Grant: Subatomic Physics Envelope - Project

The discovery of the Higgs boson at CERN was the long-awaited validation of the Standard Model, which summarizes our current understanding of fundamental particles and their interactions. The four fundamental forces are the gravitational, electromagnetic, weak and strong nuclear forces. Ordinary matter consists of a subset of the fundamental particles, electrons and up- and down- quarks bound together as protons and neutrons in the nucleus. The electromagnetic and weak forces have been successfully unified into a quantum electroweak theory, and it is a goal of subatomic theorists to unify the other two forces as well. Despite the success of the Standard Model, it is not complete. Approximately 97% of the matter and energy in the universe is unknown; this ""dark"" matter and energy is needed to explain the rotation rates of galaxies from astrophysical observations, but is not yet included in the Standard Model. In addition, the two most precise measurements of the weak mixing angle at the mass of the Z boson, which is the mediator of the weak force, do not agree with each other or the measured mass of the Higgs. The MOLLER collaboration will measure the parity-violating asymmetry in polarized electron-electron (Moller) scattering in order to make an ultra-precise measurement of the weak mixing angle. This is a quantity with a definite prediction in the Standard Model and due to the precision of the measurement any deviation from the predicted value is a sign of new physics. The asymmetry arises due to the interference of scattering via a photon (mediator of the electromagnetic force) and a Z boson. This asymmetry is expected to be on the order of 35 parts per billion (ppb), and the collaboration will measure it to within 0.73 ppb, a fractional accuracy of about 2%. The measurement will be carried out in Hall A of Jefferson Lab, using 11 GeV longitudinally polarized electrons incident on a 1.5 m liquid hydrogen target. Atoroidal spectrometer will be used to separate the Moller electrons from those which scatter from the protons in the hydrogen, and focus them at the detector plane 28 m downstream of the target. The integrated yield of the scattered electrons will be measured by an array of 224 quartz Cherenkov detectors. The MOLLER experiment will be as good as either of the two most precise measurements of the weak mixing angle made at colliders, and will help to resolve the discrepancy with the Higgs mass. It serves as a complement to direct searches for new physics at high energy colliders. Due to its precision MOLLER has discovery potential for physics beyond the Standard Model with contact interaction scales well over 10 TeV, a level of sensitivity which will not be accessible, even at colliders, for the next decade.