Using the SLAC 8 GeV Spectrometer to Probe Nucleon Structure, 1968–1986

The year 2019 can be viewed as the 50th anniversary of the discovery of quarks, as two pivotal papers on deep-inelastic electron-proton scattering were published in Physical Review Letters that October. But it would take another five years or more before the physics community became fully convinced that quarks existed. A pivotal detector facility involved in this discovery process was the SLAC 8 GeV Spectrometer, on which I performed my MIT Ph.D. and postdoctoral research. Unlike the 20 GeV Spectrometer used in the initial deep-inelastic scattering experiments, it could readily roll out to large angles and detect electrons that had scattered at high momentum transfers Q², enabling experimenters to test and confirm the structure-function scaling predictions of Bjorken and Feynman, which proved crucial in verifying the suggested point-like nucleon substructure. This highly flexible detector allowed physicists their first detailed look at the new “hard-scattering” regime discussed by Andrew Pickering in his 1984 book Constructing Quarks. If time permits, I will discuss the use of this spectrometer in separating the two nucleon structure functions W₁ and W₂ and, equivalently, determining the ratio R = σ_L/σ_T, which was the subject of my Ph.D. thesis and later research.