Critical structure factor in Ising systems

We perform a large-scale Monte Carlo simulation of the three-dimensional Ising model on simple cubic lattices of size L^3 with L=128 and 256. We determine the corresponding structure factor (Fourier transform of the two-point function) and compare it with several approximations and with experimental results. We also compute the turbidity as a function of the momentum of the incoming radiation, focusing in particular on the deviations from the Ornstein-Zernicke expression of Puglielli and Ford.Comment: 16 page

Interstellar MHD Turbulence and Star Formation

This chapter reviews the nature of turbulence in the Galactic interstellar medium (ISM) and its connections to the star formation (SF) process. The ISM is turbulent, magnetized, self-gravitating, and is subject to heating and cooling processes that control its thermodynamic behavior. The turbulence in the warm and hot ionized components of the ISM appears to be trans- or subsonic, and thus to behave nearly incompressibly. However, the neutral warm and cold components are highly compressible, as a consequence of both thermal instability in the atomic gas and of moderately-to-strongly supersonic motions in the roughly isothermal cold atomic and molecular components. Within this context, we discuss: i) the production and statistical distribution of turbulent density fluctuations in both isothermal and polytropic media; ii) the nature of the clumps produced by thermal instability, noting that, contrary to classical ideas, they in general accrete mass from their environment; iii) the density-magnetic field correlation (or lack thereof) in turbulent density fluctuations, as a consequence of the superposition of the different wave modes in the turbulent flow; iv) the evolution of the mass-to-magnetic flux ratio (MFR) in density fluctuations as they are built up by dynamic compressions; v) the formation of cold, dense clouds aided by thermal instability; vi) the expectation that star-forming molecular clouds are likely to be undergoing global gravitational contraction, rather than being near equilibrium, and vii) the regulation of the star formation rate (SFR) in such gravitationally contracting clouds by stellar feedback which, rather than keeping the clouds from collapsing, evaporates and diperses them while they collapse.Comment: 43 pages. Invited chapter for the book "Magnetic Fields in Diffuse Media", edited by Elisabete de Gouveia dal Pino and Alex Lazarian. Revised as per referee's recommendation

Adsorption of mono- and multivalent cat- and anions on DNA molecules

Adsorption of monovalent and multivalent cat- and anions on a deoxyribose nucleic acid (DNA) molecule from a salt solution is investigated by computer simulation. The ions are modelled as charged hard spheres, the DNA molecule as a point charge pattern following the double-helical phosphate strands. The geometrical shape of the DNA molecules is modelled on different levels ranging from a simple cylindrical shape to structured models which include the major and minor grooves between the phosphate strands. The densities of the ions adsorbed on the phosphate strands, in the major and in the minor grooves are calculated. First, we find that the adsorption pattern on the DNA surface depends strongly on its geometrical shape: counterions adsorb preferentially along the phosphate strands for a cylindrical model shape, but in the minor groove for a geometrically structured model. Second, we find that an addition of monovalent salt ions results in an increase of the charge density in the minor groove while the total charge density of ions adsorbed in the major groove stays unchanged. The adsorbed ion densities are highly structured along the minor groove while they are almost smeared along the major groove. Furthermore, for a fixed amount of added salt, the major groove cationic charge is independent on the counterion valency. For increasing salt concentration the major groove is neutralized while the total charge adsorbed in the minor groove is constant. DNA overcharging is detected for multivalent salt. Simulations for a larger ion radii, which mimic the effect of the ion hydration, indicate an increased adsorbtion of cations in the major groove.Comment: 34 pages with 14 figure

Measurements of exclusive B_s^0 decays at the Y(5S) resonance

Several exclusive $B_s^0$ decays are studied using a 1.86 fb-1 data sample collected at the Y(5S) resonance with the Belle detector at the KEKB asymmetric energy e^+ e^- collider. In the $B_s^0 \to D_s^- \pi^+$ decay mode we find 10 $B_s^0$ candidates and measure the corresponding branching fraction. Combining the B_s^0 -> D_s^{(*)-} \pi^+, B_s^0 -> D_s^{(*)-} \rho^+, B_s^0 -> J/\psi \phi and B_s^0 -> J/\psi \eta decay modes, a significant $B_s^0$ signal is observed. The ratio \sigma (e^+ e^- -> B_s^* \bar{B}_s^*) / \sigma (e^+ e^- -> B_s^{(*)} \bar{B}_s^{(*)}) = (93^{+7}_{-9} \pm 1)% is obtained at the Y(5S) energy, indicating that $B_s^0$ meson production proceeds predominantly through the creation of $B^*_s \bar{B}^*_s$ pairs. The $B_s^0$ and $B_s^*$ meson masses are measured to be M(B_s^0)=(5370 \pm 1 \pm 3)MeV/c^2 and M(B_s^*)=(5418 \pm 1 \pm 3)MeV/c^2. Upper limits on the B_s^0 -> \gamma \gamma, B_s^0 -> \phi \gamma, B_s^0 -> K^+ K^- and B_s^0 -> D_s^{(*)+} D_s^{(*)-} branching fractions are also reported.Comment: 9 pages, 5 figures, published in Phys. Rev. D76, 012002 (2007

Measurement of the near-threshold e+e−→DDˉe^+e^- \to D \bar D cross section using initial-state radiation

We report measurements of the exclusive cross section for $e^+e^- \to D \bar D$, where $D=D^0$ or $D^+$, in the center-of-mass energy range from the $D \bar D$ threshold to $5\mathrm{GeV}/c^2$ with initial-state radiation. The analysis is based on a data sample collected with the Belle detector with an integrated luminosity of 673 $\mathrm{fb}^{-1}$.Comment: Presented at EPS07 and LP07 conferences, published in PRD(RC

Measurement of K^+K^- production in two-photon collisions in the resonant-mass region

K^+K^- production in two-photon collisions has been studied using a large data sample of 67 fb^{-1} accumulated with the Belle detector at the KEKB asymmetric e^+e^- collider. We have measured the cross section for the process gamma gamma -> K^+ K^- for center-of-mass energies between 1.4 and 2.4 GeV, and found three new resonant structures in the energy region between 1.6 and 2.4 GeV. The angular differential cross sections have also been measured.Comment: 24 pages, 8 figures, to appear in Euro. Phys. Jour.

Search for Resonant B±→K±h→K±γγB^{\pm}\to K^{\pm} h \to K^{\pm} \gamma \gamma Decays at Belle

We report measurements and searches for resonant $B^{\pm} \to K^{\pm} h \to K^{\pm} \gamma \gamma$ decays where $h$ is a $\eta,\eta^{\prime},\eta_{c},\eta_{c}(2S),\chi_{c0},\chi_{c2},J/\psi$ meson or the X(3872) particle.Comment: accepted by Physics Letters

Study of charmonia in four-meson final states produced in two-photon collisions

We report measurements of charmonia produced in two-photon collisions and decaying to four-meson final states, where the meson is either a charged pion or a charged kaon. The analysis is based on a 395fb^{-1} data sample accumulated with the Belle detector at the KEKB electron-positron collider. We observe signals for the three C-even charmonia eta_c(1S), chi_{c0}(1P) and chi_{c2}(1P) in the pi^+pi^-pi^+pi^-, K^+K^-pi^+pi^- and K^+K^-K^+K^- decay modes. No clear signals for eta_c(2S) production are found in these decay modes. We have also studied resonant structures in charmonium decays to two-body intermediate meson resonances. We report the products of the two-photon decay width and the branching fractions, Gamma_{gamma gamma}B, for each of the charmonium decay modes.Comment: 22 pages, 12 figure

Search for Lepton-Flavor-Violating tau Decays into a Lepton and an f0(980) Meson

We search for lepton-flavor-violating tau decays into a lepton (electron or muon) and an f0(980) meson using 671 fb-1 of data collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. No events are observed and we set the following 90% C.L. upper limits on the branching fraction products: B(tau- -> e-f0(980))*B(f0(980)->pi+pi-) mu-f0(980))*B(f0(980) -> pi+pi-)<3.4x10^-8. This is the first search performed for these modes.Comment: 10 pages, 4 figures, submitted to Phys. Lett.