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Search Results: 1 - 10 of 208470 matches for " L. Nellen "
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Neutrino production through hadronic cascades in AGN accretion disks
L. Nellen,K. Mannheim,P. L. Biermann
Physics , 1992, DOI: 10.1103/PhysRevD.47.5270
Abstract: We consider the production of neutrinos in active galactic nuclei (AGN) through hadronic cascades. The initial, high energy nucleons are accelerated in a source above the accretion disk around the central black hole. From the source, the particles diffuse back to the disk and initiate hadronic cascades. The observable output from the cascade are electromagnetic radiation and neutrinos. We use the observed diffuse background X-ray luminosity, which presumably results {}from this process, to predict the diffuse neutrino flux close to existing limits from the Frejus experiment. The resulting neutrino spectrum is $E^{-2}$ down to the $\GeV$ region. We discuss modifications of this scenario which reduce the predicted neutrino flux.
Unfolding a degeneracy point of two unbound states: Crossings and anticrossings of energies and widths
E. Hernandez,A. Jauregui,A. Mondragon,L. Nellen
Physics , 2005,
Abstract: We show that when an isolated doublet of unbound states of a physical system becomes degenerate for some values of the control parameters of the system, the energy hypersurfaces representing the complex resonance energy eigenvalues as functions of the control parameters have an algebraic branch point of rank one in parameter space. Associated with this singularity in parameter space, the scattering matrix, S_l(E), and the Green's function, G_l^(+)(k; r,r'), have one double pole in the unphysical sheet of the complex energy plane. We characterize the universal unfolding or deformation of a typical degeneracy point of two unbound states in parameter space by means of a universal 2-parameter family of functions which is contact equivalent to the pole position function of the isolated doublet of resonances at the exceptional point and includes all small perturbations of the degeneracy condition up to contact equivalence.
G. Braun, K. Goronzy, A. Tr ster, A. Oschmann, Die franz sischen Korrespondenzen, Band V, 1. Teil: 1646-1647, 2. Teil: 1647
H. Nellen
BMGN : Low Countries Historical Review , 2003,
Abstract:
A. J nsson, Letters from sir James Spens and Jan Rutgers
Henk Nellen
BMGN : Low Countries Historical Review , 2009,
Abstract:
Azimuthal signal variations in the engineering array of the Pierre Auger Observatory
Caballero-Mora, K.S.;Nellen, L;Valdés-Galicia, J.F.;
Revista mexicana de física , 2008,
Abstract: we study the azimuthal variation of the signal in selected tanks in the engineering array of the pierre auger observatory. we fi a parametrization to the observed variations. we also note that the observation of the signal variations can be used to detect mistakes in the wireing from the pmts to the control board.
Local topology and universal unfolding of the energy surfaces at a crossing of unbound states
Hernández, E;Jáuregui, A;Mondragón, A;Nellen, L;
Revista mexicana de física , 2006,
Abstract: we show that when an isolated doublet of unbound states of a physical system becomes degenerate, the eigenenergy surfaces have an algebraic branch point of rank one and branch cuts in its real and imaginary parts starting at the same exceptional point but extending in opposite directions in parameter space. associated with this singularity in parameter space, the scattering matrix, sl (e), and the green's function, gl (+) (k; r, r'), have one double pole in the unphysical sheet of the complex energy plane. we characterize the universal unfolding or deformation of a typical degeneracy point of two unbound states in parameter space by means of a universal 2-parameter family of functions which is contact equivalent to the pole position function of the isolated doublet of resonances at the exceptional point and includes all small perturbations of the degeneracy condition up to contact equivalence. the rich phenomenology of crossings and anticrossings of energies and widths, as well as the sudden change in shape of the s(e)-matrix pole trajectories, observed in an isolated doublet of resonances when one control parameter is varied, is fully explained in terms of the topological properties of the energy hypersurfaces close to the degeneracy point.
Local topology and universal unfolding of the energy surfaces at a crossing of unbound states
E. Hernández,A. Jáuregui,A. Mondragón,L. Nellen
Revista mexicana de física , 2006,
Abstract: Demostramos que, cuando un doblete aislado de estados no-ligados de un sistema físico esta degenerado, las superfices de la autoenergía tienen un punto ramal de rango uno y cortes ramales en las partes real e imaginaria que empiezan en el mismo punto excepcional pero se extienden en direcciones opuestas en el espacio de parámetros. Asociado a esta singularidad en el espacio de parámetros, la matriz de dispersión, S (E), y la función de Green, Gl (+) (k; r, r'), tienen un polo doble en la hoja no física del plano complejo de la energía. Caracterizamos el despliegue universal o deformacion de un punto de degeneración de dos estados no ligados típico, en el espacio de los parámetros, por medio de una familia universal de funciones que depende de dos parámetros y que es equivalente por contacto a la función de posición del polo del doblete aislado de resonancias en el punto excepcional e incluye todas las perturbaciones peque as de las condiciones de degeneración, hasta equivalencia por contacto. La rica fenomenología de cruces y anticruces de energías y semianchuras, así como el cambio repentino de la forma de las trayectoria de los polos de la matriz S(E), que se observa en un doblete aislado de resonancias cuando un parámetro de control se hace variar, se explica completamente en términos de las propiedades topológicas de las hipersuperficies de la energía cerca del punto de la degeneración.
Azimuthal signal variations in the engineering array of the Pierre Auger Observatory
K.S. Caballero-Mora,L. Nellen,J.F. Valdés-Galicia
Revista mexicana de física , 2008,
Abstract: Estudiamos la variación azimutal de las se ales detectadas en tanques seleccionados de la red de ingeniería del Observatorio Pierre Auger. Ajustamos una parametrización a las variaciones observadas. También notamos que es posible analizar las variaciones para detectar errores en el cableado entre los tubos fotomultiplicadores y la tarjeta controladora.
Critical Casimir effect in classical binary liquid mixtures
A. Gambassi,A. Maciolek,C. Hertlein,U. Nellen,L. Helden,C. Bechinger,S. Dietrich
Physics , 2009, DOI: 10.1103/PhysRevE.80.061143
Abstract: If a fluctuating medium is confined, the ensuing perturbation of its fluctuation spectrum generates Casimir-like effective forces acting on its confining surfaces. Near a continuous phase transition of such a medium the corresponding order parameter fluctuations occur on all length scales and therefore close to the critical point this effect acquires a universal character, i.e., to a large extent it is independent of the microscopic details of the actual system. Accordingly it can be calculated theoretically by studying suitable representative model systems. We report on the direct measurement of critical Casimir forces by total internal reflection microscopy (TIRM), with femto-Newton resolution. The corresponding potentials are determined for individual colloidal particles floating above a substrate under the action of the critical thermal noise in the solvent medium, constituted by a binary liquid mixture of water and 2,6-lutidine near its lower consolute point. Depending on the relative adsorption preferences of the colloid and substrate surfaces with respect to the two components of the binary liquid mixture, we observe that, upon approaching the critical point of the solvent, attractive or repulsive forces emerge and supersede those prevailing away from it. Based on the knowledge of the critical Casimir forces acting in film geometries within the Ising universality class and with equal or opposing boundary conditions, we provide the corresponding theoretical predictions for the sphere-planar wall geometry of the experiment. The experimental data for the effective potential can be interpreted consistently in terms of these predictions and a remarkable quantitative agreement is observed.
The potential of the HAWC Observatory to observe violations of Lorentz Invariance
Lukas Nellen,for the HAWC collaboration
Physics , 2015,
Abstract: The framework of relativistic quantum-field theories requires Lorentz Invariance. Many theories of quantum gravity, on the other hand, include violations of Lorentz Invariance at small scales and high energies. This generates a lot of interest in establishing limits on such effects, and, if possible, observing them directly. Gamma-ray observatories provide a tool to probe parts of the parameter space of models of Lorentz Invariance Violation that is not accessible in terrestrial laboratories and man-made accelerators. Transients, especially gamma-ray bursts, are a particularly promising class of events to search for such phenomena. By combining cosmological distances with high energy emission and short duration, emitting photons up to 30 GeV in less than a second, one can measure the energy dependence of the speed of photons to one part in $10^{16}$. We will discuss the potential of HAWC to detect effects of the violation of Lorentz Invariance and place its sensitivity in the context of existing limits.
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