Tests of Lorentz symmetry

Abstract

A number of approaches to fundamental physics can lead to the violation of Lorentz and CPT symmetry. This talk discusses the low-energy phenomenology associated with such eects and reviews various sample experiments within this c ontext. Introduction.—The Standard Model (SM) and General Relativity (GR) provide and excel- lent phenomenological description of nature. However, from a theoretical viewpoint these two theories leave unanswered a variety of key conceptual questions. It is therefore believed that the SM and GR merge into a single unified theory at high energies that resolves these issues. One possibility for experimental research in this field is to increase the energy in experiments and hope to excite new degrees of freedom, which can give insight into such a unified theory. A complementary experimental approach is characterized by tests at comparatively low or moderate energies, but with ultra-high precision. Various eort s along these lines, such as searches for axions, axion-like particles, weakly interacting massive particles, and weakly interacting sub-eV particles, have already been discussed at this meeting. This presentation is focused on another class of precision experiments, namely tests of Lorentz and CPT symmetry. The special theory of relativity and its underlying Lorentz symmetry have been established over a century ago. Since that time, Lorentz symmetry has been subjected to numerous tests, but no credible experimental evidence for departures from Lorentz symmetry has been found. In fact, special relativity has matured into one of the most important cornerstones of physics. It provides not only the basis for present-day physics, but it is also the starting point for most theoretical approaches to new physics beyond the SM and GR. In recent years, however, it has been realized that various of these approaches to new physics (although built on Lorentz invariance) can accommodate mild, minuscule deviations from this symmetry in the ground state (1). Examples of candidate underlying models with the possibility of Lorentz violation are strings, loop quantum gravity, cosmologically varying scalars, non-commutative geometry, and multiverses (2). A further motivation for Lorentz and CPT tests is provided by the fundamental character of these symmetries: they should be backed by experimental evidence of steadily increasing quality. At energy regimes below the Planck scale, such departures from Lorentz and CPT symmetry can be described in great generality by the Standard-Model Extension (SME) (3). The SME is an eective field theory that contains both the usual SM and GR. Th e remaining terms in the SME Lagrangian control the extent of Lorentz and CPT breakdown; they are constructed to involve all operators for Lorentz and CPT violation that are scalars under coordinate changes. This broad scope guarantees widest applicability: it eliminates the association to a particular underlying theory and ensures that practically all present and near-future experiments can be