Nicola Cabibbo 
Earlier this week, Nicola Cabibbo passed away. The flags at CERN were lowered to half mast. Around the world people were discussing this man’s life and the 2008 Nobel Prize. Cabibbo was one of the most influential physicists of the 20th century. His work has influenced, and continues to influence, all the areas of particle physics, unifying interactions between all the fundamental fermions, the building blocks of all the matter we see around us.
Cabibbo’s main contribution to modern particle physics was the discovery of flavor mixing, in 1963. At the time, we knew of three quarks. The lightest quarks were called up and down, and they are the building blocks of protons and neutrons. In 1947 a new particle called the kaon was discovered, which seemed to be very similar to a lighter particle, known as the pion. The decays of the kaon seemed to be more subtle and peculiar than those of the pion, so the quark associated with these particles was named the strange quark. The more physicists studied the kaons, the more complicated the situation became, and the best models still failed to described the observations.
Cabibbo’s stroke of genius was to suggest that quarks could mix. The strange quark and down quark have the same charge, the same coupling to the strong force, but different masses. In a paper less than two pages long, Cabibbo suggested that they down and strange quarks are in fact two other particles, the strong force eigenstates, manifesting in two different ways . He described the quark mixing in terms of a rotation, where the down and strange quarks have a different preference for each strong force eigenstate. This implied that the mixing was the result of some deeper symmetry, so physicists should expect to see yet another quark, which would be the partner of the up quark. Cabibbo showed that by allowing the quark flavors to mix, he could explain the couplings of the down and strange quarks, while also predicting another quark to complete the symmetry. Over a decade later this fourth quark was discovered in three separate experiments.
A graphical representation of the CKM matrix .
However, this was just the beginning. Since Cabibbo’s 1963 paper his ideas have been taken and extended. Maskawa and Kobayashi took Cabibbo’s paper and added another pair of quarks, called the bottom quark and top quark. This may seem like an arbitrary extension of the model, but it predicts CP violation, the key to understanding why matter and antimatter behave differently. The theory was developed into a piece of mathematical machinery that describes the mixing of the quark states called the Cabibbo-Kobayashi-Maskawa matrix, or CKM matrix. So far the CKM matrix has passed all the experimental tests that have been performed, and offer a unique probe into new physics. Following the success of the CKM matrix, the same principle was applied to neutrinoes, in the Pontecorvo-Maki-Nakagawa-Sakata, or PMNS matrix. In addition, the mixing matrix has been used to describe several models of new physics, and flavor mixing is now seen as essential part of any new interaction.
Cabibbo’s contributions to particle physics have been far reaching. They took the confusion and frustration of the 1960s and turned them into a fertile theory that has stood the test of time, while explaining how both the strong and weak forces affect quarks. In 2008, Kobayashi and Maskawa were awarded the Nobel Prize for their contribution to the development of the CKM matrix. Some feel that Cabibbo was overlooked, and the discussions of the decision are heated. Cabibbo was later recognized with the award of the Dirac Medal, one of the most prestigious accolades in the field of physics. While opinion is divided on the Nobel Prize decision, nobody doubts that Cabibbo’s work was one of the most important breakthroughs of modern physics. It’s a sad week and although Cabibbo will be missed, his work continues to live on.