Location: Jefferson Lab, Virginia, USA
Participating Canadian institutions: Manitoba, Memorial, TRIUMF, UNBC, Winnipeg
International partners: USA, France, Germany, Italy, Mexico
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Within the Standard Model, the combined electro-weak interaction strength of electrons is parameterized by the so-called electron weak charge, which is itself related to the weak mixing angle, a fundamental parameter of the Standard Model that sets the degree of mixing between the electromagnetic and weak interactions.
While design efforts for MOLLER have been ramping up over the past years, the predecessor Qweak, measuring the weak charge of the proton, saw data analysis finalized and the publication of the results 1. The weak charge of the proton was determined as \(0.0719 \pm 0.0045\), derived from the parity-violating asymmetry in the scattering of polarized electrons on protons, measured at the 9 ppb level. The result is in excellent agreement with the SM prediction, and sets multi-TeV-scale constraints on any semi-leptonic parity-violating physics not described within the SM.
The MOLLER experiment aims to make the world’s most precise off-resonance measurement of the weak mixing angle, using polarized electron-electron scattering at Jefferson Laboratory to observe the parity violating asymmetry at the percent level (see Figure 1). The experiment will test the interaction of electrons with respect to a number of new physics models and will search for electron substructure at the \(10^{-21}\) meter scale. The electron-electron interaction is the cleanest process that exists among fundamental particles, uncontaminated by nuclear interactions and is experimentally extremely well controlled, and therefore a prime candidate to search for new physics. MOLLER is an international effort, presently involving more than 120 collaborators from the USA and Canada (the lead countries), as well as Germany, Italy, France, and Mexico. The current Canadian group includes about 20 people, which makes it the second strongest contiguous group in the project, behind Jefferson Lab. MOLLER employs elastic scattering using the 11 GeV electron beam at Jefferson Lab, and it requires a large, highly efficient, and precise array of electron detectors. The Standard Model prediction for parity violating asymmetry \(A_{PV}\), for the proposed experimental design, is \(\approx 33 \times 10^{-9}\) and the 2.4 % relative error goal corresponds to an overall precision of 0.7 ppb on the asymmetry measurement, which in turn corresponds to a \(\approx 0.1\) % determination of the weak mixing angle.
The Canadian team is leading the design of the magnetic spectrometer and the integrating detectors plus associated electronics (shown in Figure 2), and further established leadership roles in simulation and analysis software. Design completion and prototype testing will finalize in 2021, and construction is scheduled to begin in 2022, with commissioning around 2025. MOLLER will take data until 2030 with a few years of data analysis to follow.
The group is also involved in the P2 experiment at the MESA facility in Mainz, Germany. The synergies with MOLLER in terms of detector electronics are substantial, and the timeline is expected to be comparable to MOLLER.