The MOLLER Experiment: An Ultra-Precise Measurement of the Weak Mixing Angle Using Møller Scattering

The MOLLER (Measurement Of a Lepton-Lepton Electroweak Reaction) experiment at Jefferson Lab is a next-generation parity-violating electron scattering experiment designed to precisely measure the weak charge of the electron via Møller scattering.  Using the high intensity, high precision electron beam at Jefferson Lab, MOLLER will measure sin²(θ_w) at low momentum transfer (Q²), matching precision obtained from Z-pole measurements. This will be achieved by measuring the parity-violating asymmetry, A_PV, in the scattering of longitudinally polarized electrons off unpolarized electrons at accuracies down to 0.8 parts per billion (ppb). The resulting discovery reach is unmatched by any proposed experiment measuring a flavor- and CP-conserving process at low energy in the near future, and yields a unique window to new physics at MeV and multi-TeV scales, complementary to direct searches at high energy colliders such as the Large Hadron Collider (LHC). Assembly of the experiment is expected to finish in Hall A in late 2026. The physics motivation will be discussed, along with a review of the experimental design and the latest report on the progress of assembly.