Professor RH ("Dick") Dalitz, FRS (1925-2006)

It was with great sadness that we received the news that our esteemed and respected colleague Professor RH ("Dick") Dalitz passed away on Friday 13^th January 2006.

Dick was born in Dimboola, Victoria Australia, on February 28^th 1925, and gained degrees in Mathematics and Physics at Melbourne University. He came to Britain in 1946 to do his Ph.D. at Cambridge, and then worked in Bristol University before joining Rudolf Peierls in Birmingham in 1949. After completing his thesis, he moved to the US in 1953, firstly at Cornell and then in Chicago, returning to Oxford in 1963 as a Royal Society Research Fellow, a post he held until his retirement in 1990.

Dick made many contributions to particle physics, beyond the Dalitz Plot, Dalitz Pair and CDD (Castillejo, Dalitz, Dyson) poles that bear his name.

His Cambridge thesis was on "Zero-zero transitions in nuclei". Primarily it was a study of the transitions from the first level of oxygen, which has spin-parity 0+, to the ground state, which also has 0+, which cannot occur with real photoemission due to angular momentum but can occur if the photon converts into an electron-positron pair. It was this early insight that would later bear fruit with the concept of Dalitz pairs in particle physics.

It was during this period, when he spent a year working alongside Cecil Powell’s cosmic ray group at Bristol, that he took particular interest in the strange "tau" meson (today known as the K meson), which on decay transformed into three pions and which would soon play a major role in his scientific career. That was to be later; it was after joining Rudolf Peierls’ group at Birmingham University in 1949 and completing his thesis that, in 1951, that he made his first seminal contribution, with his demonstration that the electrically neutral pion could decay into a photon and an electron-positron pair - the eponymous Dalitz pair. The technique of measuring Dalitz pairs was used to measure the parity both of the pion and the tau meson.

Among the strange particles were two that appeared to be like identical twins, named tau and theta, but for one feature: tau decayed into three pions, while theta turned into two. The law of parity conservation implied that such particles could decay into either an even or into an odd number of pions, but not into both. Consequently it was believed that theta and tau were different.

In 1954, Dalitz looked at the decays of the tau into three pions and in doing so introduced the Dalitz plot into physics. In the subsequent 50 years, Dalitz plots led to the discovery of scores of ephemeral particles, many living no longer than the time it would take for a light-beam to cross an atomic nucleus. His first use of the Dalitz plot revealed that the theta particle appeared to be the same as the tau, which was paradoxical.

The puzzle persisted for two years: Dalitz even musing to colleagues that perhaps the law of odds and evens, "parity", was not true, even though all the evidence said otherwise. It was TD Lee and CN Yang, who in 1956 realised the fine print in nature’s contract: the law of parity had been tested only for the strong and electromagnetic forces. For the weak force, it was an open question; and it was the weak force that was at work in the theta-tau decays. They were proved to be right, and in 1957 won a well-deserved Nobel Prize.

From 1953, Dalitz was in the US, first at Cornell University and then, from 1956, as professor at the Enrico Fermi Institute in Chicago. Around 1960 Dalitz plots of the data coming from the new high-energy particle accelerators began to reveal short lived particles – the resonances, such as the omega meson and strange baryons.

Dalitz returned to Britain in 1963 joining Rudolf Peierls at Oxford as Royal Society research professor. It was around this time that the puzzle of the proliferating particles began to give way with Murray Gell Mann’s "Eightfold Way" and the idea that a more fundamental level of reality existed in what Gell Mann called "quarks".

What was less clear was whether these quarks were just a mathematical convenience or were themselves real particles. A problem with the latter was that the quarks most naturally would have electric charges that were smaller than the proton, 2/3 or 1/3 fractions of a proton’s charge, and none had ever been seen.

Dalitz took the idea of physical quarks very seriously and proposed that they were basic blocks acting within the proton analogous to the way that an electron acts within the hydrogen atom. The laws that determine the rotary and spinning states that electrons can take up within atoms were in Dalitz’s model applied to the quarks within the proton and the resonances. In a remarkable talk in Tokyo in 1965, he showed how the idea explained properties of a proton and neutron, such as their response to a magnetic field, and then took his more radical step. The quarks in Dalitz’s scheme could be raised into different energy states, following the established rules of non-relativistic quantum mechanics. This implied the existence of many short-lived resonance states similar to, but heavier than, the proton. Some of these were already known and fitted in to the scheme perfectly. Over the following decades many other examples were discovered, in many cases by application of Dalitz plots, such that the quark model became established as the explanation of what had hitherto been a menagerie of particles.

Dalitz was also interested in hypernuclei, atomic nuclei where a nucleon had been replaced by a strange baryon. He collaborated with Avraham Gal in this area for many years. He was also intimately involved with the identification of the top quark where he thought about the problems of how one might identify it from the decay processes that seemed most natural for it. With Gary Goldstein he worked out a geometrical method by which experimental data could be used to deduce the top mass. They applied the method to an early possible event from Fermilab and concluded that if this event indeed signalled top production, the top quark mass must exceed 130 GeV. This was regarded as an unexpectedly large value at the time. This one event might not even have been due to a top quark and the confirmation could only be decided on the basis of a large number of observed events, all of them being consistent with a unique mass. This was the case later when two experimental groups came to conclude that the top quark mass was about 180GeV.

He brought scholars to Oxford, which became a centre for the quark model, and trained generations of students, including Chris Llewellyn Smith a future Director-General of CERN. Following retirement he remained an inspirational figure to students new and old, continuing to work on theoretical physics with undiminished enthusiasm. With Dick’s death, international physics has lost a major figure and Britain one of its greatest unsung scientists.

As the next phase in the quest for the ultimate nature of reality begins at CERN’s Large Hadron Collider in 2007, it is likely that evidence for Higgs Bosons, supersymmetric particles, or whatever other surprises may await, will be revealed by Dalitz plots.