Requirements for contractility in disordered cytoskeletal bundles

Actomyosin contractility is essential for biological force generation, and is well understood in highly organized structures such as striated muscle. Additionally, actomyosin bundles devoid of this organization are known to contract both in vivo and in vitro, which cannot be described by standard muscle models. To narrow down the search for possible contraction mechanisms in these systems, we investigate their microscopic symmetries. We show that contractile behavior requires non-identical motors that generate large enough forces to probe the nonlinear elastic behavior of F-actin. This suggests a role for filament buckling in the contraction of these bundles, consistent with recent experimental results on reconstituted actomyosin bundles.Comment: 10 pages, 6 figures; text shortene

The optimal P3M algorithm for computing electrostatic energies in periodic systems

We optimize Hockney and Eastwood's Particle-Particle Particle-Mesh (P3M) algorithm to achieve maximal accuracy in the electrostatic energies (instead of forces) in 3D periodic charged systems. To this end we construct an optimal influence function that minimizes the RMS errors in the energies. As a by-product we derive a new real-space cut-off correction term, give a transparent derivation of the systematic errors in terms of Madelung energies, and provide an accurate analytical estimate for the RMS error of the energies. This error estimate is a useful indicator of the accuracy of the computed energies, and allows an easy and precise determination of the optimal values of the various parameters in the algorithm (Ewald splitting parameter, mesh size and charge assignment order).Comment: 31 pages, 3 figure

Scaling Invariance in a Time-Dependent Elliptical Billiard

We study some dynamical properties of a classical time-dependent elliptical billiard. We consider periodically moving boundary and collisions between the particle and the boundary are assumed to be elastic. Our results confirm that although the static elliptical billiard is an integrable system, after to introduce time-dependent perturbation on the boundary the unlimited energy growth is observed. The behaviour of the average velocity is described using scaling arguments

Exclusive rare Bs→(K,η,η′)ℓ+ℓ−B_s\to (K,\eta,\eta')\ell^+\ell^- decays in the light-front quark model

Using the light-front quark model, we calculate the transition form factors, decay rates, and longitudinal lepton polarization asymmetries for the exclusive rare $B_s\to (K,\eta^{(\prime)})(\ell^+\ell^-,\nu_{\ell}\bar{\nu_{\ell}}$ ($\ell=e,\mu,\tau$) decays within the standard model, taking into account the $\eta-\eta'$ mixing angle. For the mixing angle $\theta=-20^{\circ}$ ($-10^{\circ}$) in the octet-singlet basis, we obtain ${\rm BR}(B_s\to \eta\sum\nu_{\ell}\bar{\nu}_{\ell})=1.1 (1.7)\times 10^{-6}$, ${\rm BR}(B_s\to \eta\mu^+\mu^-)=1.5 (2.4)\times 10^{-7}$, ${\rm BR}(B_s\to \eta\tau^+\tau^-)=3.8 (5.8)\times 10^{-8}$, ${\rm BR}(B_s\to \eta'\sum\nu_{\ell}\bar{\nu}_{\ell})=1.8 (1.3)\times 10^{-6}$, ${\rm BR}(B_s\to \eta'\mu^+\mu^-)=2.4 (1.8)\times 10^{-7}$, and ${\rm BR}(B_s\to \eta'\tau^+\tau^-)=3.4 (2.6)\times 10^{-8}$, respectively. The branching ratios for the $B_s\to K(\nu_{\ell}\bar{\nu_{\ell}},\ell^+\ell^-)$ decays are at least an order of magnitude smaller than those for the $B_s\to \eta^{(\prime)}(\nu_{\ell}\bar{\nu_{\ell}},\ell^+\ell^-)$ decays. The averaged values of the lepton polarization asymmetries for $B_s\to (K,\eta^{(\prime)})\ell^+\ell^-$ are obtained as \la P^K_L\ra_\mu=\la P^\eta_L\ra_\mu=\la P^{\eta'}_L\ra_\mu=-0.98, \la P^K_L\ra_\tau=-0.24, \la P^\eta_L\ra_\tau=-0.20 and \la P^{\eta'}_L\ra_\tau=-0.14, respectively.Comment: 20 pages, 6 figures, minor revision. version to appear in Journal of Physics

KIC 10080943: a binary star with two γ Doradus/δ Scuti hybrid pulsators. Analysis of the g modes

We use 4 yr of Kepler photometry to study the non-eclipsing spectroscopic binary KIC 10080943. We find both components to be γ Doradus/δ Scuti hybrids, which pulsate in both p and g modes. We present an analysis of the g modes, which is complicated by the fact that the two sets of l = 1 modes partially overlap in the frequency spectrum. Nevertheless, it is possible to disentangle them by identifying rotationally split doublets from one component and triplets from the other. The identification is helped by the presence of additive combina- tion frequencies in the spectrum that involve the doublets but not the triplets. The rotational splittings of the multiplets imply core rotation periods of about 11 and 7 d in the two stars. One of the stars also shows evidence of l = 2 modes

Hamiltonian approach to the bound state problem in QCD_2

Bosonization of the two-dimensional QCD in the large N_C limit is performed in the framework of Hamiltonian approach in the Coulomb gauge. The generalized Bogoliubov transformation is applied to diagonalize the Hamiltonian in the bosonic sector of the theory, and the composite operators creating/annihilating bosons are obtained in terms of dressed quark operators. The bound state equation is reconstructed as a result of the generalized Bogoliubov transformation, and the form of its massless solution, chiral pion, is found explicitly. Chiral properties of the theory are discussed.Comment: 9 pages, LaTeX2

Quantum Mechanics of the Vacuum State in Two-Dimensional QCD with Adjoint Fermions

A study of two-dimensional QCD on a spatial circle with Majorana fermions in the adjoint representation of the gauge groups SU(2) and SU(3) has been performed. The main emphasis is put on the symmetry properties related to the homotopically non-trivial gauge transformations and the discrete axial symmetry of this model. Within a gauge fixed canonical framework, the delicate interplay of topology on the one hand and Jacobians and boundary conditions arising in the course of resolving Gauss's law on the other hand is exhibited. As a result, a consistent description of the residual $Z_N$ gauge symmetry (for SU(N)) and the ``axial anomaly" emerges. For illustrative purposes, the vacuum of the model is determined analytically in the limit of a small circle. There, the Born-Oppenheimer approximation is justified and reduces the vacuum problem to simple quantum mechanics. The issue of fermion condensates is addressed and residual discrepancies with other approaches are pointed out.Comment: 44 pages; for hardcopies of figures, contact [email protected]

SU(N)-Gauge Theories in Polyakov Gauge on the Torus

We investigate the Abelian projection with respect to the Polyakov loop operator for SU(N) gauge theories on the four torus. The gauge fixed $A_0$ is time-independent and diagonal. We construct fundamental domains for $A_0$. In sectors with non-vanishing instanton number such gauge fixings are always singular. The singularities define the positions of magnetically charged monopoles, strings or walls. These magnetic defects sit on the Gribov horizon and have quantized magnetic charges. We relate their magnetic charges to the instanton number.Comment: 11 pages, 2 figure