The SLAC-MIT experiment

  With the new SLAC electron accelerator, the intention was to continue and, hopefully, improve of Hofstadter’s investigations of the charge structure of the proton. Following initial experiments in which the research team studied “elastic” collisions without finding anything exciting, the part of the team which included this year’s Nobel Laureates started to investigate “inelastic collisions“. In this, the SLAC-MIT experiment, the scattering angle and energy loss of the electron after the collision were observed: these together give a measure of whether the collision really is inelastic, and also of the degree of inelasticity. By selecting electrons, which scattered at increasingly large angles and with increasing energy losses (corresponding to photons of decreasing wavelengths), the research team analysed deeper and deeper layers of the proton. Earlier investigations of the proton at low energies had shown that this ought to be “soft” with a relatively even internal distribution of its electrical charge. This year’s Laureates therefore had reason to believe there would be a decline in the probability of photon absorption (low number of events). But they found instead a high probability level (many events), i.e. there seemed to be something small and “hard” inside the proton.
     Thus the new investigations gave the surprising result that the electrical charge within the proton is concentrated to smaller components of negligible size.
     This unexpected discovery by the 1990 Laureates was noted immediately by certain skilled theoreticians, chiefly R.P. Feynman and J.D. Bjorken. The result was first interpreted within the framework of what is termed the parton model, which, however, soon came to be identified with the quark model. Very thorough analysis of the experimental results also gave the first hint of the existence of the neutral “glue particles”, later to be called gluons.  

Large magnetic spectrometers were designed and built to achieve high quality in the measurements. Taylor directed the work on the mechanics and the magnets while Friedman and Kendall designed the detectors that were to record the scattered electrons after collisions in a target of liquid hydrogen or deuterium. The spectrometer arms are turned on rails around the target to select different angles.

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