Measurement of a Lepton-Lepton Electroweak Reaction (MOLLER): An Ultra-precise Measurement of the Weak Mixing Angle using Møller Scattering at Jefferson Laboratory

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The Standard Model of Particles and Interactions summarizes our current understanding of the fundamental particles in nature, and the interactions they undergo. There are four forces in nature: the gravitational force, the electromagnetic force and the weak and strong nuclear forces. Some fundamental particles include electrons and the quarks that make up protons and neutrons. The strength with which these particles interact via a particular force depends on the size of the “charge” for that force. The weak and electromagnetic forces have been unified and it is a goal of subatomic theorists to unify the other two forces as well. The discovery of the Higgs boson at CERN validates the Standard Model to a certain extent, but there are still things we do not know. Dark matter and dark energy, for example, are not yet included in the Standard Model. Therefore testing the Standard Model has a high priority within the subatomic physics community. The MOLLER collaboration proposes to measure the parity-violating asymmetry in polarized electron-electron (Moller) scattering. In the Standard Model, this asymmetry is due to the interference between the electromagnetic and the weak forces, the latter being mediated by the Z0 boson. The asymmetry is predicted to be ~35 parts per billion (ppb) at the kinematics of the MOLLER experiment. The goal is to measure its value to a precision of 0.73 ppb. The result would yield a measurement of the weak charge of the electron to a fractional accuracy of 2.3% at an average Q2 of 0.0056 GeV^2. The measurement is sensitive to the interference of the electromagnetic amplitude with new neutral current amplitudes as weak as ~10^-3 G_F from as yet undiscovered high energy dynamics. Such a level of sensitivity is unlikely to be matched by any experiment measuring a flavor- and CP-conserving process over the next decade, and results in a unique window to new physics at the multi-TeV scale in a manner complementary to direct searches at high energy colliders. In the Standard Model, the measurement of the weak charge of the electron yields a determination of the weak mixing angle with an uncertainty of +/- 0.00026 (stat) +/- 0.00013 (syst), similar to the accuracy of the single best such determination from high energy colliders. Thus, our result could potentially influence the central value of this fundamental electroweak parameter, a critical input to deciphering signals of any physics beyond the Standard Model that might be observed at the Large Hadron Collider (LHC). The measurement will be carried out in Hall A at Jefferson Laboratory, using 11 GeV longitudinally polarized electrons incident on a liquid hydrogen target. A spectrometer will be used to separate the Moller electrons (beam electrons scattering off target electrons) from background and focus them ~ 30 m downstream of the target. The integrated flux of scattered electrons will be measured by a system of quartz Cherenkov detectors.