Search Results (28 total)

Semiexclusive measurements of the two-pion-production p(α,α^')pππ reaction have been carried out at an energy of E_α=4.2 GeV at the Saturne-II (Saclay) accelerator with the SPES4-π installation. This reaction was investigated by simultaneous registration of the scattered α particle and the secondary proton. The obtained results show that the two-pion production in inelastic α-particle scattering on the proton at the energy of the experiment proceeds mainly through excitation in the target proton of the Roper resonance and its decay with emission of two pions in the isospin I=0,S-wave state.

The HERMES experiment has measured the transverse polarization of Λ and Λ̅ hyperons produced inclusively in quasireal photoproduction at a positron beam energy of 27.6 GeV. The transverse polarization P_n^Λ of the Λ hyperon is found to be positive while the observed Λ̅ polarization is compatible with zero. The values averaged over the kinematic acceptance of HERMES are P_n^Λ=0.078±0.006(stat)±0.012(syst) and P_n^Λ̅ =-0.025±0.015(stat)±0.018(syst) for Λ and Λ̅ , respectively. The dependences of P_n^Λ and P_n^Λ̅ on the fraction ζ of the beam’s light-cone momentum carried by the hyperon and on the hyperon’s transverse momentum p_T were investigated. The measured Λ polarization rises linearly with p_T and exhibits a different behavior for low and high values of ζ, which approximately correspond to the backward and forward regions in the center-of-mass frame of the γ^*N reaction.

The first observation of an azimuthal cross section asymmetry with respect to the charge of the incoming lepton beam is reported from a study of hard exclusive electroproduction of real photons. The data have been accumulated by the HERMES experiment at DESY, in which the HERA 27.6 GeV electron or positron beam scattered off an unpolarized hydrogen gas target. The observed asymmetry is attributed to the interference between the Bethe-Heitler process and the deeply virtual Compton scattering (DVCS) process. The interference term is sensitive to DVCS amplitudes, which provide the most direct access to generalized parton distributions.

Precise measurements of the spin structure functions of the proton g₁^p(x,Q²) and deuteron g₁^d(x,Q²) are presented over the kinematic range 0.0041≤x≤0.9 and 0.18  GeV²≤Q²≤20  GeV². The data were collected at the HERMES experiment at DESY, in deep-inelastic scattering of 27.6 GeV longitudinally polarized positrons off longitudinally polarized hydrogen and deuterium gas targets internal to the HERA storage ring. The neutron spin structure function g₁ⁿ is extracted by combining proton and deuteron data. The integrals of g₁^p,d at Q²=5  GeV² are evaluated over the measured x range. Neglecting any possible contribution to the g₁^d integral from the region x≤0.021, a value of 0.330±0.011(theo)±0.025(exp)±0.028(evol) is obtained for the flavor-singlet axial charge a₀ in a leading-twist next-to-next-to-leading-order analysis.

The transfer of polarization from a high-energy positron to a Λ0 hyperon produced in semiinclusive deep-inelastic scattering has been measured. The data have been obtained by the HERMES experiment at DESY using the 27.6 GeV longitudinally polarized positron beam of the HERA collider and unpolarized gas targets internal to the positron (electron) storage ring. The longitudinal spin-transfer coefficient is found to be DLL′Λ=0.11±0.10(stat)±0.03(syst) at an average fractional energy carried by the Λ0 hyperon ⟨z⟩=0.45. The dependence of DLL′Λ on both the fractional energy z and the fractional longitudinal momentum xF is presented.

The first measurements of double-hadron production in deep-inelastic scattering within the nuclear medium were made with the HERMES spectrometer at DESY HERA using a 27.6 GeV positron beam. By comparing data for deuterium, nitrogen, krypton, and xenon nuclei, the influence of the nuclear medium on the ratio of double-hadron to single-hadron yields was investigated. Nuclear effects on the additional hadron are clearly observed, but with little or no difference among nitrogen, krypton, or xenon, and with smaller magnitude than effects seen on previously measured single-hadron multiplicities. The data are compared with models based on partonic energy loss or prehadronic scattering and with a model based on a purely absorptive treatment of the final-state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter.

The Hermes experiment has investigated the tensor spin structure of the deuteron using the 27.6  GeV/c positron beam of DESY HERA. The use of a tensor-polarized deuteron gas target with only a negligible residual vector polarization enabled the first measurement of the tensor asymmetry A_zz^d and the tensor structure function b₁^d for average values of the Bjorken variable 0.01<⟨x⟩<0.45 and of the negative of the squared four-momentum transfer 0.5  GeV²<⟨Q²⟩<5  GeV². The quantities A_zz^d and b₁^d are found to be nonzero. The rise of b₁^d for decreasing values of x can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering.

A search for an exotic baryon resonance with S=-2, Q=-2 has been performed in quasireal photoproduction on a deuterium target through the decay channel Ξ^-π^-→Λπ^-π^-→pπ^-π^-π^-. No evidence for a previously reported Ξ^--(1860) resonance is found in the Ξ^-π^- invariant mass spectrum. An upper limit for the photoproduction cross section of 2.1 nb is found at the 90% confidence level. The photoproduction cross section for the Ξ⁰(1530) is found to be between 9 and 24 nb.

Polarized deep-inelastic scattering data on longitudinally polarized hydrogen and deuterium targets have been used to determine double-spin asymmetries of cross sections. Inclusive and semi-inclusive asymmetries for the production of positive and negative pions from hydrogen were obtained in a reanalysis of previously published data. Inclusive and semi-inclusive asymmetries for the production of negative and positive pions and kaons were measured on a polarized deuterium target. The separate helicity densities for the up and down quarks and the anti-up, anti-down, and strange sea quarks were computed from these asymmetries in a “leading order” QCD analysis. The polarization of the up-quark is positive and that of the down-quark is negative. All extracted sea quark polarizations are consistent with zero, and the light quark sea helicity densities are flavor symmetric within the experimental uncertainties. First and second moments of the extracted quark helicity densities in the measured range are consistent with fits of inclusive data.

Single-spin asymmetries for semi-inclusive electroproduction of charged pions in deep-inelastic scattering of positrons are measured for the first time with transverse target polarization. The asymmetry depends on the azimuthal angles of both the pion (ϕ) and the target spin axis (ϕ_S) about the virtual-photon direction and relative to the lepton scattering plane. The extracted Fourier component ⟨sin⁡(ϕ+ϕ_S)⟩_UT^π is a signal of the previously unmeasured quark transversity distribution, in conjunction with the Collins fragmentation function, also unknown. The component ⟨sin⁡(ϕ-ϕ_S⟩_UT^π arises from a correlation between the transverse polarization of the target nucleon and the intrinsic transverse momentum of quarks, as represented by the previously unmeasured Sivers distribution function. Evidence for both signals is observed, but the Sivers asymmetry may be affected by exclusive vector meson production.

Double-spin asymmetries of semiinclusive cross sections for the production of identified pions and kaons have been measured in deep inelastic scattering of polarized positrons on a polarized deuterium target. Five helicity distributions including those for three sea quark flavors were extracted from these data together with reanalyzed previous data for identified pions from a hydrogen target. These distributions are consistent with zero for all three sea flavors. A recently predicted flavor asymmetry in the polarization of the light quark sea appears to be disfavored by the data.

Exclusive coherent and incoherent electroproduction of the ρ⁰ meson from ¹H and ¹⁴N targets has been studied at the HERMES experiment as a function of coherence length (l_c), corresponding to the lifetime of hadronic fluctuations of the virtual photon, and squared four-momentum of the virtual photon (-Q²). The ratio of ¹⁴N to ¹H cross sections per nucleon, called nuclear transparency, was found to increase (decrease) with increasing l_c for coherent (incoherent) ρ⁰ electroproduction. For fixed l_c, a rise of nuclear transparency with Q² is observed for both coherent and incoherent ρ⁰ production, which is in agreement with theoretical calculations of color transparency.

A study of inelastic scattering of polarized 3.73-GeV/c deuterons on protons in the energy region of the Roper N(1440) and the Δ(1232) resonances excitation has been performed in an exclusive experiment at LNS (Laboratoire National SATURNE, Saclay, France) using the SPES4-π setup. Tensor and vector analyzing powers of pion production for the reactions: d+p→d+n+π⁺, d+p→d+p+π⁰, d+p→d+N+ππ have been measured as functions of the squared deuteron four-momentum transfer t, of the effective mass of the subsystems Nπ, Nππ, and of the pion emission angle. A strong dependence of these analyzing powers upon the pion emission angle is observed. It is found that A_yy values for the considered reaction channels are systematically larger than the known inclusive p(d,d^′)X world data at the nearest beam energy.

The spectra of α particles in (n,α) reactions induced on ^144,147,149Sm by neutrons with energy between ≊12 and 20 MeV are greatly affected by the shell structure of the target nucleus. Semimicroscopical calculations made in the framework of the pickup model, assuming dominance of transitions to neutron-hole states of the residual nuclei and the homology of high-energy states of ^144,146Nd excited in the reactions induced on the nonmagic ^147,149Sm isotopes with the low-energy states of ¹⁴¹Nd excited in the reaction on the magic ¹⁴⁴Sm isotope, reproduce satisfactorily the data.

We have measured surface stresses on clean Si(111) 7×7 by comparing this surface to a reference surface on which gallium atoms are adsorbed under UHV conditions. Stresses are determined by optically measuring the macroscopic strain induced in thin samples. We find a surface stress of 2.37 eV/(1×1 cell) for Si(111) 7×7, and a stress in the range 0.90–1.09 eV/(1×1 cell) for the Si(Ga) (111) superlattice associated with one monolayer Ga coverage. Comparison with theory suggests that our technique will be a powerful tool to measure equilibrium stresses in atomically clean surfaces.

Measurements of α-particle spectra in (n,α) reactions induced on ¹⁴²,143,144Nd by neutrons with energy between ≊12 and 20 MeV are presented. The spectra are greatly affected by the shell structure of the target nucleus. A semi-microscopic calculation made in the framework of the pickup model, assuming dominance of transitions to neutron-hole states of the residual nucleus, provides a satisfactory reproduction of the data. Transitions to single neutron states through a knockout process have also been considered, but it is found that they may only slightly contribute to the measured α-particle yield.

Electrochemical coloration mechanisms of electrochromic (EC), amorphous tungsten and iridium oxide films have been investigated by direct elemental analysis of their compositions in oxidized and reduced states. Combined Rutherford backscattering and nuclear-reaction-analysis techniques were employed to determine the identities and concentrations of ions injected as charge compensators from the contacting electrolyte during EC redox reactions. In aqueous electrolytes, the cathodic coloration of an anodically formed, hydrous tungsten oxide film (composition ∼WO₃·H₂O), is accompanied by injection of protons, in accordance with mechanisms determined for anhydrous WO₃. The anodic coloration and coloration bleaching of an anodically formed, hydrous iridium oxide (IROX) film, which has a varying degree of hydration throughout its thickness, is accompanied by ejection and injection of H⁺ and H₃O⁺ ions. In nonaqueous aprotic electrolytes, the slow but reversible insertion of Li⁺ (r=0.60 Å) and Na⁺ (r=0.95 Å) ions into IROX films has been successfully demonstrated for the first time. Li⁺ ions were found to exchange with H⁺ ions in these hydrous films. No evidence could be found for insertion of K⁺ (r=1.33 Å), F^- (r=1.36 Å), or OH^- (r=1.55 Å) ions as charge compensators. These results provide a measure of the size and concentration of vacancies and defects in amorphous IROX films.

The erosion of water ice films by fast H and He ions has been found to depend on the electronic energy loss of the ion. At low temperatures the erosion yield is temperature independent and varies quadratically with the electronic stopping power, consistent with a Coulomb repulsion model of ejection. At temperatures above ∼100 °K the erosion is found to be temperature dependent and linear with the electronic stopping power. The erosion in this regime can be associated with defect formation and migration in the ice.

The various amorphous films (Ni, Fe, and Au) were deposited by getter sputtering in helium gas with liquid helium circulating through the substrate table. This technique is effective in producing amorphous films as a result of the low deposition temperature and the incorporation of large amounts of helium gas into the film. The amount of helium gas which was determined by three techniques (Rutherford backscattering, mass spectrometry, and annealing in vacuo) ranges between 10 and 46 at.%. The most interesting feature of this new technique is that the amorphous films thus produced are pure in the sense that the amorphous state was achieved only by the incorporation of a noninteracting noble gas. Indeed, upon annealing in vacuo, which drives the helium out of the films, the films regain all the properties (x-ray structure, resistivity, Rutherford-backscattering yield) of the bulk crystalline material.

We have measured the rate of erosion of thin films of water ice at low temperatures by bombardment with MeV hydrogen, helium, carbon, and oxygen ions. The effective "sputtering coefficients" are orders of magnitude higher than those anticipated from conventional sputtering theories. For example, for helium at 1.5 MeV, ∼10 H₂O molecules are removed for each incident ion. We believe that the erosion process is closely associated with atomic ejection following ionization in the region near the surface.

⁴He channeling measurements of minimum yields in single-crystal V₃Si have been compared with those calculated with the Karlsruhe phonon density of states. Atomic displacements versus temperature and defect level have been so obtained. In situ radiation-damaged crystals show ∼ 10^-1-Å displacements and significant phonon stiffening. The observations explain the large changes in electrical resistivity in the defect state, and suggest that both thermal and static (defect) displacements diminish the electron-phonon/defect interactions, and to equivalent degrees.

Defects introduced into superconducting Nb-Ge films by 2-MeV ⁴He particles produce a T_c-resistance-ratio correlation similar to that for as-grown films and a T_c-lattice-parameter correlation resembling both film and bulk behavior. It is suggested that simple antistructure defects in a perfect lattice are not primarily responsible for lowering T_c for the case of ⁴He damage in the sputtered films.

The channeling technique has been used to study the lattice location of Hf implanted in single-crystal Ni samples. Results indicate that about one-half the Hf atoms occupy regular Ni lattice sites, the remainder being in some form of precipitate. It was found that annealing the sample above 600 °C promotes further precipitation of the impurity. These results are compared with recent hyperfine-interaction experiments on the same system.

The channeling technique has been employed to study the lattice location of Tl, Pb, and Bi implanted into iron single crystals. The results, when compared with measured magnetic hyperfine fields acting on Pb in ferromagnetic iron, provide evidence for the association of high-field sites with Pb in nonsubstitutional positions.

Axial and planar channeling in thin single-crystalline gold foils has been investigated by wide-angle scattering of monoenergetic positrons. The beam was obtained by accelerating the positrons emitted from a Co⁵⁸ source in a 1-MeV Van de Graaff. The results are in good agreement with corresponding measurements for protons. For the planar case, classical calculations are compared to calculations based on the dynamical theory of diffraction. The results are very similar except for the "wiggles" due to wave interference, which appear in the quantum-mechanical calculation. These, however, are difficult to resolve experimentally.