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Experiment Research

E07-003: Spin Asymmetries of the Nucleon Experiment

Protons and neutrons, collectively known as nucleons, are the building blocks of all atomic nuclei. Nucleons have a complicated internal structure, which gradually is being unveiled through the use of powerful electron "microscopes" that illuminate the interior of the nucleons with very short wavelength "light." The microscopes" are the particle accelerators like CEBAF (the Continuous Electron Beam Accelerator Facility) at Jefferson Lab, or the LHC (the Large Hadron Collider) in Europe. The "light" is composed of powerful real or virtual photons that, when polarized, can resolve not only the small components inside the nucleons, called partons, but can also tell us about their intrinsic motions.

SANE, the Spin Asymmetries of the Nucleon Experiment, will use the highest-energy polarized electrons and photons produced by CEBAF to illuminate a target of polarized hydrogen nuclei, or protons, in frozen ammonia. The electrons recoil in different numbers after interacting with the target, depending on the orientation of their intrinsic sense of rotation, or spin, relative to the spin of the target protons. The difference in numbers for parallel versus anti-parallel spins can be directly connected to the state of rotation of the proton’s partons.

There are two kinds of partons: quarks and gluons. The exchange of gluons is what "glues" the constituents of the proton together. In SANE, we'll explore the likelihood of interactions between two quarks and a gluon, which can be observed by counting the differential rates of recoiling electrons when the electron spins are perpendicular or anti-perpendicular to the proton spins. Powerful theoretical tools, such as computer calculations of the nucleon structure from first principles, called Lattice QCD, and others, are available to compare the measurements to be made in SANE to models of the distribution of spinning quarks and gluons inside the proton. We'll try to see the insides of the proton in motion.

About 90 physicists from 22 U.S. and overseas institutions are working on SANE, including six Ph.D. and M.S. thesis students.

E07-003 Technical Paper