Measurement of single- and double-spin asymmetries in deep inelastic pion electroproduction with a longitudinally polarized target

We report the first measurement of the transverse momentum dependence of double-spin asymmetries in semi-inclusive production of pions in deep-inelastic scattering off the longitudinally polarized proton. Data have been obtained using a polarized electron beam of 5.7 GeV with the CLAS detector at the Jefferson Lab (JLab). Modulations of single spin asymmetries over the azimuthal angle between lepton scattering and hadron production planes ? have been measured over a wide kinematic range in Bjorken x and virtual photon squared four-momentum Q2. A significant nonzero sin2? single spin asymmetry was observed for the first time indicating strong spin-orbit correlations for transversely polarized quarks in the longitudinally polarized proton.     

The new frozen spin target at MAMI

The new frozen spin polarized target for experiments at the polarized beam of the real photon facility A2 of the MAMI accelerator is described. The A2-collaboration at the Mainz Microtron MAMI is measuring photon absorption cross section using circularly and linearly polarized photons up to the energy of 1.5 GeV. The photons are produced in the’ Bremsstrahlungs’ process. In the years 2005/2006 the Crystal Ball detector with its unique capability to cope with multi photon final states was set up in Mainz. Since 2010 the experimental apparatus has been completed by a polarized target. The horizontal dilution refrigerator of the Frozen-Spin Target has been constructed and is operated in close cooperation with the Joint Institute for Nuclear Research in Dubna, Russia. The system offers the opportunity to provide longitudinally and transversely polarized protons and deuteron. In this paper the operation experience of this new Frozen-Spin Target and first results from the runs in 2010 and 2011 are presented.     

First Measurement of the Gerasimov-Drell-Hearn Sum Rule for 1H from 0.7 to 1.8 GeV at ELSA

To verify the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the first time experimentally, we measured the helicity dependent total photoabsorption cross section with circularly polarized real photons and longitudinally polarized nucleons in the photon energy range 0.68-1.82 GeV with the tagged photon facility at ELSA. The experiment was carried out with a 4pi detection system, a circularly polarized tagged photon beam, and a frozen spin polarized proton target. The contribution to the GDH sum rule in this photon energy range is [49.9+/-2.4(stat)+/-2.2(syst)] microb.     

Helicity amplitudes A1/2 and A3/2 for the D13(1520) resonance obtained from the gamma-->p-->-->ppi(0) Reaction

The helicity dependence of the gamma-->p-->-->ppi(0) reaction has been measured for the first time in the photon-energy range from 550 to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4pi-detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. These data are predominantly sensitive to the D13(1520) resonance and are used to determine its helicity amplitudes.     

Drell-Yan representation for vector-meson spin-density matrix elements in semi-inclusive electroproduction

Polarized vector-meson production in semi-inclusive electron-nucleon scattering with longitudinally polarized electron beam has been investigated. A Drell-Yan-like representation for the spin-density matrix elements of a vector meson that takes into account the leading radiative corrections is derived. The calculations have been performed for two widely used reference systems, the Gottfried-Jackson and the helicity systems.     

Tagged-photon events in polarized deep inelastic scattering

Deep inelastic scattering events of a longitudinally polarized electron by a polarized proton with a tagged collinear photon radiated from the initial-state electron are considered. The corresponding cross section is derived in the Born approximation. The model-independent radiative corrections to the Born cross section are also calculated. The obtained result is applied to the elastic scattering.     

Photoproduction of ω-Mesons at ELSA and the role of azimuthal asymmetries

Measurements of ω photoproduction of the Crystal-Barrel/TAPS experiment at the ELSA accel-erator of Bonn University are presented which used linearly polarized tagged photon beam from threshold to E_γ = 1700 MeV. The azimuthal asymmetries Σ and Σ_π indicate s-channel resonance contributions on top of the established t-channel exchange processes. These findings are further enhanced by a very first measurement of the G-asymmetry which, in addition to the polarized photon beam, also requires a longitudinally polar-ized proton target. An intuitive interpretation of the specific sensitivity of the azimuthal asymmetries to the reaction mechanisms involved is given.     

Compton tensor with heavy photon for longitudinally polarized electron with next-to-leading accuracy

Compton tensor for scattering of a longitudinally polarized electron by a heavy photon is considered. The result obtained can be used for the analysis of electromagnetic corrections to the spin-spin correlation in quasi-elastic and deep inelastic scattering with next-to-leading accuracy.     

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.     

On the theory of generation of a polarized muon-photon shower in crystals under different initial conditions

The process of formation of a polarized muon-photon shower in crystals has been investigated under the following initial conditions: (i) a shower is generated by a high-energy longitudinally polarized initial muon and (ii) a shower is generated by a circulaly polarized high-energy γ photon. Analytical expressions have been obtained for the distribution functions of polarized shower muons and γ photons in the medium and comparative analysis of the number of shower particles under different initial conditions has been performed.     

First measurement of the helicity-dependent γ p?pη differential cross-section

The helicity dependence of the γ p↦pη reaction has been measured for the first time at a center-of-mass angle θ^* _η = 70^° in the photon energy range from 780 MeV to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4π-detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. The helicity 3/2 cross-section is found to be small and the results for helicity 1/2 agree with predictions from the MAID analysis. Received: 19 December 2002 / Accepted: 10 March 2003 / Published online: 20 May 2003     

Achievement of longitudinally polarized focusing with long focal depth by amplitude modulation

The longitudinal component of a focused beam is split into two parts along the optical axis to obtain a longitudinally polarized long focal depth using amplitude filtering based on Euler transformation and a radially polarized Bessel-Gaussian beam. Numerical results indicate that long focal depth and FWHM can be easily achieved with 9λ and 0.8λ, respectively. A radially polarized beam can be converted into a longitudinally polarized beam with a conversion efficiency of 51.0%. It can therefore be believed that the proposed scheme can be widely used to generate a longitudinally polarized beam for particle acceleration, laser cutting, and optical trapping.     

Single-spin asymmetries in the Bethe-Heitler process e − + p → e − + γ + p induced by loop corrections

The single-spin beam and target asymmetries in the hard electroproduction process e ⁻ + p → e ⁻ + γ + p induced by the loop radiative corrections to the vertex part of lepton interaction are considered. The physical reason for the appearance of such asymmetries is the nonzero imaginary part of the amplitude (on the level of radiative corrections) caused by diagrams with photon radiation from the outgoing electron. We calculate the single-spin beam and target asymmetries at a longitudinally polarized electron beam or at arbitrary polarizations of the target proton for the CLAS and HERMES experimental conditions. The text was submitted by the authors in English.     

The E166 experiment: Development of an undulator-based polarized positron source for the international linear collider

A longitudinal polarized positron beam is foreseen for the international linear collider (ILC). A proof-of-principle experiment has been performed in the final focus test beam at SLAC to demonstrate the production of polarized positrons for implementation at the ILC. The E166 experiment uses a 1 m long helical undulator in a 46.6 GeV electron beam to produce a few MeV photons with a high degree of circular polarization. These photons are then converted in a thin target to generate longitudinally polarized e ⁺ and e ⁻. The positron polarization is measured using a Compton transmission polarimeter. The data analysis has shown asymmetries in the expected vicinity of 3.4% and ∼1% for photons and positrons respectively and the expected positron longitudinal polarization is covering a range from 50% to 90%.      

Measurement of the G asymmetry for the γp↦Nπ channels in the Δ(1232) resonance region

The G asymmetry of the γp↦Nπ reaction has been measured for the first time for E_γ = 340±14 MeV. This observable, for which very scarce published data exist, plays an important role to disentangle the contributions of the various nucleon resonances. The experiment, performed at the Mainz microtron MAMI, used a 4π-detector system, a linearly polarized, tagged photon beam, and a longitudinally polarized proton target.     

Current tensor with a heavy photon for pair production by a longitudinally polarized electron

The electron current tensor for the scattering of a heavy photon on a longitudinally polarized electron accompanied with an additional electron-positron pair is considered. The contribution of soft and hard collinear and quasi-collinear pairs is taken into account. The full analysis of the spin-independent and spin-dependent parts of the electron current tensor is performed. The results allow us to calculate the corresponding contribution into the second-order radiative correction to cross sections for different processes with the next-to-leading accuracy.     

Measurement of the Gerasimov-Drell-Hearn Integrand for 2H from 200 to 800 MeV

A measurement of the helicity dependence of the total inclusive photoabsorption cross section on the deuteron was carried out at MAMI (Mainz) in the energy range 200相似文献   

Messung der Energie- und Winkelabhängigkeit der Asymmetrie der äußeren Bremsstrahlung

In order to obtain an azimuthal dependence of the external bremsstrahlung produced by electrons, the electron beam has to be transversely polarized. In first-order Born approximation the differential cross section does not depend on the azimuthal angle φ¹, but the second-order approximation includes the spin vector of the electrons^2,3 and yields, consequently, a φ-dependence. This paper deals with a measurement of the bremsstrahlung asymmetry as function of the photon energy and the emission angle Θ. The transversely polarized electron beam was produced by momentum deflection of longitudinally polarizedβ-decay electrons emitted from a⁹⁰Sr source. With a kinetic energy of 300±10 keV and a degree of transverse polarization of 74% the electrons hit a Pb target. Because of the high γ-background it was necessary to detect the emitted bremsstrahlung in coincidence with the incident electrons. We found effects of about 3% in contrast to considerably larger values (13%) published in an earlier paper. The asymmetry was numerically calculated according to Johnson and Rozics. Since at an electron energy of 300 keV the Born approximation gives not yet good results, theory and measurement agree only in order of magnitude, particularly at highγ-energies.     

Measurements of spin observables in pseudo-scalar meson photo-production using polarized neutrons in solid HD

A measurement of psuedo-scalar meson photo production from longitudinally polarized solid HD has been carried out with the CLAS at Thomas Jefferson National Accelerator Facility (Jlab) with circularly and linearly polarized photon beams. Its aim is to measure a complete set of spin observables for the neutron simultaneously from the same experiment. As a polarized neutron, deutron in HD was used. Preliminary asymmetries are shown for the π^? channel.     

Experiment studies on the polarized gamma-rays generation at KEK-ATF

Polarized positrons can be created through electron-positron pair creation from circularly polarized gamma-rays. Laser-Compton scattering is an efficient method to generate circularly polarized gamma-rays. A high finesse 2-mirror optical stacking cavity had been installed on the straight section of the electron storage ring at KEK-ATF. A 1064 nm circularly polarized pulsed laser beam was stacked in the cavity. Polarized gamma-rays with a maximum energy of 28.3 MeV were produced via inverse Compton scattering of the enhanced laser pulse off an electron beam of 1.28 GeV. The number of generated gamma photons per collision was estimated by a photon detector. It was found that the experimental result was in agreement with the simulated value.