SLAC Experiment E155

The nucleon spin structure functions g₁ and g₂ are important tools for testing models of nucleon structure and QCD. Experiments at CERN, DESY, and SLAC have measured g₁ and g₂ using deep inelastic scattering of polarized leptons on polarized nucleon targets.

The results of these experiments have established that the quark component of the nucleon helicity is much smaller than naive quark-parton model predictions. The Bjorken sum rule has been confirmed within the uncertainties of experiment and theory.

The experiment E155 at SLAC collected data in March and April of 1997. Approximately 170 million scattered electron events were recorded to tape. (Along with several billion inclusive hadron events.) The data were collected using three independent fixed-angle magnetic spectrometers, at approximately 2.75, 5.5, and 10.5 degrees. The momentum acceptance of the 2.75 and 5.5 degree spectrometers ranged from 10 to 40 GeV, with momentum resolution of 2-4%. The 10.5 degree spectrometer, new for E155, accepted events of 7 GeV to 20 GeV. Each spectrometer used threshold gas Cerenkov counters (for particle ID), a segmented lead-glass calorimeter (for energy measurement and particle ID), and plastic scintillator hodoscopes (for tracking and momentum measurement).

The polarized targets used for E155 were ¹⁵NH₃ and ⁶LiD, as targets for measuring the proton and deuteron spin structure functions respectively.

Experiment E155x recently concluded a successful two-month run at SLAC. The experiment was designed to measure the transverse spin structure functions of the proton and deuteron. The E155 target was also recently in use at TJNAF's Hall C (E93-026) and was returned to SLAC for E155x. E155x hopes to reduce the world data set errors on g₂ by a factor of three.