Explore the vast range of titles available.

A bstract A comprehensive set of azimuthal single-spin and double-spin asymmetries in semi-inclusive leptoproduction of pions, charged kaons, protons, and antiprotons from transversely polarized protons is presented. These asymmetries include the previously published HERMES results on Collins and Sivers asymmetries, the analysis of which has been extended to include protons and antiprotons and also to an extraction in a three-dimensional kinematic binning and enlarged phase space. They are complemented by corresponding results for the remaining four single-spin and four double-spin asymmetries allowed in the one-photon-exchange approximation of the semi-inclusive deep-inelastic scattering process for target-polarization orientation perpendicular to the direction of the incoming lepton beam. Among those results, significant non-vanishing cos ( ϕ−ϕ S ) modulations provide evidence for a sizable worm-gear (II) distribution, g 1 T q x p T 2 . Most of the other modulations are found to be consistent with zero with the notable exception of large sin ( ϕ S ) modulations for charged pions and K + .

A bstract Beam-helicity and beam-charge asymmetries in the hard exclusive leptoproduction of real photons from an unpolarised hydrogen target by a 27.6 GeV lepton beam are extracted from the H ermes data set of 2006-2007 using a missing-mass event selection technique. The asymmetry amplitudes extracted from this data set are more precise than those extracted from the earlier data set of 1996-2005 previously analysed in the same manner by H ermes . The results from the two data sets are compatible with each other. Results from these combined data sets are extracted and constitute the most precise asymmetry amplitude measurements made in the H ermes kinematic region using a missing-mass event selection technique.

A bstract The beam-helicity asymmetry in exclusive electroproduction of real photons by the longitudinally polarized Hera positron beam scattering off an unpolarized hydrogen target is measured at Hermes. The asymmetry arises from deeply virtual Compton scattering and its interference with the Bethe-Heitler process. Azimuthal amplitudes of the beam-helicity asymmetry are extracted from a data sample consisting of ep → epγ events with detection of all particles in the final state including the recoiling proton. The installation of a recoil detector, while reducing the acceptance of the experiment, allows the elimination of background from ep → eN πγ events, which was estimated to contribute an average of about 12% to the signal in previous Hermes publications. The removal of this background from the present data sample is shown to increase the magnitude of the leading asymmetry amplitude by 0 . 054 ± 0 . 016 to −0 . 328 ± 0 . 027 (stat . ) ± 0 . 045 (syst . ).

The processes of coherent bremsstrahlung (CB) and coherent pair production (CPP) based on aligned crystal targets have been studied in the energy range 20–170 GeV. The experimental arrangement allowed for measurements of single photon properties of these phenomena including their polarization dependences. This is significant as the theoretical description of CB and CPP is an area of active debate and development. With the approach used in this paper, both the measured cross sections and polarization observables are predicted very well. This indicates a proper understanding of CB and CPP up to energies of 170 GeV. Birefringence in CPP on aligned crystals is applied to determine the polarization parameters in our measurements. New technologies for high-energy photon beam optics including phase plates and polarimeters for linear and circular polarization are demonstrated in this experiment. Coherent bremsstrahlung for the strings-on-strings (SOS) orientation yields a larger enhancement for hard photons than CB for the channeling orientations of the crystal. Our measurements and our calculations indicate low photon polarizations for the high-energy SOS photons.

Polarisation asymmetries are measured for the hard exclusive leptoproduction of real photons from a longitudinally polarised hydrogen target. These asymmetries arise from the deeply virtual Compton scattering and Bethe-Heitler processes. From the data are extracted two asymmetries in the azimuthal distribution of produced real photons about the direction of the exchanged virtual photon: with respect to the target polarisation and with respect to the product of the beam and target polarisations. Results for both asymmetries are compared to the predictions from a generalised parton distribution model. The sin φ and cos(0φ) amplitudes observed respectively for the and asymmetries are compatible with the sizeable predictions from the model. Unexpectedly, a sin(2φ) modulation in the asymmetry with a magnitude similar to that of the sin φ modulation is observed.

Results of inclusive measurements of inelastic electron and positron scattering from unpolarized protons and deuterons at the HERMES experiment are presented. The structure functions F 2 p and F 2 d are determined using a parameterization of existing data for the longitudinal-to-transverse virtual-photon absorption cross-section ratio. The HERMES results provide data in the ranges 0.006 ≤  x  ≤ 0.9 and 0.1 GeV 2  ≤  Q 2  ≤ 20 GeV 2 , covering the transition region between the perturbative and the non-perturbative regimes of QCD in a so-far largely unexplored kinematic region. They are in agreement with existing world data in the region of overlap. The measured cross sections are used, in combination with data from other experiments, to perform fits to the photon-nucleon cross section using the functional form of the ALLM model. The deuteron-to-proton cross-section ratio is also determined.

Longitudinal double-spin asymmetries of charged hadrons with high transverse momentum p T have been measured in electroproduction using the H ermes detector at H era . Processes involving gluons in the nucleon have been enhanced relative to others by selecting hadrons with p T typically above 1 GeV. In this kinematic domain the gluon polarization has been extracted in leading order making use of the model embedded in the Monte Carlo Generator P ythia 6.2. The gluon polarization obtained from single inclusive hadrons in the p T range 1 GeV < p T < 2.5 GeV using a deuterium target is at a scale and . For different final states and kinematic domains, consistent values of have been found within statistical uncertainties using hydrogen and deuterium targets.

Bose-Einstein correlations of like-sign charged hadrons produced in deep-inelastic electron and positron scattering are studied in the HERMES experiment using nuclear targets of H, [Formula omitted]H, He, He, N, Ne, Kr, and Xe. A Gaussian approach is used to parametrize a two-particle correlation function determined from events with at least two charged hadrons of the same sign charge. This correlation function is compared to two different empirical distributions that do not include the Bose-Einstein correlations. One distribution is derived from unlike-sign hadron pairs, and the second is derived from mixing like-sign pairs from different events. The extraction procedure used simulations incorporating the experimental setup in order to correct the results for spectrometer acceptance effects, and was tested using the distribution of unlike-sign hadron pairs. Clear signals of Bose-Einstein correlations for all target nuclei without a significant variation with the nuclear target mass are found. Also, no evidence for a dependence on the invariant mass W of the photon-nucleon system is found when the results are compared to those of previous experiments.

Bose–Einstein correlations of like-sign charged hadrons produced in deep-inelastic electron and positron scattering are studied in the HERMES experiment using nuclear targets of 1 H, 2 H, 3 He, 4 He, N, Ne, Kr, and Xe. A Gaussian approach is used to parametrize a two-particle correlation function determined from events with at least two charged hadrons of the same sign charge. This correlation function is compared to two different empirical distributions that do not include the Bose–Einstein correlations. One distribution is derived from unlike-sign hadron pairs, and the second is derived from mixing like-sign pairs from different events. The extraction procedure used simulations incorporating the experimental setup in order to correct the results for spectrometer acceptance effects, and was tested using the distribution of unlike-sign hadron pairs. Clear signals of Bose–Einstein correlations for all target nuclei without a significant variation with the nuclear target mass are found. Also, no evidence for a dependence on the invariant mass W of the photon-nucleon system is found when the results are compared to those of previous experiments.

The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep-inelastic scattering has been studied by the HERMES experiment at DESY using a 27.5 GeV positron beam. The differential multiplicity of charged hadrons and identified charged pions from nitrogen relative to that from deuterium has been measured as a function of the virtual photon energy \\(\\nu\\) and the fraction z of this energy transferred to the hadron. There are observed substantial reductions of the multiplicity ratio \\(R_M^{h}\\) at low \\(\\nu\\) and at high z, both of which are well described by a gluon-bremsstrahlung model of hadronization. A significant difference of the \\(\\nu\\)-dependence of \\(R_M^{h}\\) is found between positive and negative hadrons. This is interpreted in terms of a difference between the formation times of protons and pions, using a phenomenological model to describe the \\(\\nu\\)- and z-dependence of \\(R_M^{h}\\).