In 1961 he moved back to Europe, attracted by the newly founded CERN where he worked on experiments on the structure of weak interactions. CERN had just commissioned a new type of accelerator, the Intersecting Storage Rings, using counter-rotating beams of protons colliding against each other. Rubbia and his collaborators conducted experiments there, again studying the weak force. The main results in this field were the observation of the structure in the elastic scattering process and the first observation of the charmed baryons. These experiments were crucial in order to perfect the techniques needed later for the discovery of more exotic particles in a different type of particle collider.
Early in 1983 at CERN, an international team of more than 100 physicists headed by Rubbia detected the intermediate vector bosons, the W and Z bosons, which had become a cornerstone of modern theories of elementary particle physics, long before they were observed by Rubbia and collaborators. They are believed to carry the weak force that causes radioactive decay in the atomic nucleus and controls the combustion of the Sun, just as photons, massless particles of light, carry the electromagnetic force which causes most physical and biochemical reactions. It is also believed that the weak force has played a fundamental role in the nucleosynthesis of the elements, as studied in cosmology and the big bang. These particles have a mass almost 100 times greater than the proton.
To achieve energies high enough to create these particles, Rubbia proposed, together with David Cline and Peter McIntyre, a radically new particle accelerator design. They proposed to use a beam of protons and a beam of antiprotons, their antimatter twins, counter rotating in the vacuum pipe of the accelerator and colliding head-on. As a result, scientists had to develop a number of techniques for creating antiprotons. These techniques were developed with Simon van der Meer, with whom Rubbia shared the 1984 Nobel Prize for Physics.
In addition to the observation of the intermediate vector mesons, the CERN proton-antiproton collider dominated the scene of high energy physics from its first operation in 1981 until its close in 2002, when the Tevatron at Fermilab took over this role. An entirely new phenomenology of high energy collisions has resulted, in which strong interaction phenomena are dominated by the exchange of the quanta of the strong force, the gluons, particles which are similar to the intermediate vector bosons, although, like the photons, they are apparently massless. Instead, the W and Z particles are among the heaviest particles so far produced in a particle accelerator.
Together, these discoveries provide strong evidence that theoretical physicists are on the right track in their efforts to describe Nature at its most basic level through the so-called "Standard Model". The data on the intermediate vector bosons confirm the predictions included in the "electroweak" theory, which gained the 1979 Nobel Prize for Physics to Steven Weinberg, Sheldon Glashow and Abdus Salam. The "electroweak" theory attempts to unite two of the four forces of Nature - the weak and the electromagnetic forces - under the same set of equations. It provides the basis for work on the long-standing dream of the theoretical physicists, a "unified field theory", encompassing also the strong force which binds together the atomic nucleus, and ultimately, gravity.
In 1970 Rubbia was appointed Higgins Professor of Physics at Harvard University (Cambridge, Massachusetts), where he spent one semester per year, while continuing his reserch activities at CERN. In 1989, he was appointed Director-General of the CERN Laboratory.
Rubbia has also been one of the leaders in a collaboration effort deep in the Gran Sasso Laboratory, designed to detect any sign of decay of the proton. The experiment seeks evidence that would disprove the conventional belief that matter is stable. The most widely accepted version of the unified field theories predicts that protons do not last forever, but gradually decay into energy after an average lifetime of at least 10^32 years. The same experiment, known as ICARUS and based on a new technique of electronic detection of ionizing events in ultra-pure liquid Argon, is aiming at the direct detection of the neutrinos emitted from the Sun, a first rudimentary neutrino telescope to explore neutrino signals of cosmic nature. This innovative detector is now operational at the University of Pavia, awaiting for its transfer to the Gran Sasso Laboratory, where it will start collecting data in 2008.
Prof. Rubbia further proposed the concept of an energy amplifier – a novel and safe way of producing nuclear energy exploiting present-day accelerator technologies, which is actively being studied worldwide in order to incinerate high activity waste from accelerators, and produce energy from natural thorium and depleted uranium. The energy resources potentially deriving from these fuels will be practically unlimited and comparable to those from Fusion.
His research activities are presently concentrated on the problem of energy supply for the future, with particular focus on the development of new technologies for renewable energy sources. During his term as President of ENEA (1999 - 2005) he has developed a novel method for concentrating solar power at high temperatures for energy production, known as the Archimedes Project, which is presently being developed by industry for commercial use.
Carlo Rubbia is currently principal Scientific Adviser of CIEMAT (Spain), Adviser of the Italian Minister of the Environment, Land and Sea and one of the members of the high-level Advisory Group on Climate Change set up by EU's President Barroso in 2007.