SLAC E158

SLAC E158
Name	E158
Facility	Stanford Linear Accelerator Center
Location	Menlo Park, California
Field	Particle physics
Status	Completed
Start	1997
End	2004
Spokesperson	Anthony W. Thomas
Collaborators	SLAC National Accelerator Laboratory; Lawrence Berkeley National Laboratory; California Institute of Technology; University of California, Berkeley; University of Oxford

SLAC E158 SLAC E158 was a parity-violating electron scattering experiment carried out at the Stanford Linear Accelerator Center to measure the weak mixing angle at low momentum transfer. The collaboration used a high-current polarized electron beam incident on a liquid hydrogen target to probe electroweak radiative corrections and test the Standard Model in a regime complementary to results from LEP, SLC, and the Tevatron. The measurement provided a precision determination that constrained new physics scenarios and informed global fits alongside results from CERN, Fermilab, and DESY.

Introduction

The E158 program aimed to determine the electroweak parameter sin^2θ_W through parity-violating asymmetry in Møller scattering, connecting experimental techniques developed at SLAC with theoretical predictions from groups associated with CERN, Fermilab, and Jefferson Lab. The collaboration drew on expertise from institutions such as Lawrence Berkeley National Laboratory, Caltech, MIT, Princeton, and the University of Oxford while interfacing with precision electroweak analyses linked to results from the Large Electron–Positron Collider, the Stanford Linear Collider, and the Tevatron Collider. The goal was to compare the low-Q^2 extraction of the weak mixing angle with high-energy determinations by the ALEPH, DELPHI, L3, and OPAL collaborations and with polarized beam results from SLD.

Experimental Setup

The experimental apparatus employed the SLAC linear accelerator and polarized electron sources similar to those used for SLC operation, benefiting from developments at institutions like SLAC, Stanford University, and Lawrence Livermore National Laboratory. Polarized electrons produced by a strained GaAs photocathode in a high-voltage photoinjector were accelerated and steered with beamline components developed with contributions from Brookhaven National Laboratory, Jefferson Lab, and TRIUMF. The target system used a cryogenic liquid hydrogen vessel engineered with collaboration inputs from Fermilab, DESY, and Rutherford Appleton Laboratory. Beam monitoring and polarization measurement relied on Møller and Compton polarimeters incorporating designs and analysis techniques from Argonne National Laboratory, University of Michigan, and Indiana University, while detectors and data acquisition hardware built on instrumentation advances from CERN’s NA48, SLAC’s SLD, and KEK. Computing for data reconstruction and simulation used software frameworks influenced by GEANT, ROOT, and Grid technologies propagated by collaborations at CERN, Fermilab, and RAL.

Measurement and Data Analysis

E158 measured the tiny parity-violating asymmetry between right- and left-handed polarized electrons scattering off electrons in hydrogen, extracting an asymmetry of order parts per billion by integrating signals in calorimeters and suppressing backgrounds informed by studies at DESY, JLab, and Fermilab. The analysis chain incorporated radiative correction calculations performed in parallel with theoretical work from groups associated with MIT, Columbia University, the University of Chicago, and the Institute for Advanced Study. Systematic checks compared results against auxiliary measurements from SLAC End Station A studies, beam modulation techniques developed at Jefferson Lab, and luminosity monitors with heritage from the HERA experiments H1 and ZEUS. Statistical treatment used methods employed in precision efforts at the SLC and LEP programs and employed global electroweak fits coordinated with teams at CERN and Fermilab.

Systematic Uncertainties

Quantifying systematic uncertainties invoked expertise from polarimetry research by teams at the University of Illinois, Yale University, and the University of Colorado, and from background estimation informed by results at the Paul Scherrer Institute and TRIUMF. Radiative corrections and electroweak theory uncertainties were evaluated against calculations from groups working with the Particle Data Group, the Max Planck Institute for Physics, and the Perimeter Institute. Beam-related helicity-correlated systematics were mitigated using techniques pioneered at SLAC and Jefferson Lab and cross-checked using instrumentation developed at LBNL and ANL. Uncertainties from detector linearity and calibration referenced practices established in experiments such as CMS, ATLAS, BaBar, and Belle.

Results and Interpretation

The final E158 result provided a precise low-Q^2 determination of the weak mixing angle, offering a test of Standard Model running predicted by electroweak theory and comparing to high-energy extractions from ATLAS, CMS, ALEPH, SLD, and CDF. The measurement constrained extensions to the Standard Model, including scenarios involving Z′ bosons, supersymmetric models motivated by collaborations at CERN and theoretical groups at Harvard, Princeton, and Caltech, and models of lepton compositeness studied at DESY and SLAC. Global fits incorporating E158 results were performed alongside inputs from neutrino scattering experiments at Fermilab and MINERvA, from muon g–2 efforts at Brookhaven and Fermilab, and from parity-violation studies at Jefferson Lab, impacting parameter spaces explored by model builders at Perimeter Institute, Kavli Institute for Theoretical Physics, and the Institute for Nuclear Theory.

Impact and Legacy

E158 influenced subsequent parity-violation experiments and polarimetry developments at Jefferson Lab, Mainz Microtron, and future proposals at the Electron-Ion Collider, and informed precision electroweak programs at CERN and Fermilab. Techniques and instrumentation refined during E158 contributed to detector and beam diagnostics used by ATLAS, CMS, BaBar, Belle II, and neutrino experiments such as NOvA and DUNE. The collaboration’s engagement with theoretical and experimental groups including the Particle Data Group, SLAC, CERN, Fermilab, JLab, MIT, and Oxford helped integrate low-energy electroweak constraints into ongoing searches for physics beyond the Standard Model pursued by collaborations across the particle physics community.

Category:Particle physics experiments