The electron magnetic moment, $-\mu /{\mu }_B=g/2=1.00115965218059(13)$ [0.13 ppt], is determined 2.2 times more accurately than the value that stood for fourteen years. The most precisely determined property of an elementary particle tests the most precise prediction of the standard model (SM) to 1 part in ${10}^{12}$. The test would improve an order of magnitude if the uncertainty from discrepant measurements of the fine structure constant $\alpha$ is eliminated since the SM prediction is a function of $\alpha$. The new measurement and SM theory together predict ${\alpha }^{-1}=137.035999166(15)$ [0.11 ppb] with an uncertainty 10 times smaller than the current disagreement between measured $\alpha$ values.

We propose using trapped electrons as high-$Q$ resonators for detecting meV dark photon dark matter. When the rest energy of the dark photon matches the energy splitting of the two lowest cyclotron levels, the first excited state of the electron cyclotron will be resonantly excited. A proof-of-principle measurement, carried out with one electron, demonstrates that the method is background free over a 7.4 day search. It sets a limit on dark photon dark matter at 148 GHz (0.6 meV) that is around 75 times better than previous constraints. Dark photon dark matter in the 0.1–1 meV mass range (20–200 GHz) could likely be detected at a similar sensitivity in an apparatus designed for dark photon detection.

The electron and positron magnetic moments are the most precise prediction of the standard model of particle physics. The most accurate measurement of a property of an elementary particle has been made to test this result. A new experimental method is now being employed in an attempt to improve the measurement accuracy by an order of magnitude. Positrons from a “student source” now suffice for the experiment. Progress toward a new measurement is summarized.

Lasers are used to choose the binding energies of highly excited positronium atoms (Ps*) created via charge exchange collisions. The lasers directly excite cesium (Cs) atoms to highly excited states (Cs*). These large Cs*atoms have a large cross section for resonant charge exchange collisions with trapped positrons. Highly excited Ps* is formed with a binding energy that is determined by the initial laser excitation. These Ps* should have a large cross section for resonant charge exchange collisions with trapped antiprotons—suggesting a possible new way to produce cold antihydrogen

We propose using highly excited cyclotron states of a trapped electron to detect meV axion and dark-photon dark matter, marking a significant improvement over our previous proposal and demonstration [One-electron quantum cyclotron as a milli-ev dark-photon detector, .]. When the axion mass matches the cyclotron frequency ω c , the cyclotron state is resonantly excited, with a transition probability proportional to its initial quantum number, n c . The sensitivity is enhanced by taking n c ∼ 10 6 ( 0.1 meV ω c ) 2 . By optimizing key experimental parameters, we minimize the required averaging time for cyclotron detection to t ave ∼ 10 − 6 s, permitting detection of such a highly excited state before its decay. An open–end-cap trap design enables the external photon signal to be directed into the trap, rendering our background-free detector compatible with large focusing cavities, such as the BREAD proposal, while capitalizing on their strong magnetic fields. Furthermore, the axion conversion rate can be coherently enhanced by incorporating layers of dielectrics with alternating refractive indices within the cavity. Collectively, these optimizations enable us to probe the QCD axion parameter space from 0.1 to 2.3 meV (25–560 GHz), covering a substantial portion of the predicted postinflationary QCD axion mass range. This sensitivity corresponds to probing the kinetic mixing parameter of the dark photon down to ε ≈ 2 × 10 − 16 .