NATURE|Vol 440|9 March 2006

Home , Graham Ross (physicist), Richard Dalitz

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NEWS & VIEWS NATURE|Vol 440|9 March 2006

OBITUARY Richard Dalitz (1925–2006) Particle physicist and creator of the Dalitz plot.

Richard Henry Dalitz was a giant in the field it decayed into two pions, whereas the tau of particle physics. A theorist who always decayed into three. Using his plot, he was endeavoured to work close to experiment, able to establish that the tau, like the theta, his contributions over 60 years shed vital has zero spin, so that the two particles light on the nature of the fundamental could not be identical if parity holds. forces and the constituents of matter. (Parity is the idea that reactions proceed Born in the small wheat-belt town the same way when all spatial coordinates of Dimboola, northwest of Melbourne, are reversed.) Indeed, this ‘tau–theta’ puzzle Australia, Dalitz gained degrees in both was the first indication that parity did not mathematics and physics at the University of hold for interactions involving the weak Melbourne. Awarded a travelling scholarship, nuclear force. The subsequent realization he moved in 1946 with his wife Valda to by T. D. Lee and C. N. Yang that this could England, to study for a PhD under Nicholas indeed be the case won them the 1957 Kemmer at Trinity College, Cambridge. Nobel Prize in Physics. There he benefited from the teaching of After brief periods at Cornell University in Paul Dirac, whose lecture course on quantum Ithaca, New York, and back at Birmingham, mechanics he attended twice. After two years, in 1956 Dalitz accepted a professorship at the he ran short of funds and, with a small child Enrico Fermi Institute in Chicago. Working to support, moved to a one-year post as with Riccardo Levi-Setti, he developed what assistant to Nevill Mott at the University became a lifetime interest, often pursued strange particles, whose unusual properties of Bristol. In retrospect, Dalitz considered in collaboration with Avraham Gal, in are now attributed to the presence of at this year at Bristol vital to his subsequent the recently discovered hypernuclei, in which least one ‘strange’ quark in their make-up

research: there he first learned from the a strange particle takes the place of a proton that hampers their decay, as well as states G.-C. WICK/AIP group of Cecil Powell about elementary or neutron. that contain the heavier quarks ‘charm’ particle physics and the ‘strange’ particles In 1963, Dalitz was persuaded by Peierls and ‘bottom’. (so named because they left unusual tracks in to return to Britain and join him at the The quark model was the foundation the emulsions used to detect them) produced University of Oxford as a Royal Society of quantum chromodynamics (QCD), in cosmic-ray collisions. He started to think research professor and, from 1964, a fellow the comprehensive theory of the strong particularly about the nature of one of these, of All Souls College. At Oxford, Dalitz nuclear force that depicts quarks as being the ‘tau meson’, or , as it is now known. continued his work on the resonant states held together by the exchange of gluons Dalitz was invited by Rudolf Peierls to the that were being discovered in ever-larger — analogues of the photons of the University of Birmingham in 1949, where numbers — often through the use of electromagnetic force — which are coupled his encounters with Freeman Dyson Dalitz plots. These states could be grouped to a new ‘colour’ charge. Dalitz first heard of proved most useful for his mastery of according to their mass, spin and parity into colour in a talk by Gell-Mann and recognized the quantum-electrodynamical methods families of eight and ten. Murray Gell-Mann that it resolved some profound problems that Richard Feynman was then developing dubbed this the ‘eightfold way’, and proposed associated with the quark model. The success to describe electromagnetic interactions. that the pattern could be explained if of this model once colour was included set After completing his thesis, which was on the resonances were combinations of the scene for the subsequent development transitions between spin states in the oxygen fundamental building-blocks of fractional of QCD. Dalitz’s interest in the quark model nucleus, Dalitz was finally able to concentrate electric charge — quarks, as he named them. continued all his working life. In 1992, with on particle physics and study of the tau. At the time, most physicists considered Gary Goldstein, he developed a method The result was two seminal contributions quarks as merely a mathematical tool. still in use today, to determine the detailed that bear his name: the study of the decay of Boldly, Dalitz pursued the possibility that properties of the ‘top’ quark, the last of the the neutral pion (one of the lightest mesons, they might be real dynamical objects, and, six types of quark to be found. a class of particles now identified as in a remarkable paper presented in Tokyo A fellow of the Royal Society and recipient quark–antiquark pairings) to a photon in 1965, showed how different combinations of many honours, among them the Hughes and an electron–positron, or ‘Dalitz’, pair; of three tightly bound ‘up’ and ‘down’ quarks Medal and Royal Medal of the Royal Society, and the development of the ‘Dalitz plot’. explained many properties of the proton and ‘Dick’ Dalitz never lost his appetite for This plot presents the kinematic variables neutron. He further proposed that the quark physics. He remained an active member of of the three-body final state of a reaction model could describe not only these ground the theoretical-physics department at Oxford in two dimensions. This allowed so-called states, but also the multitude of higher- after his retirement in 1990, always keen to resonances — transient states that flag energy resonant states. encourage students to share in his passion for their existence through their decay to At Oxford, Dalitz established a flourishing physics. He died on 13 January 2006, and is final-state particles of definite total energy research programme to study the quark survived by his wife, son and three daughters. — to be readily visible. model, and attracted many senior physicists Graham Ross Dalitz was interested in the difference and students to it. Their work, and work Graham Ross is in the Rudolf Peierls Centre for between the tau and another strange worldwide, has shown that all particles found Theoretical Physics, University of Oxford, particle, the theta, which seemed to be the then and since fit well with quark-model 1 Keble Road, Oxford OX1 3NP, UK. tau’s identical twin except for the fact that predictions. This encompasses the original e-mail: [email protected]