The theories of Gell-Mann

The pioneering particle physicist, Murray Gell-Mann (1929-2019) died last week. Gell-Mann is best known for his contribution to the theory of quarks, and his classification, Eightfold Way. Those names indicate the wide range of Dr Gell-Mann’s interests. 

The “Eightfold Way” is the less obscure reference, coming straight from Buddhist philosophy and the Eightfold Path of right thought, right action, and so on. The quark’s etymology is more complicated. Sometime in 1964, Gell-Mann and Richard Feynman, his rock-star colleague at Caltech, were discussing Gell-Mann’s idea that an unknown fundamental particle could explain experimental results, if these existed. 

Feynman referred casually to these “imagined” particles as “quacks”, while Gell-Mann thought of them as “Kworks”, for some reason. The sound tickled Gell-Mann’s memory, and he fished out a reference to “Three quarks for Muster Mark” in James Joyce’s magnum opus Finnegan’s Wake. Georg Zweig, who had independently developed a similar hypothesis, referred to these particles as “aces” but the odder name, quark, stuck. 

The classification of the Eightfold Way came earlier, in 1961. Gell-Mann was looking to find some way of keeping track of many different subatomic particles. H was examining the hadrons, a group of 100-odd particles that had some similarity in strong nuclear interactions. He classified them into eight different groups, hence, the octo-name.

It was in 1964 that he postulated that hadron behaviours could be explained if each was composed by the merger of two or more fundamental particles. No such particle was known to exist, but the hypothesis worked as a mathematical explanation of hadron behaviour. This unknown particle later turned out to be the “quark”. Gell-Mann went further in postulating the existence of “gluons”, particles that “glue” hadrons together because gluons contain the strong nuclear force in analogy to the way that electrons contain electromagnetic force. 

The existence of quarks was confirmed in 1968 by data derived at the Stanford Linear Accelerator Center. That led to Gell-Mann receiving the Nobel Prize in 1969. The theory of quarks and gluons has held up ever since in thousands of experiments at high-energy particle colliders where subatomic particles are smashed together at high speed. 

Physicists now say there are six different types of quark, (each has an antimatter counterpart). Gell-Mann is considered one of the founders of the theory of quantum chromodynamics, which details the ways in which strong nuclear interactions affect particles. 

Gell-Mann was born in New York in 1929. After doing his graduation at Yale, he received his Phd at the Massachusetts Institute of Technology in 1951. He joined the California Institute of Technology, Pasadena, to give Caltech its proper name, in 1955 and taught there until 1993, as the emeritus professor holding the Robert Millikan chair. He also co-founded the Santa Fe Institute. 

Thanks to Gell-Mann and his generation, we know that atomic particles are all either quarks, or leptons. Indeed, all matter is composed of quarks and leptons (with corresponding anti-matter particles). There are six quarks and six leptons. Leptons such as the electron, the muon and the neutrinos can exist in isolation and they don’t experience strong nuclear interactions. 

Quarks only exist in combination with other quarks. The taxonomy and nomenclature of types of quarks is also quirky, due to the influence of Gell-Mann. They exist in pairs called up/ down, top/bottom and charm/strange. While leptons either have an integer charge (the electron is minus 1), or no charge at all, quarks have fractional charges. The sum of the fractional charges of several combined quarks create the integer charge of a particle like the proton. All quarks undergo strong interactions. 

Quarks also have a “colour charge” as it’s called and these are classified as red, green, and blue. This has nothing to do with colour as such. It is an analogy used by physicists (Feynman called them “idiot physicists”) to describe strong nuclear interactions. Each colour charge is linked to the antimatter “anti-colour” charge carried by the equivalent antimatter quark. 

Our understanding of how quarks interact started in the 1960s, with Gell-Mann’s generation and the mathematical predictions often preceded the experimental verification by decades. The top quark was only found in the 1990s, more than 20 years after its existence had been predicted by theory. 

The theories and hypotheses of Gell-Mann and the other particle physicists of 50 years ago, led directly to the Large Hadron Collider project, which discovered the Higgs Boson. There are plenty of hypothetical particles still out there, predicted in various hypotheses but not found. None of them have the exotic names that Gell-Mann and his followers coined.

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