Resonances and higher twist in polarized lepton-nucleon scattering

We present a detailed analysis of resonance contributions in the context of higher twist effects in the moments of the proton spin structure function g_1. For each of these moments, it is found that there exists a characteristic Q^2 region in which (perturbative) higher twist corrections coexist with (non-perturbative) resonance contribution of comparable magnitude.Comment: 17 pages LaTe

DVCS on the nucleon : study of the twist-3 effects

We estimate the size of the twist-3 effects on deeply virtual Compton scattering (DVCS) observables, in the Wandzura-Wilczek approximation. We present results in the valence region for the DVCS cross sections, charge asymmetries and single spin asymmetries, to twist-3 accuracy.Comment: 20 pages, 6 figure

Theory of deeply virtual Compton scattering on the nucleon

We compute the cross section for leptoproduction of the real photon off the nucleon, which is sensitive to the deeply virtual Compton scattering amplitude with power accuracy. Our considerations go beyond the leading twist and involve the complete analysis in the twist-three approximation. We discuss consequences of the target and lepton beam polarizations for accessing the generalized parton distributions from experimental measurements of the azimuthal angular dependence of the final state photon or nucleon. We introduce several sets of asymmetries, defined as Fourier moments with respect to the azimuthal angle, which allow for a clear separation of the twist-two and -three sectors. Relying on a simple ansatz for the generalized parton distributions, we give quantitative estimates for azimuthal and spin asymmetries, discuss the uncertainties of these predictions brought in by radiative corrections, and compare them with experimental data as well as other theoretical expectations. Furthermore, we derive a general parametrization of the DVCS amplitudes in the region of small Bjorken variable.Comment: 76 pages, LaTeX, 16 figures, 3 tables, minor correction

Hard exclusive electroproduction of two pions and their resonances

We study the hard exclusive production of two pions in the virtual photon fragmentation region with various invariant masses including the resonance region. The amplitude is expressed in terms of two-pion light cone distribution amplitudes ($2\pi$DA's). We derive dispersion relations for these amplitudes, which enables us to fix them completely in terms of $\pi\pi$ scattering phases and a few low-energy subtraction constants determind by the effective chiral lagrangian. Quantitative estimates of the resonance as well $\pi\pi$ background DA's at low normalization point are made. We also prove certain new soft pion theorem relating two-pion DA's to the one-pion DA. Crossing relations between $2\pi$DA's and parton distributions in a pion are discussed. We demontrate that by studying the shape of the $\pi\pi$ mass spectra (not absolute cross section!) in a diffractive electroproduction one can extract the deviation of the meson ($\pi,\rho,$ etc.) wave functions from their asymptotic form $6 z(1-z)$ and hence to get important information about the structure of mesons. Also we discuss how the moments of quark distributions in a pion can be measured in the hard diffraction. We suggest (alternative to S\"oding's) parametrization of $\pi\pi$ spectra which is suitable for large photon virtuality.Comment: 30 pages, one figure. Discussion of crossing relations between meson distribution amplitudes and quark densities is adde

Subacromial impingement in patients with whiplash injury to the cervical spine

<p>Abstract</p> <p>Background</p> <p>Impingement syndrome and shoulder pain have been reported to occur in a proportion of patients following whiplash injuries to the neck. In this study we aim to examine these findings to establish the association between subacromial impingement and whiplash injuries to the cervical spine.</p> <p>Methods and results</p> <p>We examined 220 patients who had presented to the senior author for a medico-legal report following a whiplash injury to the neck. All patients were assessed for clinical evidence of subacromial impingement. 56/220 patients (26%) had developed shoulder pain following the injury; of these, 11/220 (5%) had clinical evidence of impingement syndrome. Only 3/11 patients (27%) had the diagnosis made prior to evaluation for their medico-legal report. In the majority, other clinicians had overlooked the diagnosis. The seatbelt shoulder was involved in 83% of cases (p < 0.03).</p> <p>Conclusion</p> <p>After a neck injury a significant proportion of patients present with shoulder pain, some of whom have treatable shoulder pathology such as impingement syndrome. The diagnosis is, however, frequently overlooked and shoulder pain is attributed to pain radiating from the neck resulting in long delays before treatment. It is important that this is appreciated and patients are specifically examined for signs of subacromial impingement after whiplash injuries to the neck. Direct seatbelt trauma to the shoulder is one possible explanation for its aetiology.</p

Lorentz invariance relations between parton distributions and the Wandzura-Wilczek approximation

The violation of the so-called Lorentz invariance relations between parton distribution functions is considered in a model independent way. It is shown that these relations are not violated in a generalized Wandzura-Wilczek approximation, indicating that numerically their violation may be small.Comment: 13 pages, added references, minor changes, to appear in Phys. Lett.

Twist-4 contribution to unpolarized structure functions F_L and F_2 from instantons

We compute in the instanton vacuum the nucleon matrix elements of the twist-4 QCD operators describing power corrections to the second moments of the unpolarized structure functions, F_L and F_2. Our approach takes into account the leading contribution in the packing fraction of the instanton medium, rho / R << 1. Parametrically leading are the matrix elements of a twist-4 quark-gluon operator, which are of the order of the inverse instanton size, 1/rho^2 = (600 MeV)^2. The matrix elements of the four-fermion (diquark) operators are suppressed by a factor (rho / R)^4 and numerically small. These results are in agreement with the pattern of phenomenological 1/Q^2-corrections to R = sigma_L / sigma_T and F_2 found in QCD fits to the data. In particular, the rise of R at low Q^2 can be obtained from instanton-type vacuum fluctuations at a low scale.Comment: 38 pages, LaTeX. 4 figures included using eps

Twist-3 contribution to the γ∗γ→ππ\gamma^*\gamma\to \pi\pi amplitude in the Wandzura-Wilczek approximation

We have calculated the Wandzura-Wilczek contribution to the twist-3 part of $\gamma^*\gamma\to 2\pi$ amplitude. It describes interaction of the longitudinally polarized virtual photon with the real one, and it is suppressed by 1/Q, where $Q^2$ is the virtuality of the $\gamma^*$, as compared to the twist-2 contribution. We have found that, in the Wandzura-Wilczek approximation, factorization applies to the twist-3 amplitude.Comment: 13 pages, 4 figure

Spin-dependent twist-4 matrix elements from the instanton vacuum: Flavor-singlet and nonsinglet

We estimate the twist-4 spin-1 nucleon matrix element f_2 in an instanton-based description of the QCD vacuum. In addition to the flavor-nonsinglet we compute also the flavor-singlet matrix element, which appears in next-to-leading order of the (1/N_c)-expansion. The corresponding twist-3 spin-2 matrix elements d_2 are suppressed in the packing fraction of the instanton medium, (\bar \rho)/(\bar R) << 1. We use our results to estimate the leading (1/Q^2) power corrections to the first moment of the proton and neutron spin structure functions G_1, as well as the intrinsic charm contribution to the nucleon spin.Comment: 17 pages, 4 eps figures include

Azimuthal single spin asymmetries in SIDIS in the light of chiral symmetry breaking

An attempt is made to understand the z-dependence of the azimuthal single spin asymmetries observed by the HERMES collaboration in terms of chiral models based on effective quark and Goldstone boson degrees of freedom. The effects of respectively neglecting and considering Gaussian intrinsic parton transverse momenta and the Sivers effect are explored. Predictions for the transverse target polarization experiment at HERMES are presented.Comment: 14 pages, 18 figure