Abstract
An overview of more recent and important results from the HERMES experiment is presented in this paper. HERMES collected a wealth of data using the 27.6 GeV polarized HERA lepton beam and various polarized and unpolarized gaseous targets. This allows for a series of unique measurements of observables sensitive to the multidimensional structure of the nucleon. Amongst them are the semi-inclusive deep-inelastic scattering measurements of azimuthal modulations sensitive to the leading-twist transverse momentum distributions and the distributions sensitive to convolutions of the twist-2 and twist-3 distribution and fragmentation functions. They all provide information on the three-momentum-dependent quark distributions. Knowledge about the quark distribution as a function of longitudinal momentum and transverse position in impact-parameter space can be accessed, e.g., through exclusive vector–meson leptoproduction. Particularly, the measurement of spin-density-matrix elements on an unpolarized hydrogen target and the measurement of azimuthal modulations on transversely polarized hydrogen target for the ω–meson production are described in this paper. The results of the measurement of ratios of helicity amplitudes for the ρ0 –meson leptoproduction on transversely polarized hydrogen target are also presented.
Highlights
In case of longitudinal beam (L) and unpolarized target (U) only target spin-independent parts can contribute to the asymmetry
The solid lines show the calculation of the GK model for a positive πω transition form factor Dashed lines are the model results without the pion pole
With 0.4 fm < rG < 1.0 fm Hermes & BBCNC agree well Method of unlike–sign pairs (MUS) values are higher than Method of event mixing (MEM)
Summary
In case of longitudinal beam (L) and unpolarized target (U) only target spin-independent parts can contribute to the asymmetry. The role of the twist-3 DF or FF is sizeable ω –meson production: SDMEs & AUT from exclusive DIS. Unpolarized & longitudinally polarized e+/e- beam Unpolarized H & D targets Transversely polarized H targets. Exclusive ω - meson production at HERMES e(k) + N(p) ⤍ e(k’) + N(p’) + ω ω ⤍ π+ π− π0 , π0 ⤍ 2 γ. Kinematic conditions: 1 GeV2 < Q2 < 10 GeV2 , 0.01 < x B < 0.35 , 3.0 GeV < W < 6.3 GeV , 0 ≤ −t’ = − (t – tmin) < 0.2 GeV2
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