Where quarks began to seem real

20 October 1999

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On 20 October 1969, two important papers appeared in the journal Physical Review Letters*. The papers described an experiment at the Stanford Linear Accelerator Center (SLAC), which fired a high-energy beam of electrons at a target of liquid hydrogen. The electrons, from the 2-mile long linear accelerator, were scattered by collisions with protons in the hydrogen and were detected as they flew off at a variety of angles. The team, from SLAC and MIT (Massachusetts Institute of Technology), found that more electrons bounced back at wide angles and with high energies than they would expect if the protons were uniform spheres of matter. Instead, their results suggested that matter in the proton is concentrated in much smaller particles, the quarks.

This week's image shows the experiment in End Station A at SLAC that made the historic discovery. The electrons were scattered in a hydrogen "target" to the left of the image, and detected at different angles by these huge "sepctrometers", which use magnets (the blue structures) to bend the electrons, in order to measure their momentum before they are detected (within the big yellow "containers"). The spectrometers could move through different angles on the rails visible in the foreground. The people in front of the near instrument, show the scale of the apparatus. The far instrument is the one used in the experiment reported in 1969. In 1990, Jerome Friedman (MIT), Henry Kendall (MIT), and Richard Taylor (SLAC) were rewarded with the Nobel prize for their pioneering work on this experiment.

(*see E.D.Bloom et al, Physical Review Letters 23, 930, (1969), and M.Breidenbach et al, Physical Review Letters 23, 935, (1969))

Credit: Stanford Linear Accelerator Center