted experiments such as the UA-1 collaboration at the proton-antiproton collider. He served as director general of CERN from 1989 to 1993. Married to Marisa Rubbia, a high school physics teacher, he is the father of two and a grandfather.
In November 1993, he proposed his Energy Amplifier project, a search for new sources of nuclear energy, exploiting knowledge and skills from high-energy physics that suggested the development of a power plant in which energy would be produced in a subcritical reactor. In 1994 he began to explore a route to energy production through controlled nuclear fission.
Rubbia believes that future experiments will lead to the discovery of an ever-finer definition of nature’s fundamental building blocks:
I think that an elementary particle is something much more complex than a mathematical point…. For a long time we thought nuclei were elementary particles. Even the word “atom” means “that which cannot be divided.” Later, we thought that protons were elementary particles. Now we know that the proton is made of quarks. In the history of physics, we have often over-simplified the structure of nature.
The fundamental impact of Carlo Rubbia’s Nobel Prize-winning work, in verifying Glashow, Weinberg, and Salam’s unified elec-troweak SU(2) X U(1) theory, was to open the door for physicists to begin searching for a global unification of the electroweak theory with the quantum chromodynamic SU(3) strong interaction theory of murray gell-mann. The search for a “theory of everything” to complete this unification is one of the leading directions in particle physics today.
