Five decades ago, Lee explained: 'We knew a form of neutrino, but we didn’t know how to make a coherent mixture of all these three.' Now, one of Lee’s main interests lies in the phenomenon of mixing in the leptons and quarks, described by two 3 × 3 matrices, which he calls the cornerstones of particle physics. Since then, the field has blossomed with the discovery of a total of six kinds of quark, a third charged lepton (the τ) and three kinds of neutrino. Quarks lay in the future, and the neutrino associated with nuclear beta-decay had been detected for the first time only the previous year. In 1957, however, physicists had a clear knowledge of only two of these – the electron and the muon, both charged leptons. They are what students first learn about the subject. The constituents of all matter – not dark matter, not dark energy but our kind of matter – every star, our Milky Way, all the galaxies in the universe are made of these 12.' These 12 particles, divided into four families, each with three particles of the same charge, form the basis of the current Standard Model of particle physics. 'Today, we now know that all matter is made of 12 particles: six quarks and six leptons. 'Our concept of what matter is made of, 50 years later, is very different,' he points out. Half a century later, Lee continues to focus on understanding the basic constituents of matter and, in particular, symmetry in fundamental particles, though much has changed in the intervening years.
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