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Salam and Ward


The fourth quark
Murray Gell-Mann Scientist
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Why did I resist the idea of a fourth quark? Why didn't I immediately embrace it? You remember that back in 1954 in Glasgow, we had presented something very like strangeness and charm. The reason was the following: if the charges of the fundamental objects were two-thirds minus a third and minus a third then they summed to zero, and then the charge could be part of a simple group–one wouldn't need a—or product of simple groups--one wouldn't need a U(1) factor. If the charges average to something non-zero, as would be the case with two-thirds minus a third and two-thirds minus a third, then I was worried about the character of the whole theory. They… you would be having a charge operator with a… with a non-zero average value. What I didn't realize was that the… it was only in the context of quarks and leptons together that we would have this situation, and indeed with charm, two-thirds and minus a third, two-thirds and minus a third times three. That gives you two, which just balances minus one for the electron and minus one for the muon. And of course when we add in the top and bottom quarks and the tau lepton, the… the situation is unchanged. So these families of quarks and leptons, with the charges adding up to zero for each family was something I hadn't considered seriously. Maybe I thought about it for a moment at some time, but I didn't take it seriously. And otherwise I probably would have embraced this idea of… of a fourth flavor because it did solve so many problems. In any case, by around 1970 the electroweak theory, the original proposal of Glashow 1959-- ’60, plus the Weinberg suggestion of the soft mass mechanism, that… you… of Anderson, Higgs, Kibble and so on, plus the charm had pretty much taken care of all the problems. But there was still the question of whether the theory was fully renormalizable. And around 1970 or ’71 I believe, 't Hooft, as a student of Tini Veltman, showed that that was the case. And that was plausible because we knew that the Yang-Mills theory without symmetry breaking was renormalizable, and here the soft mass mechanism had been supplied and so it was a plausible supposition that the whole thing would be renormalizable, but it was nice to have it shown.

New York-born physicist Murray Gell-Mann (1929-2019) was known for his creation of the eightfold way, an ordering system for subatomic particles, comparable to the periodic table. His discovery of the omega-minus particle filled a gap in the system, brought the theory wide acceptance and led to Gell-Mann's winning the Nobel Prize in Physics in 1969.

Listeners: Geoffrey West

Geoffrey West is a Staff Member, Fellow, and Program Manager for High Energy Physics at Los Alamos National Laboratory. He is also a member of The Santa Fe Institute. He is a native of England and was educated at Cambridge University (B.A. 1961). He received his Ph.D. from Stanford University in 1966 followed by post-doctoral appointments at Cornell and Harvard Universities. He returned to Stanford as a faculty member in 1970. He left to build and lead the Theoretical High Energy Physics Group at Los Alamos. He has numerous scientific publications including the editing of three books. His primary interest has been in fundamental questions in Physics, especially those concerning the elementary particles and their interactions. His long-term fascination in general scaling phenomena grew out of his work on scaling in quantum chromodynamics and the unification of all forces of nature. In 1996 this evolved into the highly productive collaboration with James Brown and Brian Enquist on the origin of allometric scaling laws in biology and the development of realistic quantitative models that analyse the influence of size on the structural and functional design of organisms.

Tags: Glasgow, Sheldon Lee Glashow, Steven Weinberg, Phil Anderson, Peter Higgs, Tom WB Kibble, Gerard 't Hooft, Martinus JG Veltman

Duration: 3 minutes, 17 seconds

Date story recorded: October 1997

Date story went live: 29 September 2010