Insights on scalar mesons from their radiative decays

We estimate the rates for radiative transitions of the lightest scalar mesons f_0(980) and a_0(980) to the vector mesons rho and omega. We argue that measurements of the radiative decays of those scalar mesons can provide important new information on their structure.Comment: 20 pages, 5 figures; appendix added, to be published in Phys. Rev.

2-elementary subgroups of the space Cremona group

We give a sharp bound for orders of elementary abelian 2-groups of birational automorphisms of rationally connected threefolds

Long-time evolution of magnetic fields in relativistic GRB shocks

We investigate the long-time evolution of magnetic fields generated by the two-stream instability at ultra- and sub-relativistic astrophysical collisionless shocks. Based on 3D PIC simulation results, we introduce a 2D toy model of interacting current filaments. Within the framework of this model, we demonstrate that the field correlation scale in the region far downstream the shock grows nearly as the light crossing time, lambda(t) ~ ct, thus making the diffusive field dissipation inefficient. The obtained theoretical scaling is tested using numerical PIC simulations. This result extends our understanding of the structure of collisionless shocks in gamma-ray bursts and other astrophysical objects.Comment: 5 pages. 2 figures. Submitted to ApJ

Modeling Phase-resolved Observations of the Surfaces of Magnetic Neutron Stars

Recent observations by XMM-Newton detected rotational pulsations in the total brightness and spectrum of several neutron stars. To properly interpret the data, accurate modeling of neutron star emission is necessary. Detailed analysis of the shape and strength of the rotational variations allows a measurement of the surface composition and magnetic field, as well as constrains the nuclear equation of state. We discuss our models of the spectra and light curves of two of the most observed neutron stars, RX J1856.5-3754 and 1E 1207.4-5209, and discuss some implications of our results and the direction of future work.Comment: 5 pages, 6 figures; Proceedings of "40 Years of Pulsars", eds. C. Bassa, Z. Wang, A. Cumming, V. Kaspi, AIP, submitte

Designing Dirac points in two-dimensional lattices

We present a framework to elucidate the existence of accidental contacts of energy bands, particularly those called Dirac points which are the point contacts with linear energy dispersions in their vicinity. A generalized von-Neumann-Wigner theorem we propose here gives the number of constraints on the lattice necessary to have contacts without fine tuning of lattice parameters. By counting this number, one could quest for the candidate of Dirac systems without solving the secular equation. The constraints can be provided by any kinds of symmetry present in the system. The theory also enables the analytical determination of k-point having accidental contact by selectively picking up only the degenerate solution of the secular equation. By using these frameworks, we demonstrate that the Dirac points are feasible in various two-dimensional lattices, e.g. the anisotropic Kagome lattice under inversion symmetry is found to have contacts over the whole lattice parameter space. Spin-dependent cases, such as the spin-density-wave state in LaOFeAs with reflection symmetry, are also dealt with in the present scheme.Comment: 15pages, 9figures (accepted to Phys. Rev. B

Localized boundary layer below the mid-Pacific velocity anomaly identified from a PeP precursor

Dense record sections from deep earthquakes in Fiji and Argentina recorded on hundreds of short-period stations in California at distances of 81° to 85° are used to investigate the detailed P wave velocity structure above the core-mantle boundary (CMB). In the Fiji data a secondary phase arriving 2 to 4 s after the direct P is identified as a precursor to PcP. This phase provides good evidence for a reflection off the top of a thin low-velocity layer above the CMB. Comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5%–10% compared to the overlying mantle. A record section from an Argentina event does not show the PcP precursor, indicating that the low-velocity layer is not a global feature. This thin low-velocity layer is in the same place as a much larger S wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the CMB under the mid-Pacific

Evidence for a Mid-Atomic-Number Atmosphere in the Neutron Star 1E1207.4-5209

Recently Sanwal et al. (2002) reported the first clear detection of absorption features in an isolated neutron star, 1E1207.4-5209. Remarkably their spectral modeling demonstrates that the atmosphere cannot be Hydrogen. They speculated that the neutron star atmosphere is indicative of ionized Helium in an ultra-strong (~1.5x10^{14} G) magnetic field. We have applied our recently developed atomic model (Mori & Hailey 2002) for strongly-magnetized neutron star atmospheres to this problem. We find that this model, along with some simp le atomic physics arguments, severely constrains the possible composition of the atmosphere. In particular we find that the absorption features are naturally associated with He-like Oxygen or Neon in a magnetic field of ~10^{12} G, comparable to the magnetic field derived from the spin parameters of the neutron star. This interpretation is consistent with the relative line strengths and widths and is robust. Our model predicts possible substructure in the spectral features, which has now been reported by XMM-Newton (Mereghetti et al. 2002). However we show the Mereghetti et al. claim that the atmosphere is Iron or some comparable high-Z element at ~ 10^{12} G is easily ruled out by the Chandra and XMM-Newton data.Comment: 5 pages, AASTeX, Revised version. Accepted for publication in ApJ Letter

Steric Hindrance as a Mechanistic Probe for Olefin Reactivity: Variability of the Hydrogenic Canopy over the Isomeric Adamantylideneadamantane/Sesquihomoadamantene Pair (A Combined Experimental and Theoretical Study)

Access to each CC face of adamantylideneadamantane (AA) and sesquihomoadamantene (SA) is hindered by the hydrogenic canopy consisting of four β-hydrogens; otherwise, these olefins have quite normal environments. X-ray crystallography and density functional (DFT) calculations show a 0.5 Å larger annular opening in the protective cover of AA than that in SA. This contributes to the remarkable differences in reactivity toward various reagents, not only by limiting access to the olefin site in SA but also by inhibiting reactions which force these hydrogens closer together. Thus, AA is subject to typical olefin-addition reactions with bromine, sulfuryl chloride, m-chloroperbenzoic acid, dioxygen, and so forth, albeit sometimes at attenuated rates. On the other hand, SA is singularly unreactive under identical reaction conditions, except for the notable exceptions that include Brønsted (protonic) acids, a nitrosonium cation, and dichlorine. The exceptions are characterized as three sterically limited (electrophilic) reagents whose unique reactivity patterns are shown to be strongly influenced by steric access to the CC center. As such, the different degrees of steric encumbrance in the isomeric donors AA and SA shed considerable light on the diverse nature of olefinic reactions. In particular, they evoke mechanistic features in electrophilic addition versus electron transfer, which are otherwise not readily discernible with other less hindered olefinic donors. Transient structures of the olefinic-reaction intermediates such as the protonated carbocations AA−H+ and SA−H+ as well as the cation radicals AA•+ and SA•+ are probed by the combination of X-ray crystallographic analyses and density functional theoretical computations

Structure of the baryonic flux tube in N_{f}=2 lattice QCD at finite temperature

We study the flux tube profile in the baryonic system in full QCD at finite temperature on $N_{t}=8$ lattice. We fix the maximally Abelian gauge and measure the monopole and the photon parts of the Abelian action density, the color electric field and the monopole current on both sides of the finite temperature transition. We demonstrate the disappearance of the flux tube in the high temperature phase.Comment: 3 pages, 4 figures, Lattice2003 topolog

Magnetically assisted self-injection and radiation generation for plasma based acceleration

It is shown through analytical modeling and numerical simulations that external magnetic fields can relax the self-trapping thresholds in plasma based accelerators. In addition, the transverse location where self-trapping occurs can be selected by adequate choice of the spatial profile of the external magnetic field. We also find that magnetic-field assisted self-injection can lead to the emission of betatron radiation at well defined frequencies. This controlled injection technique could be explored using state-of-the-art magnetic fields in current/next generation plasma/laser wakefield accelerator experiments.Comment: 7 pages, 4 figures, accepted for publication in Plasma Physics and Controlled Fusio