New Clock Comparison Tests of Lorentz Symmetry

Lorentz symmetry is a fundamental feature of modern descriptions of nature. Lorentz transformations include both spatial rotations and boosts. Therefore, experimental investigations of rotation symmetry provide important tests of the framework of the standard model of particle physics and single-metric theories of gravity [1]. In particular, the minimal SU(3)×SU(2)×U(1) standard model successfully describes particle phenomenology, but is believed to be the low energy limit of a more fundamental theory that incorporates gravity. While the fundamental theory should remain invariant under Lorentz transformations, spontaneous symmetry-breaking could result at the level of the standard model in violations of local Lorentz invariance (LLI) and CPT (symmetry under simultaneous application of Charge conjugation, Parity inversion, and Time reversal) [2]. Clock comparisons provide sensitive tests of rotation invariance and hence Lorentz symmetry by bounding the frequency variation of a given clock as its orientation changes, e.g., with respect to the fixed stars [3]. In practice, the most precise limits are obtained by comparing the frequencies of two co-located clocks as they rotate with the Earth (see Fig. 1). Atomic clocks are typically used, involving the electromagnetic signals emitted or absorbed on hyperfine or Zeeman transitions. We report preliminary results from two new atomic clock tests of LLI and CPT: