October 20, 1969

Deep inelastic scattering

MIT–SLAC deep-inelastic electron–proton results are published

The papers remembered as the discovery of quarks never claimed to have seen a quark. They reported cross sections, a structure function, and electrons bouncing far too often.

Two companion papers, pages 930 and 935

On 20 October 1969, Physical Review Letters carried two companion papers from the MIT–SLAC collaboration on high-energy, highly inelastic electron–proton scattering, back to back in volume 23.

The first, “High-Energy Inelastic Scattering at 6° and 10°,” reported the measured cross sections. The second, “Observed Behavior of Highly Inelastic Electron-Proton Scattering,” estimated the proton structure function W₂ and reported scaling behaviour within experimental error.

Electrons that bounced too often

Collisions transferring large energy happened far more frequently than a featureless proton allowed — by an APS summary’s account, 10 to 100 times more often than predicted, over an electron-energy range of 7 to 17 GeV.

That pointed to small, hard, point-like charged constituents inside the proton rather than a smeared-out charge distribution. James Bjorken had predicted the scaling behaviour the data were tested against; Richard Feynman read the same picture as nearly independent constituents he called partons.

What the papers stopped short of saying

Neither paper announced that the constituents had been identified as quarks. Quarks had already been proposed on paper by Murray Gell-Mann and George Zweig in 1964, and the match between partons and quarks firmed up through later exchange between experiment and theory.

The decisive detector readings were also older than the publication: the runs were made principally in 1967–68. October 20 is the date the analysis reached the refereed literature.

Jerome Friedman, Henry Kendall and Richard Taylor received the 1990 Nobel Prize in Physics for the deep-inelastic investigations.

Sources

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What the sources leave uncertain

The publication date and papers are unambiguous. What is retrospective is the label “discovery of quarks”: the papers directly established point-like proton constituents and scaling, while their identification as quarks developed gradually.
The experiments were performed before October 1969—principally in 1967–68. October 20 is the publication date, not the date on which the decisive detector readings were first obtained.