[en] The authors have computed the helicity amplitudes for both the real and virtual nucleon Compton scattering at large momentum transfer in leading order perturbative QCD. The angular dependence of both the phases of the amplitudes and the cross sections are presented as a function of photon virtuality with different model distribution amplitudes. The singular numerical integration involved are handled by the method suggested by Kronfeld and Nizic. As pointed out before, at sufficiently high energies these predictions for virtual Compton Scattering can provide a stringent test of QCD

[en] Highlights: • Rutherford backscattering spectrometry (RBS) to determine the change in depth. • Rutherford backscattering spectrometry (RBS) to determine the change in depth. • Rutherford backscattering spectrometry (RBS) to determine the change in depth. -- Abstract: A novel, high-resolution technique has been developed for the measurement of erosion and deposition in solid material surfaces. The technique uses a combination of nuclear reaction analysis (NRA) and Rutherford backscattering spectrometry (RBS) to determine the change in depth of a previously implanted marker layer consisting of ⁷Li. A scoping study shows that ⁷Li is an ideal marker candidate due to a high Q (∼18 MeV) nuclear reaction, ⁷Li(p,α)⁴He. Net erosion or deposition is measured by NRA of modified alpha energy passing through the bulk material. The reaction’s high cross-section provides for the fast time resolution needed to measure erosion from high flux plasmas, and a highly penetrating proton beam provides for a large range of erosion/deposition measurements. Additionally, the implantation of low-Z Li leads to relatively low vacancy concentrations in the solid material due to implantation. This technique thus provides greater assurance that the measured erosion rate is indicative of the solid material: due to both the low vacancy production and the fact that no films or deposits are involved. Validation was performed by comparing the measured and predicted amount of erosion based on previously measured sputtering yields; the two were found to agree, within the uncertainty of the experiment. The depth resolution of the techniques is ∼60 nm at a net erosion depth of about 1 μm. The benefits of this technique are summarized as: short time scales (minutes) to obtain results, the marker layer can be used in any solid material, greater assurance that the measured erosion is indicative of the unperturbed solid material, and the continuous monitoring of the surface composition for contaminants and/or identification of deposited species using RBS simultaneous with the NRA

[en] It is pointed out that the correct criterion to define the legal PQCD contribution to the exclusive processes in the lightcone perturbative expansion should be based on the large off-shellness of the lightcone energy in the intermediate states. In the lightcone perturbative QCD calculation of the pion form factor, the authors find that the legal PQCD contribution defined by the lightcone energy cut saturates in the smaller Q² region compared to that defined by the gluon four-momentum square cut. This is due to the contribution by the highly off-energy-shell gluons in the end point regions of the phase space, indicating that the gluon four-momentum-square cut may have cut too much to define the legal PQCD

[en] It is pointed out in the example of the pion form factor that the usual factorized hard scattering amplitude T_H in perturbative QCD is derived from the light-cone time-ordered perturbative expansion. In the light-cone perturbative expansion, the natural variable to make a separation of perturbative contributions from contributions intrinsic to the bound-state wave function itself is the light-cone energy, rather than the gluon virtuality of T_H. We find that the ''legal'' PQCD contribution defined by the light-cone energy cut saturates to the full PQCD prediction without any cut in the smaller Q² region as compared to that defined by the gluon four-momentum square cut. This is due to the contribution from highly off-energy-shell gluons in the end-point regions of the phase space

[en] In the PQCD analyses of the exclusive production of higher generation hadrons, the quark distribution amplitude of the heavy quark system has often been approximated by a δ function from the nonrelativistic consideration. Going beyond the peaking approximation, the factorization of the covariant hard scattering amplitude from the nonperturbative quark distribution amplitude is no longer valid. We therefore use the light-cone time-ordered perturbation theory which is the step prior to the usual factorization formula and calculate the form factor of a pseudoscalar meson composed of a heavy quark and antiquark. However, we find that the numerical results for the cross section of exclusive heavy meson pair production in e⁺e^- annihilation are not much different from those of the peaking approximation. copyright 1997 The American Physical Society

[en] Linear low-density polyethylene (LLDPE)/poly (vinyl alcohol) (PVOH) filled with untreated kenaf (UT-KNF) and eco-friendly coupling agent (ECA)-treated kenaf (ECAT-KNF) were prepared using ThermoHaake internal mixer, respectively. Filler loadings of UT-KNF and ECAT-KNF used in this study are 10 and 40 parts per hundred parts of resin (phr). The effect of ECA on tensile properties and water absorption of LLDPE/PVOH/KNF composites were investigated. Field emission scanning electron microscopy (FESEM) analysis was applied to visualize filler-matrix adhesion. The results indicate LLDPE/PVOH/ECAT-KNF composites possess higher tensile strength and tensile modulus, but lower elongation at break compared to LLDPE/PVOH/UT-KNF composites. The morphological studies of tensile fractured surfaces using FESEM support the increment in tensile properties of LLDPE/PVOH/ECAT-KNF composites. Nevertheless, LLDPE/PVOH/UT-KNF composites reveal higher water absorption compared to LLDPE/PVOH/ECAT-KNF composites. (paper)

[en] A Fourier optics calculation of image formation in low energy electron microscopy (LEEM) is presented. The adaptation of the existing theory for transmission electron microscopy to the treatment of LEEM and other forms of cathode lens electron microscopy is explained. The calculation incorporates imaging errors that are caused by the objective lens (aberrations), contrast aperture (diffraction), imperfect source characteristics, and voltage and current instabilities. It is used to evaluate the appearance of image features that arise from phase objects such as surface steps and amplitude objects that produce what is alternatively called amplitude, reflectivity or diffraction contrast in LEEM. This formalism can be used after appropriate modification to treat image formation in other emission microscopies. Implications for image formation in the latest aberration-corrected instruments are also discussed.

[en] Recent experiments by the HERMES group at DESY HERA are measuring semi-inclusive electroproduction of pions from deuterium. We point out that by comparing the production of π⁺ and π^- from an isoscalar target, it is possible, in principle, to measure charge symmetry violation in the valence quark distributions of the nucleons. It is also possible in the same experiments to obtain an independent measurement of the quark fragmentation functions. We review the information which can be deduced from such experiments and show the open-quote open-quote signature close-quote close-quote for charge symmetry violation in such experiments. Finally, we predict the magnitude of the charge symmetry violation, from both the valence quark distributions and the pion fragmentation function, which might be expected in these experiments. copyright 1996 The American Physical Society

[en] We use the Brodsky-Lepage-Mackenzie (BLM) method to fix the renormalization scale of the QCD coupling in exclusive hadronic amplitudes such as the pion form factor and the photon-to-pion transition form factor at large momentum transfer. Renormalization-scheme-independent commensurate scale relations are established which connect the hard scattering subprocess amplitudes that control exclusive processes to other QCD observables such as the heavy quark potential and the electron-positron annihilation cross section. The commensurate scale relation connecting the heavy quark potential, as determined from lattice gauge theory, to the photon-to-pion transition form factor is in excellent agreement with γe→π⁰e data assuming that the pion distribution amplitude is close to its asymptotic form √ (3) f_πx(1-x). We also reproduce the scaling and normalization of the γγ→π⁺π^- data at large momentum transfer. Because the renormalization scale is small, we argue that the effective coupling is nearly constant, thus accounting for the nominal scaling behavior of the data. However, the normalization of the space-like pion form factor F_π(Q²) obtained from electroproduction experiments is somewhat higher than that predicted by the corresponding commensurate scale relation. This discrepancy may be due to systematic errors introduced by the extrapolation of the γ^*p→π⁺n electroproduction data to the pion pole. copyright 1997 The American Physical Society