J. A. R. Cembranos

Publications (37)8.93 Total impact

• Article: Reliability of Monte Carlo event generators for gamma ray dark matter searches

  J. A. R. Cembranos, A. de la Cruz-Dombriz, V. Gammaldi, R. A. Lineros, A. L. Maroto
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  ABSTRACT: We study the differences in the gamma ray spectra simulated by four Monte Carlo event generator packages developed in particle physics. Two different versions of PYTHIA and two of HERWIG are analyzed, namely PYTHIA 6.418 and HERWIG 6.5.10 in Fortran and PYTHIA 8.165 and HERWIG 2.6.1 in C++. For all the studied channels, the intrinsic differences between them are shown to be significative and may play an important role in misunderstanding dark matter signals.
  05/2013;
• Article: Spectral Study of the HESS J1745-290 Gamma-Ray Source as Dark Matter Signal

  J. A. R. Cembranos, V. Gammaldi, A. L. Maroto
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  ABSTRACT: We study the main spectral features of the gamma-ray fluxes observed by the High Energy Stereoscopic System (HESS) from the J1745-290 Galactic Center source during the years 2004, 2005 and 2006. In particular, we show that these data are well fitted as the secondary gamma-rays photons generated from dark matter annihilating into Standard Model particles in combination with a simple power law background. We present explicit analyses for annihilation in a single standard model particle-antiparticle pair. In this case, the best fits are obtained for the u and d quark-antiquark channels and for the WW and ZZ gauge bosons, with background spectral index compatible with the Fermi-Large Area Telescope (LAT) data from the same region. The fits return a heavy WIMP, with a mass above 10 TeV approximately, but well below the unitarity limit for thermal relic annihilation.
  02/2013;
• Article: On the non-attractive character of gravity in f(R) theories

  F. D. Albareti, J. A. R. Cembranos, A. de la Cruz-Dombriz, A. Dobado
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  ABSTRACT: In General Relativity without a cosmological constant a non-positive contribution from the space-time geometry to Raychaudhuri equation is found provided that particular energy conditions are assumed and regardless the considered solution of the Einstein's equations. This fact is usually interpreted as a manifestation of the attractive character of gravity. Nevertheless, a positive contribution to Raychaudhuri equation from space-time geometry should occur since this is the case in an accelerated expanding Robertson-Walker model for congruences followed by fundamental observers. Modified gravity theories provide the possibility of a positive contribution although the standard energy conditions are assumed. We address this important issue in the context of f(R) theories, deriving explicit upper bounds for the contribution of space-time geometry to the Raychaudhuri equation. Then, we examine the parameter constraints for some paradigmatic f(R) models in order to ensure a positive contribution to this equation.
  12/2012;
• Article: Isotropy theorem for cosmological Yang-Mills theories

  J. A. R. Cembranos, A. L. Maroto, S. J. Núñez Jareño
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  ABSTRACT: We consider homogeneous non-abelian vector fields with general potential terms in an expanding universe. We find a mechanical analogy with a system of N interacting particles (with N the dimension of the gauge group) moving in three dimensions under the action of a central potential. In the case of bounded and rapid evolution compared to the rate of expansion, we show by making use of a generalization of the virial theorem that for arbitrary potential and polarization pattern, the average energy-momentum tensor is always diagonal and isotropic despite the intrinsic anisotropic evolution of the vector field. We consider also the case in which a gauge-fixing term is introduced in the action and show that the average equation of state does not depend on such a term. Finally, we extend the results to arbitrary background geometries and show that the average energy-momentum tensor of a rapidly evolving Yang-Mills fields is always isotropic and has the perfect fluid form for any locally inertial observer.
  12/2012;
• Article: On the collapse in fourth order gravities

  B. Montes Núñez, J. A. R. Cembranos, A. de la Cruz-Dombriz
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  ABSTRACT: The gravitational collapse in fourth order theories of gravity defined by an arbitrary action of the scalar curvature shows significant deviations with General Relativity. The presence of a new scalar mode produces a higher initial contraction that favors the reduction of the collapsing time. However, depending on the particular model, there are fundamental differences when the modifications to the General Relativity collapse leave the linear regime. These analyses can be used to exclude an important region of the parameter space associated with alternative gravitational models.
  10/2012;
• Article: Focusing of geodesic congruences in an accelerated expanding Universe

  F. D. Albareti, J. A. R. Cembranos, A. de la Cruz-Dombriz
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  ABSTRACT: We study the accelerated expansion of the Universe through its consequences on a congruence of geodesics. We make use of the Raychaudhuri equation which describes the evolution of the expansion rate for a congruence of timelike or null geodesics. In particular, we focus on the space-time geometry contribution to this equation. By straightforward calculation from the metric of a Robertson-Walker cosmological model, it follows that in an accelerated expanding Universe the space-time contribution to the Raychaudhuri equation is positive for the fundamental congruence, favoring a non-focusing of the congruence of geodesics. However, the accelerated expansion of the present Universe does not imply a tendency of the fundamental congruence to diverge. It is shown that this is in fact the case for certain congruences of timelike geodesics without vorticity. Therefore, the focusing of geodesics remains feasible in an accelerated expanding Universe. Furthermore, a negative contribution to the Raychaudhuri equation from space-time geometry which is usually interpreted as the manifestation of the attractive character of gravity is restored in an accelerated expanding Robertson-Walker space-time at high speeds.
  08/2012;
• Article: Dark matter origin of the gamma ray emission from the galactic center observed by HESS

  J. A. R. Cembranos, V. Gammaldi, A. L. Maroto
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  ABSTRACT: We show that the gamma ray spectrum observed with the HESS array of Cherenkov telescopes coming from the Galactic Center (GC) region and identified with the source HESS J1745-290, is well fitted by the secondary photons coming from dark matter (DM) annihilation over a diffuse power-law background. The amount of photons and morphology of the signal localized within a region of few parsecs, require compressed DM profiles as those resulting from baryonic contraction, which offer $\sim 10^3$ enhancements in the signal over DM alone simulations. The fitted background from HESS data is consistent with recent Fermi-LAT observations of the same region.
  04/2012;
• Article: Isotropy theorem for cosmological vector fields

  J. A. R. Cembranos, C. Hallabrin, A. L. Maroto, S. J. Núñez Jareño
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  ABSTRACT: We consider homogeneous abelian vector fields in an expanding universe. We find a mechanical analogy in which the system behaves as a particle moving in three dimensions under the action of a central potential. In the case of bounded and rapid evolution compared to the rate of expansion, we show by making use of the virial theorem that for arbitrary potential and polarization pattern, the average energy-momentum tensor is always diagonal and isotropic despite the intrinsic anisotropic evolution of the vector field. For simple power-law potentials of the form V=\lambda (A^\mu A_\mu)^n, the average equation of state is found to be w=(n-1)/(n+1). This implies that vector coherent oscillations could act as natural dark matter or dark energy candidates. Finally, we show that under very general conditions, the average energy-momentum tensor of a rapidly evolving bounded vector field in any background geometry is always isotropic and has the perfect fluid form for any locally inertial observer.
  03/2012;
• Source

  Available from: ArXiv

  Article: Indirect constraints to branon dark matter

  J. A. R. Cembranos, A. de la Cruz-Dombriz, V. Gammaldi, A. L. Maroto
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  ABSTRACT: If the present dark matter in the Universe annihilates into Standard Model particles, it must contribute to the gamma ray fluxes detected on the Earth. Here we briefly review the present constraints for the detection of gamma ray photons produced in the annihilation of branon dark matter. We show that observations of dwarf spheroidal galaxies and the galactic center by EGRET, Fermi-LAT or MAGIC are below the sensitivity limits for branon detection. However,future experiments such as CTA could be able to detect gamma-ray photons from annihilating branons of masses above 150 GeV.
  02/2012;
• Source

  Available from: ArXiv

  Article: Modified spinning black holes

  J. A. R. Cembranos, A. de la Cruz-Dombriz, P. Jimeno Romero
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  ABSTRACT: In the context of f(R) modified gravity theories we determine that the black holes existence is determined by the sign of a parameter dependent of the mass, the charge, the spin and the scalar curvature. We obtain the different astrophysical objects derived from the presence of different horizons. Thermodynamics for this kind of black holes is studied for negative values of the curvature, revealing that we can distinguish between two kinds of BH: fast and slow.
  02/2012;
• Source

  Available from: ArXiv

  Article: Gravitational collapse in f(R) theories

  J. A. R. Cembranos, A. de la Cruz-Dombriz, B. Montes Nuñez
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  ABSTRACT: We study the gravitational collapse in modified gravitational theories. In particular, we analyze a general $f(R)$ model with uniformly collapsing cloud of self-gravitating dust particles. This analysis shares analogies with the formation of large-scale structures in the early Universe and with the formation of stars in a molecular cloud experiencing gravitational collapse. In the same way, this investigation can be used as a first approximation to the modification that stellar objects can suffer in these modified theories of gravity. We study concrete examples, and find that the analysis of gravitational collapse is an important tool to constrain models that present late-time cosmological acceleration.
  01/2012;
• Source

  Available from: ArXiv

  Article: Detection of branon dark matter with gamma ray telescopes

  J. A. R. Cembranos, A. de la Cruz-Dombriz, V. Gammaldi, A. L. Maroto
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  ABSTRACT: Branons are new degrees of freedom that appear in flexible brane-world models corresponding to brane fluctuations. These new fields can behave as standard weakly interacting massive particles (WIMPs) with a significant associated thermal relic density. We analyze the present constraints from their spontaneous annihilations into photons for EGRET, Fermi-LAT and MAGIC, and the prospects for detection in future Cherenkov telescopes. In particular, we focus on possible signals coming from the Galactic Center and different dwarf spheroidals, such as Draco, Sagittarius, Canis Major and SEGUE 1. We conclude that for those targets, present observations are below the sensitivity limits for branon detection by assuming standard dark matter distributions and no additional boost factors. However, future experiments such as CTA could be able to detect gamma-ray photons coming from the annihilation of branons with masses higher than 150 GeV.
  11/2011;
• Source

  Available from: ArXiv

  Article: Kerr-Newman black holes in f(R) theories

  J. A. R. Cembranos, A. de la Cruz-Dombriz, P. Jimeno Romero
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  ABSTRACT: In the context of f(R) modified gravity theories, we study the Kerr-Newman black-hole solutions. We study non-zero constant scalar curvature solutions and discuss the metric tensor that satisfies the modified field equations. We determine that, in absence of a cosmological constant, the black holes existence is determined by the sign of a parameter dependent of the mass, the charge, the spin and the scalar curvature. We obtain that for negative values of the curvature, the extremal black hole is no longer given by a spin parameter a_max = M (as is the case in General Relativity), but by a_max < M, and that for positive values of the curvature there are two kinds of extremal black holes: the usual one, that occurs for a_max > M, and the extreme marginal one, where the exterior (but not interior) black hole's horizon vanishes provided that a < a_min. Thermodynamics for this kind of black holes is then studied, as well as their local and global stability. Finally we study different f(R) models and see how these properties manifest for their parameters phase space.
  09/2011;
• Source

  Available from: Roberto A Lineros

  Article: Fitting formulae for photon spectra from WIMP annihilation

  J. A. R. Cembranos, A. de la Cruz-Dombriz, A. Dobado, R. Lineros, A. L. Maroto
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  ABSTRACT: Annihilation of different dark matter (DM) candidates into Standard Model (SM) particles could be detected through their contribution to the gamma ray fluxes that are measured on the Earth. The magnitude of such contributions depends on the particular DM candidate, but certain imprints of produced photon spectra may be analyzed in a model-independent fashion. In this work we provide the fitting formulae for the photon spectra generated by WIMP annihilation into quarks, leptons and gauge bosons channels in a wide range of WIMP masses.
  12/2010;
• Source

  Available from: Roberto A Lineros

  Article: Photon spectra from quark generation by WIMPs

  J. A. R. Cembranos, A. de la Cruz-Dombriz, A. Dobado, R. Lineros, A. L. Maroto
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  ABSTRACT: In this work we provide the fitting formula valid for the simulated photon spectra from WIMP annihilation into light quark-anti quark (qq-) channels in a wide range of WIMP masses. We illustrate our results for the cc- channel.
  11/2010;
• Source

  Available from: Roberto A Lineros

  Article: Photon spectra from WIMP annihilation

  J. A. R. Cembranos, A. de la Cruz-Dombriz, A. Dobado, R. A. Lineros, A. L. Maroto
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  ABSTRACT: If the present dark matter in the Universe annihilates into Standard Model particles, it must contribute to the fluxes of cosmic rays that are detected on the Earth, and in particular, to the observed gamma ray fluxes. The magnitude of such contribution depends on the particular dark matter candidate, but certain features of the produced photon spectra may be analyzed in a rather model-independent fashion. In this work we provide the complete photon spectra coming from WIMP annihilation into Standard Model particle-antiparticle pairs obtained by extensive Monte Carlo simulations. We present results for each individual annihilation channel and provide analytical fitting formulae for the different spectra for a wide range of WIMP masses.
  09/2010;
• Source

  Available from: ArXiv

  Article: Is the CMB cold spot a gate to extra dimensions?

  J. A. R. Cembranos, A. de la Cruz-Dombriz, A. Dobado, A. L. Maroto
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  ABSTRACT: One of the most striking features found in the cosmic microwave background data is the presence of an anomalous Cold Spot (CS) in the temperature maps made by the Wilkinson Microwave Anisotropy Probe (WMAP). This CS has been interpreted as the result of the presence of a collapsing texture, perhaps coming from some early universe Grand Unified Theory (GUT) phase transition. In this work we propose an alternative explanation based on a completely different kind of texture which appears in a natural way in a broad class of brane-world models. This type of textures known as brane-skyrmions can be understood as holes in the brane which make possible to pass through them along the extra-dimensional space. The typical scales needed for the proposed brane-skyrmions to correctly describe the observed CS can be as low as the electroweak scale. Comment: 8 pages. New comments and references included. Final version to appear in JCAP
  03/2008;
• Article: Some model-independent phenomenological consequences of flexible braneworlds

  J A R Cembranos, A Dobado, A L Maroto
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  ABSTRACT: In this work, we will review the main properties of braneworld models with low tension. Starting from very general principles, it is possible to obtain an effective action for the relevant degrees of freedom at low energies (branons). Using the cross sections for high-energy processes involving branons, we set bounds on the different parameters appearing in these models. We also show that branons provide a WIMP candidate for dark matter in a natural way. We consider cosmological constraints on its thermal and non-thermal relic abundances. We derive direct detection limits and compare those limits with the preferred parameter region in the case in which the EGRET excess in the diffuse galactic gamma rays is due to dark matter annihilation. Finally, we will discuss the constraints coming from the precision tests of the standard model and the muon anomalous magnetic moment.
  Journal of Physics A Mathematical and Theoretical 06/2007; 40(25):6631. · 1.56 Impact Factor
• Source

  Available from: cern.ch

  Article: Catching photons from extra dimensions

  J. A. R. Cembranos, A. Dobado, A. L. Maroto
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  ABSTRACT: In extra-dimensional brane-world models with low tension, brane excitations provide a natural WIMP candidate for dark matter. Taking into account the various constraints coming from colliders, precision observables and direct search, we explore the possibilities for indirect search of the galactic halo branons through their photon producing annihilations in experiments such as EGRET, HESS or AMS2.
  12/2006;
• Source

  Available from: arxiv.org

  Article: Some model-independent phenomenological consequences of flexible brane worlds

  J. A. R. Cembranos, A. Dobado, A. L. Maroto
  [show abstract] [hide abstract]
  ABSTRACT: In this work we will review the main properties of brane-world models with low tension. Starting from very general principles, it is possible to obtain an effective action for the relevant degrees of freedom at low energies (branons). Using the cross sections for high-energy processes involving branons, we set bounds on the different parameters appearing in these models. We also show that branons provide a WIMP candidate for dark matter in a natural way. We consider cosmological constraints on its thermal and non-thermal relic abundances. We derive direct detection limits and compare those limits with the preferred parameter region in the case in which the EGRET excess in the diffuse galactic gamma rays is due to dark matter annihilation. Finally we will discuss the constraints coming from the precision tests of the Standard Model and the muon anomalous magnetic moment.
  12/2006;