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In Search of Unparticles The standard model of particle physics, which describes the basic building blocks of the universe and the interactions among them, is incomplete. Numerous theoretical extensions have been proposed, some of which predict long-range, spin-spin interactions. To test whether such interactions exist, a laboratory spin source is normally used. Hunter et al. (p. 928 ) used Earth as a polarized spin source and looked for these interactions by changing the geographical position and the orientation of the measurement apparatus. The polarized spins mainly come from the electrons in iron-containing minerals of Earth's mantle, which align in Earth's magnetic field. The large numbers of such polarized electrons allowed the extraction of upper bounds on some of the exotic spin-spin interactions far lower than those obtained in the laboratory. |
