Numerical Calculation of Normalization Factor for the Analysis of Azimuthal Asymmetry Amplitudes Related to Deeply Virtual Compton Scattering

Numerical calculation of normalization factor in a maximum likelihood technique providing a bin-free fit in azimuthal angles to the data is presented. It can be used for simultaneous extraction of asymmetry amplitudes with respect to the combined dependence of the cross section of hard exclusive leptoproduction of real photons on transversely polarized protons on beam charge and helicity and target transverse polarization. This method of data processing can reveal previously unseparated contributions from deeply virtual Comptonscattering and its interference with the Bethe-Heitler process

Overview of recent HERMES results

An overview of more recent and important results from the HERMES experiment are 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 unique data set opens the door to the measurements of observables sensitive to the multidimensional structure of the nucleon. Among them are semi-inclusive deep-inelastic scattering measurements of azimuthal modulations sensitive to the transverse momentum distributions, such as the leading-twist Sivers and Collins distributions and distributions sensitive to the convolutions of the twist-2 and twist-3 functions. They all provide an information on the three-momentum-dependent quark distributions. Knowledge on the quark distribution as a function of longitudinal momentum and transverse position in impact-parameter space can be accessed, e.g., through exclusive $\omega$−meson leptoproduction, particularly through the measurement of spin density matrix elements and the measurement of azimuthal modulations on transversely polarized proton target. The measurement of Bose-Einstein correlations of hadron pairs in quasi-real leptoproduction are also presented. The transverse polarization of $\Lambda$ hyperons measured again in quasi-real leptoproduction regime are presented as well. Finally, the new analysis for the search on pentaquark at HERMES are mentioned

Study of GPDs at HERMES

The HERMES experiment has collected a wealth of deep-inelastic sca ering data using the 27.6 GeV polarized electron or positron beams at HERA and various pure gas targets, both unpolarized and polarized. is allowed for a series of unique measurements. Among them are measurements in hard exclusive processes, such as deeply virtual Compton sca ering process or exclusive meson production processes. ey provide information on the three-dimensional structure of the nucleon both in momentum space and in mixed momentum and position space. Results on various cross- section asymmetries, sensitive to generalized parton distributions and thus to the three-dimensional nucleon structure in mixed momentum and position space, are shown. e operation of a recoil detector during the last two years of HERMES running enabled for the full kinematic reconstruction of the events of exclusively produced real photons and a clean (with a background well below the 1%) measurement of the beam-helicity asymmetry. Also, rst measurement of the beam helicity asymmetry related to the associated deeply virtual Compton sca ering process, where the proton is excited to a Δ-resonance state, was possible using the recoil detector information. Spin density matrix elements and transverse-target spin asymmetries in exclusive meson production on unpolar- ized protons and deuterons and on transversely polarized protons, respectively, as well as helicity amplitude ratios from exclusive rho production on transversely polarized protons are presented as well

Spin Density Matrix Elements in Exclusive Production of Omega Mesons at HERMES

Exclusive electroproduction of $\omega$ mesons on unpolarized hydrogen and deuterium targets is studied at HERMES in the kinematic region of $Q^2$ > 1.0 GeV$^2$ , 3.0 GeV < W < 6.3 GeV, and −t′ < 0.2 GeV$^2$ . The data were accumulated during the 1996-2007 running period using the 27.6 GeV longitudinally polarized electron or positron beams at HERA. The determination of the virtual-photon longitudinal-to-transverse cross-section ratio shows that a considerable part of the cross section arises from transversely polarized photons. Spin density matrix elements are derived and presented in projections of Q$^2$ or −t′. Violation of s-channel helicity conservation is observed for some of these elements. A sizable contribution from unnatural-parity-exchange amplitudes is found and the phase shift between those amplitudes that describe transverse $\omega$ production by longitudinal and transverse virtual photons is determined for the first time. Good agreement is found between the HERMES proton data and results of a pQCD-inspired phenomenological model that includes pion-pole contributions

Overview of recent HERMES results

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 $\omega$ meson production are described in this paper. The results of the measurement of ratios of helicity amplitudes for the $\rho^0$ meson leptoproduction on transversely polarized hydrogen target are also presented

Overview of HERMES Results on Longitudinal Spin Asymmetries

The HERMES experiment 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 (spin) structure of the nucleon, in particular semi-inclusive deep-inelastic scattering (SIDIS) measurements, for which the HERMES dual-radiator ring-imaging Cherenkov counter provided final-hadron identification between 2 to 15 GeV for pions, kaons, and (anti)protons.In this contribution, longitudinal single- and double-spin asymmetries in SIDIS will be presented. The azimuthally uniform double-spin asymmetries using longitudinally polarized nucleons constrain the flavor dependence of the quark-spin contribution to the nucleon spin. For a first time, such asymmetries are explored differential in three dimensions in Bjorken-x and in the hadron kinematics z and P$_{h⊥}$ (which respectively represent the energy fraction and transverse momentum of the final-state hadron) simultaneously. This approach increases the quark-flavor sensitivity and allows to probe the transverse-momentum dependence of the helicity distribution. The measurement of hadron charge-difference asymmetries allows, under certain simplifying assumptions, the direct extraction of valence-quark polarizations. The azimuthal modulation of this double-spin as well as of the single-(beam)spin asymmetry probe novel quark-gluon-quark correlations through twist-3 distribution and fragmentation functions. Also here, asymmetries are explored in several dimensions. Furthermore, in case of the beam-spin asymmetry, results for electro-produced protons and antiprotons have become available. The beam-spin asymmetries for pions are compared to similar measurements for pions at CLAS and unidentified hadrons at COMPASS