Light charged Higgs boson production at the Large Hadron electron Collider

We study the production of a light charged Higgs boson at the future Large Hadron electron Collider (LHeC), through the process $e^- p \to \nu_e H^- q$ considering both decay channels $H^- \to b \bar c$ and $H^- \to \tau \bar \nu_\tau$ in the final state. We analyse these processes in the context of the 2-Higgs Doublet Model Type III (2HDM-III) and assess the LHeC sensitivity to such $H^-$ signals against a variety of both reducible and irreducible backgrounds. We confirm that prospects for $H^-$ detection in the 2HDM-III are excellent assuming standard collider energy and luminosity conditions.Comment: 12 pages, 12 figures. Accepted in Physical Review

Bright and dark breathers in Fermi-Pasta-Ulam lattices

In this paper we study the existence and linear stability of bright and dark breathers in one-dimensional FPU lattices. On the one hand, we test the range of validity of a recent breathers existence proof [G. James, {\em C. R. Acad. Sci. Paris}, 332, Ser. 1, pp. 581 (2001)] using numerical computations. Approximate analytical expressions for small amplitude bright and dark breathers are found to fit very well exact numerical solutions even far from the top of the phonon band. On the other hand, we study numerically large amplitude breathers non predicted in the above cited reference. In particular, for a class of asymmetric FPU potentials we find an energy threshold for the existence of exact discrete breathers, which is a relatively unexplored phenomenon in one-dimensional lattices. Bright and dark breathers superposed on a uniformly stressed static configuration are also investigated.Comment: 11 pages, 16 figure

High temperature behavior of Sr-doped layered cobaltites Y(Ba1-xSrx)Co2O5.5: phase stability and structural properties

In this article we present a neutron diffraction in-situ study of the thermal evolution and high-temperature structure of layered cobaltites Y(Ba, Sr)Co2 O5+{\delta}. Neutron thermodiffractograms and magnetic susceptibility measurements are reported in the temperature range 20 K <= T <= 570 K, as well as high resolution neutron diffraction experiments at selected temperatures. Starting from the as-synthesized samples with {\delta} ~ 0.5, we show that the room temperature phases remain stable up to 550 K, where they start loosing oxygen and transform to a vacancy-disordered "112" structure with tetragonal symmetry. Our results also show how the so-called "122" structure can be stabilized at high temperature (around 450 K) in a sample in which the addition of Sr at the Ba site had suppressed its formation. In addition, we present the structural and magnetic properties of the resulting samples with a new oxygen content {\delta} ~ 0.25 in the temperature range 20 K <= T <= 300 K

Exploring cloudy gas accretion as a source of interstellar turbulence in the outskirts of disks

High--resolution 2D--MHD numerical simulations have been carried out to investigate the effects of continuing infall of clumpy gas in extended HI galactic disks. Given a certain accretion rate, the response of the disk depends on its surface gas density and temperature. For Galactic conditions at a galactocentric distance of ~20 kpc, and for mass accretion rates consistent with current empirical and theoretical determinations in the Milky Way, the rain of compact high velocity clouds onto the disk can maintain transonic turbulent motions in the warm phase (~2500 K) of HI. Hence, the HI line width is expected to be ~6.5 km/s for a gas layer at 2500 K, if infall were the only mechanism of driving turbulence. Some statistical properties of the resulting forcing flow are shown in this Letter. The radial dependence of the gas velocity dispersion is also discussed.Comment: 13 pages, 3 figures, accepted for publication in ApJ Letter

First-Principles Study of Substitutional Metal Impurities in Graphene: Structural, Electronic and Magnetic Properties

We present a theoretical study using density functional calculations of the structural, electronic and magnetic properties of 3d transition metal, noble metal and Zn atoms interacting with carbon monovacancies in graphene. We pay special attention to the electronic and magnetic properties of these substitutional impurities and found that they can be fully understood using a simple model based on the hybridization between the states of the metal atom, particularly the d shell, and the defect levels associated with an unreconstructed D3h carbon vacancy. We identify three different regimes associated with the occupation of different carbon-metal hybridized electronic levels: (i) bonding states are completely filled for Sc and Ti, and these impurities are non-magnetic; (ii) the non-bonding d shell is partially occupied for V, Cr and Mn and, correspondingly, these impurties present large and localized spin moments; (iii) antibonding states with increasing carbon character are progressively filled for Co, Ni, the noble metals and Zn. The spin moments of these impurities oscillate between 0 and 1 Bohr magnetons and are increasingly delocalized. The substitutional Zn suffers a Jahn-Teller-like distortion from the C3v symmetry and, as a consequence, has a zero spin moment. Fe occupies a distinct position at the border between regimes (ii) and (iii) and shows a more complex behavior: while is non-magnetic at the level of GGA calculations, its spin moment can be switched on using GGA+U calculations with moderate values of the U parameter.Comment: 13 figures, 4 tables. Submitted to Phys. Rev. B on September 26th, 200