Twist-3 single-spin asymmetries in semi-inclusive deep-inelastic scattering

The single-spin asymmetries for a longitudinally polarized lepton beam or a longitudinally polarized nucleon target in semi-inclusive deep-inelastic scattering are twist-3 observables. We study these asymmetries in a simple diquark spectator model of the nucleon. Analogous to the case of transverse target polarization, non-vanishing asymmetries are generated by gluon exchange between the struck quark and the target system. It is pointed out that the coupling of the virtual photon to the diquark is needed in order to preserve electromagnetic gauge invariance at the twist-3 level. The calculation indicates that previous analyses of these observables are incomplete.     

Meson cloud of the nucleon in polarized semi-inclusive deep-inelastic scattering

We investigate the possibility of identifying an explicit pionic component of the nucleon through measurements of polarized ⁺⁺ baryon fragments produced in deep-inelastic leptoproduction off polarized protons, which may help to identify the physical mechanism responsible for the breaking of the Gottfried sum rule. The pion-exchange model predicts highly correlated polarizations of the ⁺⁺ and target proton, in marked contrast with the competing diquark fragmentation process. Measurement of asymmetries in polarized production may also reveal the presence of a kaon cloud in the nucleon.Communicated by: W. Weise     

QCD effects in semi-inclusive deep inelastic scattering from a polarized target

The next-to-leading logarithmic corrections to cross-sections for semi-inclusive deep inelastic scattering from a polarized target are worked out. Predictions for pion asymmetries and pion charge ratios are given.     

Single spin asymmetries in charged pion production from semi-inclusive deep inelastic scattering on a transversely polarized 3He Target at Q2 = 1.4-2.7 GeV2

We report the first measurement of target single spin asymmetries in the semi-inclusive (3)He(e,e'π(±))X reaction on a transversely polarized target. The experiment, conducted at Jefferson Lab using a 5.9 GeV electron beam, covers a range of 0.16 < x < 0.35 with 1.4 < Q(2) < 2.7 GeV(2). The Collins and Sivers moments were extracted from the azimuthal angular dependence of the measured asymmetries. The π(±) Collins moments for (3)He are consistent with zero, except for the π(+) moment at x = 0.35, which deviates from zero by 2.3σ. While the π(-) Sivers moments are consistent with zero, the π(+) Sivers moments favor negative values. The neutron results were extracted using the nucleon effective polarization and measured cross section ratios of proton to (3)He, and are largely consistent with the predictions of phenomenological fits and quark model calculations.     

On recent evidence for higher-twist effects in semi-inclusive deep-inelastic neutrino scattering

We discuss recent experimental evidence for higher-twist contributions in $\text{ν}\text{N}\text{→ μ}{}^{\mathrm{?}}\text{h}{}^{\mathrm{+}}\text{X}$. We show that the observed effects are equally well reproduced by a longitudinal phase space model, with no free parameters. With an antineutrino beam instead the corresponding effect predicted by the LPS model is found to be opposite to that expected from the high-twist calculation, and thus a cleaner test of the theory is possible.     

Final-state interactions and single-spin asymmetries in semi-inclusive deep inelastic scattering

Recent measurements from the HERMES and SMC Collaborations show a remarkably large azimuthal single-spin asymmetries A_UL and A_UT of the proton in semi-inclusive pion leptoproduction γ^*(q)p→πX. We show that final-state interactions from gluon exchange between the outgoing quark and the target spectator system lead to single-spin asymmetries in deep inelastic lepton–proton scattering at leading twist in perturbative QCD; i.e., the rescattering corrections are not power-law suppressed at large photon virtuality Q² at fixed x_bj. The existence of such single-spin asymmetries requires a phase difference between two amplitudes coupling the proton target with J^z_p=±1/2 to the same final-state, the same amplitudes which are necessary to produce a nonzero proton anomalous magnetic moment. We show that the exchange of gauge particles between the outgoing quark and the proton spectators produces a Coulomb-like complex phase which depends on the angular momentum L^z of the proton's constituents and is thus distinct for different proton spin amplitudes. The single-spin asymmetry which arises from such final-state interactions does not factorize into a product of distribution function and fragmentation function, and it is not related to the transversity distribution δq(x,Q) which correlates transversely polarized quarks with the spin of the transversely polarized target nucleon.     

Effect of triple gluon coupling in semi-inclusive polarized scattering processes

We show that the gluon helicity inside a proton and a photon can be deduced from a knowledge of a special combination of cross sections of the semi-inclusive processese+p→e+π+π+... ande ⁺+e ^?→e ⁺+e ^?+π+π+... Such a measurement could thus be used to check the QCD prediction that the gluon helicity increases linearly with lnQ ².     

The strange-sea quark spin distribution in the nucleon from inclusive and semi-inclusive deep-inelastic scattering

We propose new method which allows determination of the strange-sea quark spin distribution in the nucleon through measurement of various inclusive and semi-inclusive polarized deep inelastic electron–proton or muon–proton reactions. It is shown, that using combinations of inclusive data and semi-inclusive data containing neutral pions in the final state, it is possible to extract the strange-sea quark spin distribution. Similar result can be obtained for charged pions and some other hadrons also.     

Cross-section asymmetries in vector-meson electroproduction and in inelastic ep scattering with polarized beam and target

We discuss the cross-section asymmetries for forward diffractive vector meson (ρ⁰) production by inelastic scattering of longitudinally polarized electrons (or muons) on polarized protons. Only for the case when both the beam and the target are polarized can considerable effects be expected. On the basis of the off-diagonal GVDM we give relations between the cross section asymmetries in ep → epρ⁰ (forward direction) and in ep → eX. For a longitudinally polarized target the cross-section asymmetries in ep → epρ⁰ are predicted to be roughly twice as large as in ep → eX.     

Single-Spin asymmetries in semi-inclusive pion production

For the first time single-spin asymmetries in semi-inclusive pion production are measured by the HERMES experiment with a transversely polarised hydrogen target. Two different sine-modulations are extracted which can be related to the transversity δq(x) and Sivers $f_{1T}^{ \bot q} (x)$ quark distribution functions. The extracted sine-moments still contain small sub-leading twist contributions which can be extracted from HERMES data combining the results from the transversely polarised hydrogen target with previously measured results from a longitudinally polarised hydrogen target.     

QED corrections in deep-inelastic scattering from tensor-polarized deuteron target

The leading-log model-independent radiative corrections to deep-inelastic scattering of an unpolarized electron beam from the tensor-polarized deuteron target have been considered. The calculation is based on the covariant parameterization of the deuteron quadrupole polarization tensor and the use of the Drell-Yan-like representation in electrodynamics to describe the radiation of real and virtual particles by the initial and scattered electron.