Test of spatial isotropy using a cryogenic torsion pendulum

Abstract

Motion of the earth through the cosmic neutrino background, or through certain kinds of vacuum states, produces a term of the form gσ⋅v in the energy of an electron. To search for such a term a cryogenic torsion pendulum carrying a transversely polarized magnet was used. Superconducting shields reduced magnetic torques. A σ⋅v term would produce a sinusoidal oscillation of the pendulum with a period of one sidereal day. Such an oscillation was not detected, and a new limit of 8.5×${10}^{\mathrm{-}18}$ eV for the splitting of the spin states of an electron at rest on the Earth has been set.