The Italian physicist Nicola Cabibbo, who many said should have shared the Nobel Prize for Physics in 2008 for his contribution to understanding the mechanism of quark mixing, died yesterday at the age of 75.

Cabibbo held many high-profile positions throughout his career including president of the Italian National Institute of Nuclear Physics (INFN). At the time of his death he was working at the University of Rome "La Sapienza", and was president of the Pontifical Academy of Sciences and chair of the scientific council at the Abdus Salam International Centre for Theoretical Physics (ICTP).

Only last week Cabibbo, together with Ennackal Chandy George Sudarshan of the University of Texas at Austin, were awarded the ICTP's 2010 Dirac medal for their "fundamental contributions to the understanding of weak interactions and other aspects of theoretical physics". A friend or colleague of Cabibbo will now be invited to accept the award on his behalf when it is presented in November by Irina Bokova, the director general of the United Nations Educational, Scientific and Cultural Organization.

Weak force pioneer

Cabibbo was best known for his work on the weak interaction in quarks – a fundamental particle that makes up hadrons such as protons and neutrons – and had been recognized for his contribution to "quark-mixing" between different favours of quarks. In 1963 he introduced the "Cabibbo angle" that is related to the relative probability that down and strange quarks decay into up quarks.

Cabibbo's 2×2 quark-mixing matrix was later extended to include a third generation of quarks by the Japanese physicists Makoto Kobayashi and Toshihide Maskawa, who then shared the 2008 Nobel Prize for Physics together with theorist Yoichiro Nambu. The resulting Cabibbo–Kobayashi–Maskawa (CKM) matrix describes how the strange quark and the down quark inside a kaon can switch to and fro into their antiparticles and, in doing so, occasionally violate charge–parity (CP) symmetry. This matrix also predicted the existence of new quarks – the charm, bottom and top – that were later discovered in experiments.

"Cabibbo was a giant of contemporary particle physics," says Tim Gershon, a particle physicist at the University of Warwick in the UK. "The impact of his contribution to the development of the Standard Model is almost impossible to overstate."

Some physicists, however, still feel that Cabibbo should have been awarded a share of the 2008 prize together with Kobayashi and Maskawa for laying the groundwork of the CKM matrix. "There has been an almost universal feeling in the community that Cabibbo was desperately unfortunate not to share in the prize," says Gershon.

A life in physics

Born in Rome in 1935, Cabibbo graduated in physics from the University of Rome "La Sapienza" in 1958 and then worked as a researcher at the INFN until 1962. After spending some time at the Lawrence Berkeley National Laboratory, California and at Harvard University, he returned to Italy to work at the University of Aquila in 1965. In 1966 he moved back to "La Sapienza" where he remained affiliated for rest of his career. He became president of the INFN in 1983 and had been president of the Pontifical Academy of Sciences since 1993.

"Nicola Cabibbo had a very strong influence on his colleagues. He was highly respected and considered to be a true gentleman," says theoretical physicist Giorgio Parisi from "La Sapienza" who was supervised by Cabibbo during his graduate studies. "I remember he always made Saturday morning a time to do physics. It was like a game for him to put together the pieces of a puzzle."