Testing Lorentz invariance in the weak decay of polarized atoms

Abstract

Symmetries of space time, and Lorentz and CPT symmetry in particular, are a cornerstone of modern physics and lie at the foundation of quantum field theory (QFT) and Einstein's theory of general relativity, the two most successful theories in physics which together describe the four fundamental forces of nature. However, the inability to incorporate gravity as described by general relativity into the QFT standard model of particle physics, which very successfully combines the electromagnetic, strong and weak interactions, has led to the development of alternative so-called Grand Unified Theories (GUTs) or theories of quantum gravity. Since many of these theories break Lorentz symmetry at some small level 1-4, experimental searches for Lorentz-violating effects could help shed light on new physics beyond the standard model (SM) and provide clues as to the nature of quantum gravity. Parameterization of such effects within the Lorentz-violating Standard Model Extension (SME) developed by A. Kostelecky 5 has allowed direct comparison of many experiments, ranging from table-top precision measurements to observations of ultra-high energy cosmic and gamma rays to astrophysical observations. The SME has proven to be a remarkable tool in the search for Lorentz violation across the landscape of experimental physics. Up until the present, experimental results have taken the form of upper bounds on the SME coefficients and are tabulated in the Data Tables for Lorentz and CPT Violation 6. On a closer look, however, Lorentz invariance has been tested poorly in the weak interaction 7, 8, which is all the more surprising since violations of fundamental symmetries could be observed only in the weak sector: The first discovery of a violation of presumed (discrete) space-time symmetry was that of parity violation in nuclear β decay 9. C. S. Wu and her collaborators found that when a specific nucleus (60Co) was placed in a magnetic field, electrons from the beta decay were preferentially emitted in the direction opposite that of the aligned angular momentum of the nucleus. When it is possible to distinguish these two cases in a mirror, parity is not conserved. If the mirror not only reverses spatial direction but also changes matter to antimatter, then the experiment in front of the mirror would look just like its mirror image. The separate violations of P symmetry and C symmetry cancel to preserve CP symmetry. These symmetry violations arise only from the weak interaction, not from the strong and electromagnetic interactions, and therefore shows up strongly only in beta decay. Until 1964 it was thought that the combination CP was a valid symmetry of the Universe. That year, Christenson, Cronin, Fitch, and Turlay performed their historic experiment to see if the long-lived neutral K meson could occasionally decay to π+ π−. They found that indeed it did 10! And the observed CP violation implies, by CPT invariance, violations of T-symmetry as well.