Gluon GPDs and Exclusive Photoproduction of a Quarkonium in Forward Region

Forward photoproduction of $J/\psi$ can be used to extract Generalized Parton Distributions(GPD's) of gluons. We analyze the process at twist-3 level and study relevant classifications of twist-3 gluon GPD's. At leading power or twist-2 level the produced $J/\psi$ is transversely polarized. We find that at twist-3 the produced $J/\psi$ is longitudinally polarized. Our study shows that in high energy limit the twist-3 amplitude is only suppressed by the inverse power of the heavy quark mass relatively to the twist-2 amplitude. This indicates that the power correction to the cross-section of unpolarized $J/\psi$ can have a sizeable effect. We have also derived the amplitude of the production of $h_c$ at twist-3, but the result contains end-point singularities. The production of other quarkonia has been briefly discussed.Comment: Discussions of results are adde

Frequency response in short thermocouple wires

Theoretical expressions are derived for the steady state frequency response of a thermocouple wire. In particular, the effects of axial heat conduction are demonstrated for a nonuniform wire with unequal material properties and wire diameters across the junction. The amplitude ratio at low frequency omega approaches 0 agrees with the results of Scadron and Warshawsky (1952) for a steady state temperature distribution. Moreover, the frequency response for a nonuniform wire in the limit of infinite length l approaches infinity is shown to reduce to a simple expression that is analogous to the classic first order solution for a thermocouple wire with uniform properties. Theoretical expressions are also derived for the steady state frequency response of a supported thermocouple wire. In particular, the effects of axial heat conduction are demonstrated for both a supported one material wire and a two material wire with unequal material properties across the junction. For the case of a one material supported wire, an exact solution is derived which compares favorably with an approximate expression that only matches temperatures at the support junction. Moreover, for the case of a two material supported wire, an analytical expression is derived that closely correlates numerical results. Experimental measurements are made for the steady state frequency response of a supported thermocouple wire. In particular, the effects of axial heat conduction are demonstrated for both a supported one material wire (type K) and a two material wire (type T) with unequal material properties across the junction. The data for the amplitude ratio and phase angle are correlated to within 10 pct. with the theoretical predictions of Forney and Fralick (1991). This is accomplished by choosing a natural frequency omega sub n for the wire data to correlate the first order response at large gas temperature frequencies. It is found that a large bead size, however, will increase the amplitude ratio at low frequencies but decrease the natural frequency of the wire. The phase angle data are also distorted for imperfect junctions

Origin of sawtooth domain walls in ferroelectrics

Domains and domain walls are among the key factors that determine the performance of ferroelectric materials. In recent years, a unique type of domain walls, i.e., the sawtooth-shaped domain walls, has been observed in BiFeO$_{3}$ and PbTiO$_{3}$. Here, we build a minimal model to reveal the origin of these sawtooth-shaped domain walls. Incorporating this model into Monte-Carlo simulations shows that (i) the competition between the long-range Coulomb interaction (due to bound charges) and short-range interaction (due to opposite dipoles) is responsible for the formation of these peculiar domain walls and (ii) their relative strength is critical in determining the periodicity of these sawtooth-shaped domain walls. Necessary conditions to form such domain walls are also discussed