Abstract
A measurement of the virtual-photon asymmetry A_2(x,Q^2) and of the spin-structure function g_2(x,Q^2) of the proton are presented for the kinematic range 0.004 < x < 0.9 and 0.18 GeV^2 < Q^2 < 20 GeV^2. The data were collected by the HERMES experiment at the HERA storage ring at DESY while studying inclusive deep-inelastic scattering of 27.6 GeV longitudinally polarized leptons off a transversely polarized hydrogen gas target. The results are consistent with previous experimental data from CERN and SLAC. For the x-range covered, the measured integral of g_2(x) converges to the null result of the Burkhardt-Cottingham sum rule. The x^2 moment of the twist-3 contribution to g_2(x) is found to be compatible with zero.
Highlights
The description of inclusive deep-inelastic scattering of longitudinally polarized charged leptons off polarized nucleons requires two nucleon spin-structure functions, g1(x, Q2) and g2(x, Q2), in addition to the well-known structure functions F1(x, Q2) and F2(x, Q2) [1]
Its properties can be interpreted in the framework of the operator product expansion (OPE) analysis [2,3,4], which shows that g2(x, Q2) is related to the matrix elements of both twist-2 and twist-3 operators
The moment d2 has been linked to the transverse force acting on the quark that absorbed the virtual photon in a transversely polarized nucleon, and to the Sivers effect [8,9,10]
Summary
The description of inclusive deep-inelastic scattering of longitudinally polarized charged leptons off polarized nucleons requires two nucleon spin-structure functions, g1(x, Q2) and g2(x, Q2), in addition to the well-known structure functions F1(x, Q2) and F2(x, Q2) [1]. The measurement of the structure function g2 requires a longitudinally polarized beam and a transversely polarized target. The polarization-related cross section σLT is significantly smaller than the polarization-averaged part σUU and its measurement requires high statistical precision.
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