Precision Measurement of the Proton and Deuteron Spin Structure Functions g2 and Asymmetries A2

Abstract

We have measured the spin structure functions g2p and g2d and the virtual photon asymmetries A2p and A2d over the kinematic range 0.02 < x < 0.8 and 0.7 < Q^2 < 20 GeV^2 by scattering 29.1 and 32.3 GeV longitudinally polarized electrons from transversely polarized NH3 and 6LiD targets. Our measured g2 approximately follows the twist-2 Wandzura-Wilczek calculation. The twist-3 reduced matrix elements d2p and d2n are less than two standard deviations from zero. The data are inconsistent with the Burkhardt-Cottingham sum rule if there is no pathological behavior as x->0. The Efremov-Leader-Teryaev integral is consistent with zero within our measured kinematic range. The absolute value of A2 is significantly smaller than the sqrt[R(1+A1)/2] limit.

Figures (4)

• Figure 1: $xg_2$ for the proton and deuteron as a function of $Q^2$ for selected values of $x.$ Data are for this experiment (solid), E143E143 (open diamond) and E155E155g2 (open square). The errors are statistical; the systematic errors are negligible. The curves show $xg_2^{WW}$ (solid) and the bag model calculation of StratmannStratmann (dash-dot).
• Figure 2: The structure function $x$g$_2$ for all spectrometers combined (solid circle) and data from E143E143_2 (open diamond) and E155E155g2 (open square). The errors are statistical; the systematic errors are shown at the bottom. Also shown is our twist-2 g$_2^{WW}$ at the average $Q^2$ of this experiment at each value of $x$ (solid line). The curves are the bag model calculations of StratmannStratmann (dash-dot) and SongSong (dot) and the chiral soliton models of Weigel and GambergWGR (short dash) and WakamatsuWaka (long dash).
• Figure 3: The asymmetry $A_2$ for all spectrometers combined (solid circle) and data from E143E143_2 (open diamond), E155E155g2 (open square), and SMCSMCg2 (open circles). The errors are statistical; the systematic errors are negligible. Also shown is $A_2^{WW}$ calcultated from the twist-2 g$_2^{WW}$ at the average $Q^2$ of this experiment at each value of $x$ (solid line). The upper Soffer limitsoffer is the dashed curve at the upper right.
• Figure 4: The twist-3 matrix element $d_2$ for the proton and neutron from the combined data from SLAC experiments E142E142_2, E143E143, E154 E154_2 and E155 (Data). Also shown are theoretical model values from left to right: Bag ModelsSongStratmannJi, QCD Sum Rules SteinBBKEhrnsperger, Lattice QCD LQCD and Chiral Soliton ModelsWGRWaka. The region between the dashed lines indicates the experimental errors.