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| SLAC E142 | |
|---|---|
| Name | Experiment E142 |
| Institution | Stanford Linear Accelerator Center |
| Location | Stanford University |
| Dates | 1988–1989 |
| Principal investigators | Eugene D. Bloom; Anthony W. Thomas; Bertrand L. I. |
| facility = Stanford Linear Accelerator Center | beam = polarized electron beam | target = polarized ^3He gas | published = 1989 }}
SLAC E142 was a parity-conserving deep-inelastic scattering experiment performed at the Stanford Linear Accelerator Center to measure the spin-dependent structure functions of the neutron using polarized electron scattering from polarized helium-3. The collaboration used a high-polarization continuous-wave electron beam, a cryogenic polarized target, and large-acceptance detectors to probe the Bjorken-scaling regime and test predictions of the Bjorken sum rule, the Ellis–Jaffe sum rule, and perturbative Quantum Chromodynamics in the spin sector. Results provided one of the first precise extractions of the neutron spin structure function g1^n, feeding into global analyses alongside data from European Muon Collaboration, EMC, SMC, and experiments at CERN and DESY.
The experiment was motivated by anomalies observed in polarized deep-inelastic scattering by the European Muon Collaboration that suggested the quark-spin contribution to the nucleon spin was unexpectedly small. E142 set out to determine the neutron spin-dependent structure function g1^n to confront theoretical frameworks developed by James D. Bjorken and later refinements from Steven Weinberg-inspired operator product expansion techniques and perturbative corrections from Gross–Wilczek and Politzer. Using polarized ^3He as an effective polarized neutron target tied to nuclear models from Carl Friedrich von Weizsäcker-based techniques and few-body calculations by groups at Los Alamos National Laboratory and Argonne National Laboratory, the collaboration aimed to test the Bjorken sum rule and the Ellis–Jaffe sum rule implications for singlet axial charge and polarized parton distributions extracted in global fits with inputs from Martin L. Perl-era polarized scattering data.
E142 used the high-duty-factor polarized electron beam available at the Stanford Linear Accelerator Center injector and accelerator complex, incorporating polarized electron sources developed from strained-semiconductor photocathode technology championed in developments at Lawrence Berkeley National Laboratory and Yale University. Beam polarization was monitored with a Mott/Møller polarimeter system influenced by methods from Mott scattering and implementations at Brookhaven National Laboratory. The polarized ^3He target employed spin-exchange optical pumping techniques pioneered by groups at Princeton University and University of Wisconsin–Madison, with target polarization measured by nuclear magnetic resonance diagnostics similar to those used at TRIUMF.
The detector suite comprised large solid-angle spectrometers adapted from designs used at SLAC and CERN, segmented calorimetry and gas Cherenkov counters inspired by instruments at DESY and Jefferson Lab (CEBAF), and tracking chambers descended from drift-chamber technologies refined at Fermilab. Data acquisition systems built on architectures used in earlier experiments by collaborations with ties to Columbia University and Caltech provided event triggering, electronics, and on-line monitoring.
Data-taking spanned several run periods during which beam energy, target polarization direction, and spectrometer settings were varied to map kinematic coverage in Bjorken x and four-momentum transfer Q^2. The analysis pipeline combined radiative corrections frameworks developed in part at Max Planck Institute for Nuclear Physics and unfolding procedures similar to those applied by the European Muon Collaboration. Nuclear corrections to translate ^3He asymmetries into neutron asymmetries relied on few-body calculations from groups at University of Illinois Urbana–Champaign and Tennessee Technical University as well as parametrizations of unpolarized structure functions from global fits such as those by the CTEQ and MSTW collaborations.
Systematic uncertainties were constrained by cross-calibrations with polarized proton data from complementary experiments at SLAC End Station A and by simultaneous measurements of inclusive and semi-inclusive channels to check spin-dependent fragmentation effects traced back to studies at DESY and CERN. Perturbative QCD evolution of g1^n employed calculations by theorists associated with MIT, University of Pennsylvania, and Rutgers University to evolve measured moments to common Q^2 for comparison with sum-rule predictions.
E142 reported a nonzero but small value for the integral of g1^n over the measured x-range, indicating that the net quark spin contribution to the neutron was smaller than naive constituent-quark-model expectations. When combined with contemporaneous proton data from experiments at SLAC, CERN, and DESY, the results provided evidence for a significant role of gluon spin and orbital angular momentum, consistent with theoretical developments by Xiangdong Ji and others who formulated angular-momentum decompositions in QCD. The extracted neutron moments allowed a test of the Bjorken sum rule within experimental uncertainties and suggested deviations from the simple Ellis–Jaffe sum rule expectations depending on assumptions about strange-quark polarization as discussed by theorists at Brookhaven National Laboratory and CERN.
E142 stimulated refinements in radiative correction techniques and nuclear-model treatments, leading to revised global fits of polarized parton distribution functions produced by groups at Duke University, Helsinki Institute of Physics, and University of Glasgow that incorporated the new neutron constraints.
The experiment is widely cited in the development of the modern spin program, influencing subsequent polarized scattering experiments at Jefferson Lab, the polarized proton program at Relativistic Heavy Ion Collider, and planned measurements at proposed facilities such as the Electron–Ion Collider. Methodological innovations in polarized target technology, polarimetry, and systematic-control strategies from E142 were adopted by collaborations at TRIUMF and Brookhaven National Laboratory. The E142 dataset remains part of global analyses of polarized structure functions maintained by groups such as NNPDF, DSSV, and JAM and is referenced in reviews by authors at CERN and Institute for Nuclear Theory. Its contributions helped to shift the community’s view toward a multi-component picture of nucleon spin involving quark, gluon, and orbital degrees of freedom, a theme pursued by later experiments and theoretical programs at institutions including MIT, Caltech, Princeton University, and Yale University.