Time filter

Source Type

Francia D.,CNRS Astroparticle and Cosmology Lab
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2010

Unconstrained local Lagrangians for higher-spin gauge theories are bound to involve auxiliary fields, whose integration in the partition function generates geometric, effective actions expressed in terms of curvatures. When applied to the triplets, emerging from the tensionless limit of open string field theory, the same procedure yields interesting alternative forms of geometric Lagrangians, expressible for both bosons and fermions as squares of field-strengths. This shows that higher-spin curvatures might play a role in the dynamics, regardless of whether the Fronsdal-Labastida constraints are assumed or forgone. © 2010 Elsevier B.V. All rights reserved. Source

Parentani R.,University Paris - Sud | Serreau J.,CNRS Astroparticle and Cosmology Lab
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We propose a new approach to compute correlators of quantum fields in de Sitter space. It is based on nonequilibrium field theory techniques, and exploits de Sitter symmetries so as to partially reduce the number of independent variables of n-point functions in a manner that preserves the usefulness of a momentum representation, e.g., for writing spatial convolution integrals as simple products. In this representation, the two-point function of a scalar field only depends on two physical momenta, and the corresponding Schwinger-Dyson evolution equations take the form of momentum flow equations. Moreover, standard diagrammatic rules can be entirely formulated in this representation. The method is suitable for analytical approximations as well as numerical implementations. In forthcoming publications, we apply it to resum infrared logarithmic terms appearing in the perturbative calculation of vertex and correlation functions. © 2013 American Physical Society. Source

Ferreira J.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Casse F.,CNRS Astroparticle and Cosmology Lab
Monthly Notices of the Royal Astronomical Society | Year: 2013

We investigate under which conditions cold, fan-shaped winds can be steadily launched from thin (Keplerian) accretion discs. Such winds are magnetocentrifugal winds launched from a thin annulus in the disc along open magnetic field lines that fan out above the disc. In principle, such winds could be found in two situations: (1) at the interface between an inner jet emitting disc, which is itself powering magnetocentrifugally driven winds, and an outer standard accretion disc; (2) at the interface between an inner closed stellar magnetosphere and the outer standard accretion disc. We refer to terminal or T-winds as the former kind and to magnetospheric or M-winds as the latter. The full set of resistive and viscous steady-state magnetohydrodynamic (MHD) equations are analysed for the disc (the annulus), which allow us to derive general expressions valid for both configurations. We find that, under the framework of our analysis, the only source of energy able to power any kind of fan-shaped winds is the viscous transport of rotational energy coming below the inner radii. Using standard local a prescriptions for the anomalous (turbulent) transport of angular momentum and magnetic fields in the disc, we derive the strength of the transport coefficients that are needed to steadily sustain the global configuration. It turns out that in order for these winds to be dynamically relevant and explain observed jets, the disc coefficients must be far much larger than values expected from current knowledge of turbulence occurring inside protostellar discs. Either the current view on MHD turbulence must be deeply reconsidered or steady-state fan-shaped winds are never realized in nature. The latter hypothesis seems to be consistent with current numerical simulations. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Francia D.,CNRS Astroparticle and Cosmology Lab
Journal of Physics: Conference Series | Year: 2010

Equations of motion for free higher-spin gauge fields of any symmetry can be formulated in terms of linearised curvatures. On the other hand, gauge invariance alone does not fix the form of the corresponding actions which, in addition, either contain higher derivatives or involve inverse powers of the d'Alembertian operator, thus introducing possible subtleties in degrees of freedom count. We suggest a path to avoid ambiguities, starting from local, unconstrained Lagrangians previously proposed, and integrating out the auxiliary fields from the functional integral, thus generating a unique non-local theory expressed in terms of curvatures. © 2010 IOP Publishing Ltd. Source

Volpe C.,CNRS Astroparticle and Cosmology Lab
Annalen der Physik | Year: 2013

Neutrinos of astrophysical origin are messengers produced in stars, in explosive phenomena like core-collapse supernovae, in the accretion disks around black holes, or in the Earth's atmosphere. Their fluxes and spectra encode information on the environments that produce them. Such fluxes are modified in characteristic ways when neutrinos traverse a medium. Here the current understanding of neutrino flavour conversion in media is summarized. The importance of this domain for astrophysical observations is emphasized. Examples are given of the fundamental properties that research into astrophysical neutrinos has uncovered, or might reveal in the future. © 2013 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Discover hidden collaborations