  Measurements of the fine-structure constant alpha require methods from across
subfields and are thus powerful tests of the consistency of theory and
experiment in physics. Using the recoil frequency of cesium-133 atoms in a
matter-wave interferometer, we recorded the most accurate measurement of the
fine-structure constant to date: alpha = 1/137.035999046(27) at 2.0 x 10^-10
accuracy. Using multiphoton interactions (Bragg diffraction and Bloch
oscillations), we demonstrate the largest phase (12 million radians) of any
Ramsey-Borde interferometer and control systematic effects at a level of 0.12
parts per billion. Comparison with Penning trap measurements of the electron
gyromagnetic anomaly ge-2 via the Standard Model of particle physics is now
limited by the uncertainty in ge-2; a 2.5 sigma tension rejects dark photons as
the reason for the unexplained part of the muon's magnetic moment at a 99
percent confidence level. Implications for dark-sector candidates and electron
substructure may be a sign of physics beyond the Standard Model that warrants
further investigation.

Atomic Physics