Polarization Rotation Effects Due to Parity Violation in Atoms

Abstract

We present a study of parity (P) violating polarization rotations of atoms in external electric fields. Five different types of rotations are identified and the consequences of time reversal invariance (T) are discussed. The role played by Zeldovich′s electric dipole moment of unstable states is elucidated. To calculate the effects, we use the standard model of elementary particle physics, where P violation in atoms is due to the exchange of the Z boson between the quarks in the nucleus and the atomic electrons. We consider in detail hydrogen-like systems in n = 2 states, where n is the principal quantum number, especially 1 1H, 4 2He +, and 12 6C 5+. There one has the metastable 2 S 1/2 states which are separated from the 2 P 1/2 states with opposite parity only by the small Lamb shift leading to a relatively large P-violating state mixing. The nominal order of magnitude of the polarization rotations is 10 −11 − 10 −12 but we discuss ways to obtain in some cases large enhancement factors of order 10 4. We show that, e.g., for 1 1H one could in principle observe P-violating polarization rotations as large as a few percentages, where for a statistically significant result one would need a total of 10 15 polarized atoms. We point out that some of our P-violating polarization rotations am very sensitive to the nuclear spin-dependent part of the P-violating Hamiltonian which receives a contribution from the polarized strange quark density in polarized nuclei. Thus a measurement of P-violating polarization rotations of atoms would be of great interest in at least three respects: it would demonstrate the existence of quantum mechanical effects in atoms where P violation and T conservation interplay in a subtle way; it would allow a measurement of polarized quark densities in polarized nuclei, thus shedding light on problems associated with the "spin crisis" for the nucleons; and it would allow precision determinations of the electroweak parameters related to the Z boson exchange at very low energies. This shows again the interplay of atomic, nuclear, and particle physics in P-violating effects in atoms.