Richard Dalitz

Physicist who gave 'Dalitz plots' and 'Dalitz pairs' to the language and saw his 'naïve' quark model prevail

Richard Henry Dalitz, physicist: born Dimboola, Victoria 28 February 1925; Lecturer in Mathematical Physics, Birmingham University 1949-55, Reader 1955-56; Professor of Physics, University of Chicago 1956-66; FRS 1960; Royal Society Research Professor, Oxford University 1963-90 (Emeritus); Fellow, All Souls College, Oxford 1964-90 (Emeritus); married 1946 Valda Suiter (one son, three daughters); died Oxford 13 January 2006.

"Dalitz plots" and "Dalitz pairs" have been part of the language of experimental particle physics for over 50 years. The theoretical physicist Richard Dalitz used his plot to reveal an apparent paradox that was only resolved by the shattering discovery in 1957 that nature distinguishes left from right.

Dalitz's other really seminal contribution, for which he never received adequate recognition, was to demonstrate the descriptive and predictive power of the quark model, which describes protons and neutrons (the constituents of atomic nuclei) and related particles as composites of more fundamental particles called quarks.

Dick Dalitz was born in Dimboola in western Victoria in 1925, but his family moved to Melbourne when he was two because his mother, a teacher, wanted her children to have access to good schools. After taking degrees in both mathematics and physics at the University of Melbourne, he went to Cambridge in 1946 to embark on a PhD in nuclear physics.

With his wife - Valda, who accompanied him from Australia - and new baby to support, Dalitz ran out of money after two years. He moved to Bristol, as student assistant to Neville Mott, where he became interested in C.F. Powell and collaborators' seminal discoveries and studies of cosmic-ray particles. A year later he joined Rudolf Peierls's outstanding theoretical physics group in Birmingham, but he continued to visit Bristol as a theoretical consultant to the cosmic ray group.

In 1951, pondering a new photographic image of an unusual cosmic ray event while driving back to Birmingham, it occurred to him that it could be due to a newly discovered unstable particle called the neutral pi-meson (which was known to decay into two photons - particles of light) decaying into one photon plus an electron and an anti-electron (a "Dalitz pair"). He calculated that this must happen, on average, once in 80 decays. This prediction was subsequently verified, and Dalitz pairs are commonplace in the debris of collisions of high energy particles studied today.

At this time Dalitz became interested in the so-called "tau" decay into three pi-mesons of a heavy particle (now known as the K meson) that had recently been discovered in cosmic rays, which also exhibits a "theta" mode of decay into two pi-mesons. Each time a heavy particle decays to three lighter particles they share their parent's energy in different proportions. The probabilities with which different energy partitions occur, which can be measured by observing a large number of decays, are governed by properties of the parent.

In 1953 Dalitz invented a very ingenious way of mapping the data, with one point representing each observation of a tau decay, that allows the properties of the parent particle to be read directly from the distribution of points on this map, or "Dalitz plot".

At first the tau decay data were too sparse to infer the properties of the parent heavy meson (only 13 decays had been observed in 1953). But as more decays were observed (53 by 1955, over 600 by early 1956) the Dalitz plot revealed a huge surprise. Despite having identical masses and lifetimes, and being produced in collisions at identical rates, the parents of tau and theta decays must be different particles - unless the decay process "violates parity", or in everyday language distinguishes left from right.

Up to that time it was universally believed that nature makes no such distinction, the most direct consequence being that it should be impossible to tell whether a film of any conceivable experiment had been shot directly or was a film of a mirror image (in which a right-handed person would appear to be left-handed). Dalitz speculated privately that parity might be violated, but was discouraged from pursuing this radical idea by Peierls, who in common with almost all leading theorists thought that other explanations should be sought.

It was left to Lee Tsung-Dao and Chen Ning Yang in 1956 (joint winners of the Nobel Prize the following year) to show that, in a whole class of decay processes that includes tau and theta decays, no measurements had actually tested mirror symmetry, which had been taken for granted. They proposed comparing the rates of certain non-identical mirror image process. To general astonishment they were found to be different: the "principle" of parity conservation had been only a theoretical prejudice.

Dalitz moved to Cornell in 1953 and then to Chicago in 1956. While in Chicago, he became the world's leading theoretical expert on nuclei that contain unstable heavy particles called hyperons, whose properties he helped infer from observations of these "hyper-nuclei". In 1963 Peierls, who had just moved from Birmingham, persuaded him to come as a Royal Society Research Professor to Oxford (although he retained a connection with Chicago until 1966), where he spent the rest of his life, and was the first physical scientist to be a Fellow of All Souls since the 17th century.

This was just after Murray Gell-Mann had brought some order into the host of newly discovered particles - many identified using the "Dalitz plot", which remains a vital tool to this day - by using mathematical group theory to classify particles with related properties into families. In 1964 Gell-Mann showed that this family structure would emerge naturally if the particles were composites of more elementary objects, which he called quarks.

But he equivocated over whether quarks were just a device to simplify the mathematics, or should be taken seriously as real particles. Dalitz seized on the quark model and used the idea that quarks are real not only to explain many known experimental results but also, exploiting his knowledge of nuclear physics, to predict the existence of new particles inside which the quarks rotate and vibrate in different ways.

Experimentalists quickly recognised the power and importance of this work, but most theorists looked down on the quark model. Gell-Mann himself - then the unrivalled world leader of the field - referred to it as naïve, perhaps hedging his bets, and is said to have walked out of a plenary talk by Dalitz, devoted to the quark model, at the 1966 world particle physics conference. It is not surprising that quarks were controversial. Their electrical charges are fractions of the charge of an electron, the smallest charge ever observed. They are so strongly bound together that they have never been separated, yet behave like independent particles in many ways.

By around 1970, however, the experimental evidence as interpreted by Dalitz and others, together with measurements that showed that electrical charge is distributed in fractional packets inside protons and neutrons, had convinced most physicists that quarks should be taken very seriously indeed. In 1973 a theory was developed that explains why the inter-quark force is weak when they are close together but grows without limit if attempts are made to drag them apart, and even clearer evidence for quarks followed. Almost overnight the "naïve" quark model came to be regarded as almost trivially obvious. Dalitz's key pioneering role was never adequately recognised and is hardly mentioned in textbooks. He never publicly expressed bitterness about this, although it would have been justified.

Dalitz, who made many other important contributions, was suspicious of theorising. He never got involved in constructing new theories, and showed little apparent interest in the so-called standard model of particle physics that, since the 1970s, has been the cornerstone of particle physics, although his work helped establish some of its essential ingredients. He saw his job as being to understand the data, of which he maintained an encyclopaedic knowledge until their volume became too great. I remember him once apologising for feeling that he should check an experimental result that he had quoted by consulting his meticulous notebooks (characteristically he had got it right), adding, "I used to know all the data; at one time I knew every event."

Dick Dalitz was a shy, modest person of few words, but those who got to know him appreciated his character and his low-key sense of humour. In Who's Who he described his recreations as "travelling, biographical research, study of the Wendish (Sorbian) people, especially their language and emigration". He knew that his ancestors had emigrated to Australia from Germany, but it was only after moving to Oxford that he learned of the existence of the Wends, or Sorbs - pre-German inhabitants of Brandenburg who speak a Slavonic language - and found that he was descended from them.

He became fascinated by the Wends and, despite limited linguistic abilities, published work that casts new light on aspects of their history. A personal high point came in the 1970s in Werben in East Germany, which he had identified as the village from which his family originated. Examining parish records with the local historian, they discovered that they were distant relatives.

Many younger theoretical particle physicists were out of sympathy with Dalitz's empirical approach. But he was revered in the experimental community, and all those who knew him in the 1950s or 1960s - when he was arguably the world's leading "phenomenological" particle theorist - recognise the enormous contributions that he made to unravelling the basic structure of matter.

Chris Llewellyn Smith

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