Linearly polarized gluons and the Higgs transverse momentum distribution

We study how gluons carrying linear polarization inside an unpolarized hadron contribute to the transverse momentum distribution of Higgs bosons produced in hadronic collisions. They modify the distribution produced by unpolarized gluons in a characteristic way that could be used to determine whether the Higgs boson is a scalar or a pseudoscalar particle.     

An unconditionally stable time-domain discretization on cartesian meshes for the simulation of nonuniform magnetized cold plasma

In this paper an unconditionally stable, spatially and temporally implicit time-domain discretization for nonuniform magnetized cold plasma is developed. The discrete dispersion relation is free of spurious solutions and approximates the continuous dispersion relation for well-resolved wavelengths and frequencies (kΔ ? π, ωΔ_t ? π). For a specific choice of parameters, the discrete dispersion relation approximates the continuous dispersion relation for all wavelengths and frequencies up to the Nyquist limit. A few examples, amongst them one involving mode conversion, illustrate the new method.     

Sharp Upper Bounds on the Minimum Number of Components of 2-factors in Claw-free Graphs

Let G be a claw-free graph with order n and minimum degree δ. We improve results of Faudree et al. and Gould & Jacobson, and solve two open problems by proving the following two results. If δ = 4, then G has a 2-factor with at most (5n − 14)/18 components, unless G belongs to a finite class of exceptional graphs. If δ ≥ 5, then G has a 2-factor with at most (n − 3)/(δ − 1) components, unless G is a complete graph. These bounds are best possible in the sense that we cannot replace 5/18 by a smaller quotient and we cannot replace δ − 1 by δ, respectively.     

Dynamics of the collective excitations of the quantum Hall system

Using the non linear optical technique of 3-pulse 4-wave mixing, we study the dynamics of the collective excitations of the quantum Hall system. We excite the system with 100 fs pulses propagating in directions k₁ and k₃ and then probe its time evolution with a delayed pulse k₂. We measure the non-linear optical response from the lowest Landau level along the direction k₁+k₂−k₃. As function of the time delay of pulse k₂, this signal shows striking beats for short time delays (500 fs), followed by a rise (20 ps) and then a decay (100 ps). We identify the microscopic origin of this dynamics by extending the standard theory of ultra fast nonlinear optics to include the effects of the correlations.     

Orbits of euclidean frames under discrete linear groups

We obtain certain sufficient conditions for the orbit of a (euclidean)p-frame over a vector spaceV,p<dimV, under the action of a discrete subgroup of GL(V), to be dense in the corresponding orbit of a Lie subgroup of GL(V). Using the result we classify thep-frames whose orbits under SL (n,Z) are dense in the space ofp-frames and deduce, in turn, a classification of dense orbits of certain horospherical flows. A similar result is obtained for Sp (2n,Z) forp≦n.     

Some two-step and three-step nilpotent Lie groups with small automorphism groups

We construct examples of two-step and three-step nilpotent Lie groups whose automorphism groups are ``small' in the sense of either not having a dense orbit for the action on the Lie group, or being nilpotent (the latter being stronger). From the results we also get new examples of compact manifolds covered by two-step simply connected nilpotent Lie groups which do not admit Anosov automorphisms.

     

Chemisorption determines the photovoltage of a Ti/TiO2/Au/dye internal electron emission photovoltaic cell

The Ti/TiO2/Au junction forms the basis of a promising new type of photovoltaic cell, provided that a light-harvesting antenna layer can be deposited on the thin gold film. We report that the electrical diode characteristics of the TiO2/Au Schottky barrier deteriorate by deposition of a hydrophobic quantum dot film, Merbromin dye adsorption, or electron-hole photogeneration in TiO2 under inert conditions. In the presence of oxygen and water vapor, the Schottky barrier characteristics and high photovoltage are recovered. The strong influence of the TiO2 surface chemistry on the (photo)electrical characteristics of the solar cell is discussed on the basis of an existing microscopic model.     

Using a small molecule inhibitor of peptide: N-glycanase to probe its role in glycoprotein turnover

Peptide:N-glycanase (PNGase) is ostensibly the sole enzyme responsible for deglycosylation of unfolded N-linked glycoproteins dislocated from the ER to the cytosol. Here we show the pan-caspase inhibitor, Z-VAD-fmk, to be an active site-directed irreversible inhibitor of yeast and mammalian PNGase at concentrations below those used to inhibit caspases in vivo. Through chemical synthesis we determined that the P1 residue, electrophile position, and leaving group are important structural parameters for PNGase inhibition. We show that Z-VAD-fmk inhibits PNGase in living cells and that degradation of class I MHC heavy chains and TCRalpha, in an identical cellular setting, is markedly different. Remarkably, proteasome-mediated turnover of class I MHC heavy chains proceeds even when PNGase is completely inhibited, suggesting that the function of PNGase may be to facilitate more efficient proteasomal proteolysis of N-linked glycoproteins through glycan removal.     

Anhydrodihydroartemisinin and its 10-trifluoromethyl analogue: access to novel d-ring-contracted artemisinin trifluoromethyl ketones

The preparation of the 10-trifluoromethyl hydroartemisinin, followed by dehydration, afforded the trifluoromethyl analogue 2 of anhydrodihydroartemisinin 1. The reactivity of these two glycals of artemisinin were compared in epoxidation and halogenation reactions. Iodination of glycal 1 in water and the further rearrangement of the produced iodo hemiacetal provided the new D-ring-contracted aldehyde 8alpha, where the methyl at C-9 is beta. Epoxidation of 10-trifluoromethyl anhydrodihydroartemisinin 2 stereoselectively provided the beta-epoxy ether 11 in high yield. When treated with hexafluoro-2-propanol or trifluoroethanol, 11 readily underwent a rearrangement yielding to the D-ring-contracted trifluoromethyl ketone 9alpha with retention of configuration at C-9.