BSM-PM: Precision Measurements of Weak-Force Induced Parity Violating Transitions in Atomic Cesium

Purdue University, West Lafayette IN

Investigators

Abstract

The Standard Model of particle physics is the most successful physical model describing the structure of the known matter of the universe and the interactions between these particles. Still, we know that the Standard Model is not complete, in that there are several features of the universe that it cannot explain. These include the asymmetry between matter and anti-matter, the identity of dark matter and dark energy, and the anomalous magnetic moment of the muon. A variety of theoretical models that extend the Standard Model, so-called beyond Standard Model (BSM) physics, have been proposed. Precision atomic, molecular, and optical physics measurements can be used to test the predictions of the Standard Model in small scale, table-top experiments, validating the standard model in this energy range, or helping elevate one of the various proposals for BSM physics. The goal of the proposed work is to precisely measure the strength of an extremely weak optical interaction in atomic cesium. This interaction is permitted only through the influence of the weak force interaction between electrons and the cesium nucleus, or between the nucleons themselves. A precision measurement of the strength of this interaction provides a precise value of the weak charge of the cesium nucleus, which in turn is used to determine a quantity known as the electro-weak mixing angle. The goal of the present program is to improve the precision of the value of the electro-weak mixing angle in the low energy range, testing the level of agreement with the Standard Model prediction, and possibly discriminating between various BSM models. In addition to its impact on our understanding of the universe, this program provides a valuable laboratory experience for graduate and undergraduate students. Students gain strength in physics knowledge and technical skills in lasers and optics, vacuum techniques, electronics, data acquisition and analysis, and technical writing, which they can apply to solving other important technical problems in industry or academics. One set of measurements proposed in this program will be carried out on the optical transition from the ground 6S state to the excited 7S state in atomic cesium. The weak force interaction, which is not symmetric upon spatial inversion, slightly mixes atomic states of opposite parity (S- and P-states, for example), and introduces a weak electric dipole (E1) transition moment between the 6S ground state and 7S excited state. Using a two-pathway coherent control technique, and applying a uniform static electric field to the atoms, the investigators will drive the transition concurrently with two optical interactions, one a linear interaction with 539.5 nm light (the weak-force allowed transition and a Stark-induced transition); the second a two-photon interaction with light at 852 nm and 1470 nm. The laser beams are phase locked to one another, assuring mutual coherence between the optical interactions. The interference between the strong two-photon interaction and the weak linear interactions can be controlled by varying the optical phase difference between the various laser beams, resulting in modulation of the total excitation rate. Precise measurement of the amplitude of the modulation allows a precise determination of the ratio of the E1 moment for the transition and the Stark transition polarizability. The goal of the measurement is to reduce the uncertainty of this ratio to a value approaching 0.1%. A second measurement is based on the extremely weak E1 transition between hyperfine components of the ground state. The largest contribution to this nuclear-spin-dependent interaction is due to the anapole moment of the nucleus, which results from a parity-odd nuclear current produced by the weak force interaction between nucleons. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.