Polarized Scattering Data Challenge Quantum Chromodynamics

It is a truism of elementary‐particle theory that spin effects become negligible at high energy and large momentum transfer in collisions between protons—or any other spinning hadrons. But it doesn’t appear to be true. For almost a decade now, Alan Krisch (University of Michigan) and his coworkers have been accumulating experimental evidence that spin‐spin and spin‐orbit interactions are far from negligible in large‐angle proton‐proton elastic scattering at high energies. Their most recent data with an unpolarized 28‐GeV proton beam hitting a polarized target at the Brookhaven Alternating Gradient Synchrotron indicate a quite spectacular and unanticipated growth of left‐right asymmetry (a spin‐orbit effect) as one looks at ever larger scattering angles. Two months ago this Mighigan, Brookhaven, Notre Dame, Rice, Texas A&M, Zurich collaboration reported its first data taken with a polarized proton beam at the AGS—the first time a polarized beam has been achieved in a high‐energy, strong‐focusing proton synchrotron.

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