Qweak

Qweak
Name	Qweak
Location	Jefferson Lab
Date	2007–2012
Facility	Continuous Electron Beam Accelerator Facility
Type	Parity-violating electron scattering
Primary investigator	D. H. Beck
Collaborators	University of Manitoba, William & Mary, MIT
Target	Liquid hydrogen
Beam energy	1.155 GeV
Status	Completed

Qweak Qweak was a precision parity-violating electron scattering experiment conducted at Jefferson Lab to measure the weak charge of the proton through elastic scattering from a liquid hydrogen target. The experiment used the Continuous Electron Beam Accelerator Facility and a high-current polarized electron source to achieve the statistical reach needed to test predictions of the Standard Model and to search indirectly for effects from physics beyond the Standard Model. The measurement's design emphasized control of helicity-correlated beam parameters, precision electromagnetic form-factor inputs, and comprehensive evaluation of systematic uncertainties.

Overview

Qweak aimed to determine the proton weak charge, Q_W^p, which is directly related to the electroweak mixing angle at low momentum transfer and to radiative corrections from Quantum Electrodynamics and Quantum Chromodynamics. The experiment exploited parity violation in electron-proton elastic scattering, where interference between Photon and Z boson exchange produces a small longitudinal analyzing power. The physics motivation connected to tests of the Standard Model performed at disparate energy scales, complementing results from LEP, SLAC E158, and atomic parity violation experiments such as measurements in Cesium.

Experimental Setup

The apparatus was installed in Hall C of Jefferson Lab and integrated with the High Momentum Spectrometer infrastructure for beam transport and diagnostics. Polarized electrons were produced by a strained superlattice gallium arsenide photocathode and illuminated by lasers stabilized against helicity-correlated intensity fluctuations, techniques developed at MIT and Yale University. The liquid hydrogen target was engineered by teams including University of Virginia personnel to tolerate a 180 μA continuous beam with minimal density fluctuations. Scattered electrons at a central momentum transfer of Q^2 ≈ 0.025 (GeV/c)^2 were selected by a large-acceptance toroidal magnetic spectrometer and collected in synthetic quartz Cherenkov detectors developed with institutions such as University of Manitoba and George Washington University.

Measurement and Methodology

The parity-violating asymmetry was measured as the relative difference in integrated detector signals between right- and left-handed beam helicity states, with rapid helicity reversal implemented via a Pockels cell system pioneered in polarized source programs at SLAC National Accelerator Laboratory. Simultaneous monitoring of beam position, angle, energy, and current used Beam Position Monitors and cavity current monitors designed by groups at MIT and University of Kentucky. Backgrounds from elastic scattering off target windows, inelastic processes, and neutral backgrounds were measured using dedicated calibrations and ancillary detectors modeled in Monte Carlo codes maintained by teams from Los Alamos National Laboratory and University of Winnipeg. The Q^2 calibration employed tracking detectors assembled by collaborators at University of Regina and George Mason University.

Results and Analysis

The final published asymmetry led to an extraction of the proton weak charge after applying corrections for electromagnetic form factors and radiative processes. Analysis incorporated global fits to proton and neutron electromagnetic form factors from experiments at Mainz Microtron, MIT-Bates, and Jefferson Lab itself. The extracted value for Q_W^p was compared with high-precision predictions from the Standard Model including electroweak radiative corrections computed using techniques from perturbative Quantum Field Theory and input parameters like the Fermi coupling constant and running of the weak mixing angle. Results constrained possible contributions from new neutral current interactions parameterized in model frameworks such as extra Z' boson scenarios and compositeness scales explored at CERN.

Systematic Uncertainties

Systematic studies addressed helicity-correlated beam asymmetries, detector nonlinearity, target density fluctuations, and background subtraction. Helicity control and reversal patterns minimized false asymmetries originally characterized in polarized experiments at SLAC. Detector linearity and gain stability were traced with LED systems and periodic low-current calibration runs coordinated with instrumentation from TRIUMF collaborators. Radiative corrections and two-photon exchange contributions were evaluated using theoretical input from groups at MIT and Caltech, and uncertainties associated with electromagnetic form factors were propagated using covariance matrices assembled from world data, including measurements from Saskatchewan, Mainz, and Jefferson Lab experiments.

Theoretical Implications

The Qweak result provided a low-energy determination of the weak mixing angle, complementing high-energy measurements at LEP and SLAC and atomic parity violation constraints from Cesium and Thallium experiments. Agreement with the Standard Model limited the parameter space for extensions such as extra Z' boson models, leptoquark scenarios discussed in Grand Unified Theory contexts, and certain low-mass dark sector portals considered in phenomenology studies at Fermilab. Discrepancies at the level of the experimental uncertainty would have signaled isospin-dependent new physics or underestimated hadronic effects; the Qweak analysis engaged theoretical groups specializing in electroweak radiative corrections and hadronic structure from institutions like MIT and University of Washington.

Collaboration and Timeline

The collaboration included over 100 scientists from institutions such as William & Mary, University of Manitoba, MIT, University of Virginia, and Los Alamos National Laboratory. The proposal and design phase followed commissioning of polarized source and Hall C infrastructure in the early 2000s, with data taking spanning approximately 2010–2012 and analysis continuing through the following decade. Principal investigators included experimentalists with prior leadership roles in parity-violation programs at SLAC and Jefferson Lab, and the collaboration maintained active ties with theoretical groups at Caltech and University of Washington for interpretation and radiative-correction work. The experiment's legacy informs ongoing parity-violation initiatives and constraints used by global electroweak fits performed by groups at CERN and Fermilab.

Category:Parity-violating electron scattering experiments