What Richard Henry Dalitz, who died
in Oxford, England, on 13 January 2006 following a stroke, modestly called phase space diagrams,
every other particle physicist knows as Dalitz plots. From such plots, together with Dalitz pairs
and CDD poles (named after Leonardo Castillejo, Dalitz, and Freeman Dyson), Dick's name will forever
be a byword in high-energy particle physics. (Although famous in physics, the name Dalitz is a rare
one. One of Dick's lifelong passions outside science was researching the history of his name, which
led him to become an expert historian on the Wendish—also called Sorbian—people, pre-German
settlers of Brandenburg who speak a Slavic language.)
Dick was born in Dimboola,
Australia, on 28 February 1925. After earning bachelor's degrees in mathematics and physics at
the University of Melbourne, he moved to the University of Cambridge in 1946 and in 1950 completed
his PhD on "zero–zero transitions in nuclei." In such transitions, emission of a real photon
is forbidden by angular-momentum conservation, but the transitions do allow for a longitudinally
polarized virtual photon that converts into an electron–positron pair. It was this early
insight of Dick's that would later bear fruit with the concept of Dalitz pairs—a form of internal
conversion in π0 → 2γ
decay in which one of the emitted photons becomes an e+e– pair.
While working on his PhD
thesis, Dick spent a year working alongside Cecil Powell's cosmic ray group at Bristol University
and became interested in the strange particles that were beginning to be found in cosmic rays and
at particle accelerators. Those particles included the first hypernucleus, in which one of the
nucleons is replaced by a strange baryon, and it inspired a lifelong interest in hypernuclei. The
observations also included two kinds of mesons, named θ
and τ (today known
to be the same K+ meson), with the same masses and lifetimes but distinguished by their
decays: θ into two
pions and τ into
three pions. In 1953, by then a lecturer at Birmingham University, Dick analyzed the τ
decay into three pions and in so doing introduced the Dalitz plot, a kinematic two-dimensional
diagram that reveals quantum properties of unstable particles. The plot showed that the τ
had even spin and odd parity, which is different from the θ.
The "θ-τ
puzzle"—how could two mesons have the same masses and lifetimes and yet have different quantum
numbers?—persisted for two years. Dick mused to colleagues that perhaps the law of parity
had to be abandoned, although all the evidence at the time appeared to say otherwise. The breakthrough
came from T. D. Lee and C. N. Yang, who realized in 1956 that the assumption of conserved parity in
weak interactions had not been tested, and it was the weak force that was at work in the θ-τ
decays.
Dick spent much of the next
10 years in the US, notably as a professor of physics in the Enrico Fermi Institute for Nuclear Studies
at the University of Chicago. He spent a sabbatical year at the University of California, Berkeley,
where with a hydrogen bubble chamber Luis Alvarez's team discovered the strange baryon resonance
Σ(1385) in K-p → Λπ+π-
and displayed the three final-state particles on a prominently acknowledged Dalitz plot. Dalitz
plots led to the discovery of other baryon and meson resonances, which in turn led to the idea that
a more fundamental level of reality existed in what Murray Gell-Mann called quarks in 1964. It was
initially unclear whether these fractionally charged quarks were just a mathematical convenience
for operations in SU(3) flavor symmetry or were real particles. It was around this time,
in 1963, that Dick returned to Britain and joined Rudolf Peierls at the University of Oxford as a
Royal Society Research Professor.
Dick took the idea of physical
quarks seriously and in 1965 proposed that they were the basic blocks of baryons and mesons, which
could be excited into different energy states according to the established rules of nonrelativistic
quantum mechanics and nuclear physics. To explain the pattern of the baryon SU(3) octet
and decuplet representations required assuming what became known as a symmetric quark model,
in defiance of the established antisymmetry for fermions. Over the following decades, many other
resonances were discovered for both baryons and mesons, in many cases by application of Dalitz
plots. During that time, the nonrelativistic quark model, with later incorporation of color SU(3)
effects from quantum chromodynamics, became established as an orderly description of what had
formerly been a menagerie of particles.
For Dick, quarks were real,
but as his student in 1968, I found that being asked to believe in fractionally charged particles
that no one had seen and that few outside Oxford took seriously could be demoralizing. When their
reality began to emerge in deep inelastic scattering data around that time and Richard Feynman
developed his parton model, Dick seemed hesitant to push the new area forward. With quarks as with
the θ-τ analysis
earlier and the eponymous Dalitz plot, he had helped pave the way in different manners for Nobel
Prizes, but he never made the final step himself.
Featured Jobs
