Probing New Physics with Spectroscopy of Trapped Ions

Abstract

Dark matter, a missing puzzle piece in our understanding of the Universe, remains dark despite abundant evidence for its existence and concerted experimental searches for candidate particles. In recent decades, experiments with atomic systems, driven by unprecedented developments in precision, have been providing tests for the Standard Model (SM), our current understanding of the Universe, and probing physics beyond the SM, including dark matter. In particular, it has been proposed that a new hypothetical elementary boson, a dark-matter candidate, can violate an SM prediction: linear distributions of measured isotope shifts (ISs) mapped onto graphs called King plots [1, 2, 3]. The prediction can be tested purely experimentally. If the violation is observed, however, possible new-physics contribution has to be distinguished from higher-order SM corrections originating from nuclear physics.

This thesis reports IS spectroscopy experiments with laser-cooled and trapped singly ionized Ytterbium (Yb⁺) ions to search for new physics through the proposed novel method. The King-plot nonlinearities thus observed for the optical clock transitions in Yb⁺ ions with significance up to 240 standard deviations 𝜎 and their implications to the new boson and nuclear physics are presented. In particular, there is a dominant, common source of nonlinearity originating from nuclear charge distributions and yet a small, second source of unknown origin with 4.3𝜎 significance. Pattern analysis of the nonlinearity in the King plots has been developed as a method for identifying or removing the sources of the observed nonlinearity. Atomic and nuclear structure calculations translate the measured nonlinearity patterns into bounds on new-boson interaction between subatomic particles as well as information on nuclear properties. The atomic structure calculations performed for Yb⁺ ions are illustrated in detail. Outlook and future works are discussed, including measurements for more transitions and isotopes and improving the experimental precision.