The Quantum Story: A History in 40 Moments by Jim Baggott

Author:Jim Baggott [Baggott, Jim]
Language: eng
Format: epub, mobi
Tags: History, Science, Physics, Quantum Theory - History, Time, Quantum Theory
ISBN: 9780199566846
Google: LF8nTeqmrYYC
Amazon: 0199566844
Publisher: Oxford University Press
Published: 2011-04-15T00:00:00+00:00

25

Of Charm and Weak Neutral Currents

Harvard, February 1970

The SLAC results on deep inelastic scattering of electrons from protons were still interpreted with some caution. There were many questions that remained unanswered. But, for the advocates of the quark model, here was the first evidence that these ultimate constituents of matter might actually be real.

Sheldon Glashow was an early convert to the quark model. In fact, based on little more than an appeal to an essential tidiness in nature, he had speculated with James Bjorken in 1964 that there should be a fourth quark, which they had called charm. At that time the four leptons—electron, electron neutrino, muon, and muon neutrino—seemed at odds with just three quarks—up, down, and strange. A fourth quark was surely needed to even things up.

Glashow had collaborated with Bjorken whilst visiting the Niels Bohr Institute in Copenhagen. For Bjorken, this was an unusual line of reasoning. More used to anticipating the properties and behaviour of particles likely to have implications for experiment, this was for him, at best, a highly speculative proposal. ‘You have to remember the flavour of the time,’ he explained, ‘It was an easygoing time for models. Models came and went.’ He was nevertheless rather ambivalent about the proposal. Instead of using his own name, James Daniel Bjorken, he signed the paper ‘B.J. Bjørken’, using his nickname, ‘BJ’, and a Danish interpretation of his originally Swedish surname.

Glashow’s attempts to build an SU(2) × U(1) field theory of the weak and electromagnetic forces had foundered on the problem of the Z0 boson and the weak neutral currents it implied. With Bjorken he had actually arrived at the solution to this problem, but he had failed to make the connection. It seems that Glashow had forgotten all about his earlier efforts. ‘The problem which was explicitly posed in 1961 was solved, in principle, in 1964. No one, least of all me, knew it.’

He returned to the problems of the SU(2) × U(1) theory in 1970, in the company of two Harvard postdoctoral students, Greek physicist John Iliopoulos and Italian Luciano Maiani. Glashow had first met Iliopoulos at CERN and had been impressed with his efforts to find ways to renormalize a field theory of the weak force. Maiani arrived at Harvard with some curious ideas about the strength of weak-force interactions. All three realized that their interests converged. At this stage none was aware of Weinberg’s use of spontaneous symmetry-breaking and the Higgs mechanism.

The three physicists now wrestled with the theory once again. But they kept running up against the same problems. Admitting the field particle masses by hand produced unruly divergences. Then there was the problem of the weak neutral currents. The theory predicted that a neutral kaon should in principle decay by emission of a Z0 boson, changing the strangeness of the particle in the process and producing two muons. However, this was a decay mode for which there was absolutely no experimental evidence. For some reason, this particular reaction was being suppressed. But how?

Glashow was finally ready to put two-and-two together.

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