25,453 research outputs found
Beam-Induced Nuclear Depolarization in a Gaseous Polarized-Hydrogen Target
No full textSpin-polarized atomic hydrogen is used as a gaseous polarized-proton target in high-energy and nuclear-physics experiments operating with internal beams in storage rings. When such beams are intense and bunched, this type of target can be depolarized by a resonant interaction with the transient magnetic field generated by the beam bunches. This effect has been studied with the HERA positron beam in the HERMES experiment at DESY. Resonances have been observed and a simple analytic model has been used to explain their shape and position. Operating conditions for the experiment have been found where there is no significant target depolarization due to this effect
HERMES results on hard exclusive processes
No full textHard exclusive leptoproduction of real photons or mesons on nucleons and nuclei can provide access to Generalized Parton Distributions (GPDs). The theoretical framework of GPDs includes parton distribution functions and form
factors as limiting cases and as moments of GPDs, respectively, and can provide a multidimensional representation of the structure of hadrons at the partonic level. The HERMES experiment at DESY, Hamburg, collected a wealth of data on Deeply Virtual Compton Scattering (DVCS) and exclusive meson production utilizing the HERA polarized electron or positron beams with an energy of 27.6GeV and longitudinally and transversely polarized or unpolarized gas targets (H, D or heavier nuclei). The azimuthal asymmetries measured in the DVCS process allow access to
the imaginary and/or real part of certain combinations of GPDs. For the last two years of HERA running, the HERMES collaboration installed a recoil detector to improve the selection of DVCS events by direct measurement of the recoil protons. An overview of recent HERMES results on DVCS including first results obtained with the recoil detector is presented
DVCS at HERMES
No full textDeeply virtual Compton scattering process was studied widely at the HERMES experiment through measurements of various cross-section asymmetries. These measurements exploited the rich data collected at HERMES on scattering a longitudinally polarized lepton (electron/positron) beam off unpolarized, longitudinally and transversely polarized hydrogen targets, unpolarized and longitudinally polarized deuterium targets, as well as unpolarized nuclear targets. The 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, first measurement of the beam-helicity asymmetry related to the associated deeply virtual Compton scattering process, where the proton is excited to a -resonance state, was possible using the recoil detector information
Spin transfer to and hyperons in deep inelastic scattering at HERMES
No full textPreliminary results on the spin transfer to the Λ and Λ¯ hyperons measured by the HERMES Collaboration are presented. Longitudinal spin transfer directed along the virtual-photon momentum in the Λ rest frame is found to be D LL Λ = 0.19 ± 0.04stat ± 0.02syst, the transverse component being compatible with zero. For Λ¯ both longitudinal and transverse components are compatible with zero within statistical errors of ±0.
Overview of recent HERMES results
No full textAn 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 −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 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
DVCS at HERMES
No full textThis talk explores the impact that the Hermes experiment has had regarding knowledge of the Deeply Virtual Compton Scattering process. We discuss the various measurements that HERMES has contributed to the library of DVCS knowledge [1–3], with focus in particular on the recent high-precision beam spin and charge asymmetries
Recent results on TMDs from the HERMES Experiment
No full textHERMES has taken a wealth of deep-inelastic scattering data using the 27.6 GeV polarized lepton beam at HERA and various pure gas targets, both unpolarized and polarized, which opened the door to several unique results. Among them are the first evidences for the naive-T-odd Sivers and Collins effects. An overview of recent HERMES results on measurement of azimuthal asymmetries in semi-inclusive production of pions, charged kaons and (anti)protons is presented
DVCS measurements at HERMES
No full textDeeply Virtual Compton Scattering (DVCS) is currently one of the most promising processes that provides information about the structure of the nucleon in the framework of Generalized Parton Distributions (GPDs). During its last years the deeply virtual Compton scattering process was extensively studied at the HERMES experiment through measurements of cross-section asymmetries. To the virtue of unique experimental conditions, HERMES had collected a wealth of data on scattering a longitudinally polarized lepton (electron/positron) beam off unpolarized, longitudinally and transversely polarized hydrogen targets, as well as off unpolarized and longitudinally polarized deuterium targets. During the last two years of HERA operation a recoil detector around the target area allowed for the full kinematic reconstruction of the exclusive DVCS event, leading to a clean measurement of the beam-helicity asymmetry with background well below the 1% level. The recoil detector also allowed for first measurement of the related process of associated DVCS, in which the proton is excited to a Delta resonance, giving access to transition GPDs. In this contribution these recent results using the recoil detector are presented
Overview of HERMES Results on Exclusive Processes
No full textAn overview of HERMES results on measurements of hard exclusive electroproduction of real photons and mesons on hydrogen and deuterium targets is presented. Experimental measurements of hard exclusive processes are commonly described within the formalism of generalized parton distributions (GPDs), which provide a unified description of the structure of hadrons embedding longitudinal-momentum distributions (ordinary PDFs) and transverse-position information (form factors). The HERMES experiment at DESY Hamburg studies hard exclusive processes using polarized electron or positron beams from HERA and internal gas targets. Information about GPDs is gained from the measurements of asymmetries that appear in the azimuthal distributions of produced mesons and photons, together with studies of the azimuthal distribution of the decay products via spin-density matrix elements
Exclusive meson production at HERMES
No full textThe HERMES experiment has extensively studied exclusive meson production in deep-inelastic scattering of the 27.6 GeVelectron/positron beam of HERA by various gaseous targets internal to the storage ring. Recent results on exclusive omegaproduction are presented: spin-density matrix elements using unpolarized and longitudinally polarized leptons andunpolarized hydrogen and deuterium targets, and azimuthal distributions for scattering from transversely polarizedprotons. In addition, an analysis of ratios of helicity-amplitudes for exclusive production of rho mesons fromtransversely polarized protons is presented
- …
