Session G03: Precision Tests of Fundamental Physics

Live

The standard model of particle physics -- a set of particles, interactions and symmetries knitted together by field theory -- is the great triumph and the great frustration of modern physics. The great triumph is its ability to predict the outcome of laboratory tests with exquisite precision. The great frustration is its inability to account for basic features of the universe, like how it survives annihilation after the big bang, why it is made of matter rather than antimatter, why does gravity not fit well, what is dark matter, etc. The magnetic and electric dipole moments of the electron, measured using completely differently methods, illustrate the crucial role of tabletop measurements for testing the standard model and beyond. The electron's magnetic moment, determined to 3 parts in 10 /{13}, is the most precisely determined property of an elementary particle. The measurement with a one-electron quantum cyclotron tests the most precise prediction of the standard model. A ten times more precise measurement is currently being pursued in light of an intriguing 2.4 standard deviation discrepancy that has recently emerged between the measurement and the prediction. A positron measurement at the new precision should provide a 200 times improved test of the standard model's fundamental CPT symmetry invariance with leptons. The electron electric dipole, measured using the incredibly strong internal field on valence electrons within cold molecules in a cold beam, is an extremely sensitive probe for physics beyond the standard model. The standard model requires 4th order perturbation theory to produce the CP violation needed for an electric dipole, while supersymmetric models and other proposed beyond-the-standard-model improvements predict much large electric dipoles moments from first order perturbation theory. Following two previous order-of-magnitude improvements in sensitivity, a new measurement seeks to improve the sensitivity by another order of magnitude.   

Live

We report results of a new technique to measure the electric dipole moment of $^{129}$Xe with $^3$He comagnetometry. Both species are polarized using spin-exchange optical pumping, transferred to a measurement cell, and transported into a magnetically shielded room, where SQUID magnetometers detect free precession in applied electric and magnetic fields. The result from a one week measurement campaign in 2017 and a 2.5 week campaign in 2018, combined with detailed study of systematic effects, is $d_A(^{129}\mathrm{Xe}) = (1.4 \pm 6.6_\mathrm{stat} \pm 2.0_\mathrm{syst})\times10^{-28}~e\,\mathrm{cm}$. This corresponds to an upper limit of $|d_A(^{129}\mathrm{Xe})| < 1.4 \times 10^{-27} ~e\,\mathrm{cm}~(95\%~\mathrm{CL})$, a factor of five more sensitive than the limit set in 2001.   

On Demand

A measurement almost a decade ago of the n=2 Lamb shift in muonic hydrogen allowed for a precise determination of the proton rms charge radius. This radius, however, showed a strong disagreement with radii determined from regular atomic hydrogen spectroscopy and electron scattering, and the disagreement has become known as the proton-radius puzzle. Here, we report on a new high-precision measurement of the atomic hydrogen n=2 Lamb shift. The measurement uses a new technique (Frequency Offset Separated Oscillatory Fields, or FOSOF), which is an innovation on the Ramsey technique. The measurement makes a precise determination of the proton charge radius, and this determination can be directly compared to the analogous determination using muonic hydrogen.