328 research outputs found
All-regime Lagrangian-Remap numerical schemes for the gas dynamics equations. Applications to the large friction and low Mach coefficients
In this talk, we propose an all regime Lagrange-Projection like numerical scheme for the gas dynamics
equations. By all regime, we mean that the numerical scheme is able to compute accurate
approximate solutions with an under-resolved discretization with respect to the Mach number
M, i.e. such that the ratio between the Mach number M and the mesh size or the time step is
small with respect to 1. The key idea is to decouple acoustic and transport phenomenon and
then alter the numerical flux in the acoustic approximation to obtain a uniform truncation error
in term of M. This modified scheme is conservative and endowed with good stability properties
with respect to the positivity of the density and the internal energy. A discrete entropy inequality
under a condition on the modification is obtained thanks to a reinterpretation of the modified
scheme in the Harten Lax and van Leer formalism. A natural extension to multi-dimensional
problems discretized over unstructured mesh is proposed. Then a simple and efficient semi
implicit scheme is also proposed. The resulting scheme is stable under a CFL condition driven
by the (slow) material waves and not by the (fast) acoustic waves and so verifies the all regime
property. Numerical evidences are proposed and show the ability of the scheme to deal with
tests where the flow regime may vary from low to high Mach values.Universidad de Málaga. Campus de Excelencia Internacional AndalucĂa Tec
Revisiting LHC gluino mass bounds through radiative decays using MadAnalysis 5
The ATLAS and CMS experiments at the CERN LHC have collected about 25 inverse
femtobarns (fb) of data each at the end of their 8 TeV run, and ruled out a
huge swath of parameter space in the context of Minimally Supersymmetric
Standard Model (MSSM). Limits on masses of the gluino have been pushed to above
1 TeV. These limits are however extremely model dependent and do not always
reflect the level of exclusion. So far the limits on the gluino mass using the
simplified model approach only constrained its value using its three-body
decays. We show in this work that already existing ATLAS and CMS analysis can
also constrain the radiative gluino decay mode and we derived improved mass
limits in particular when the mass difference between the LSP and the gluino is
small.Comment: 4 pages, 3 figures. To appear in the proceedings of the 27th
Rencontres the Blois on Particle Physics and Cosmology, May 31 - June 05,
201
Computing material fronts with a Lagrange-Projection approach
This paper reports investigations on the computation of material fronts in
multi-fluid models using a Lagrange-Projection approach. Various forms of the
Projection step are considered. Particular attention is paid to minimization of
conservation errors
Closing in on compressed gluino-neutralino spectra at the LHC
A huge swath of parameter space in the context of the Minimal Supersymmetric
Standard Model (MSSM) has been ruled at after run I of the LHC. Various
exclusion contours in the m_{\tilde{g}}-\m_{\tilde{\chi}_{1}^{0}} plane were
derived by the experimental collaborations, all based on three-body gluino
decay topologies. These limits are however extremely model dependent and do not
always reflect the level of exclusion. If the gluino-neutralino spectrum is
compressed, then the current mass limits can be drastically reduced. In such
situations, the radiative decay of the gluino \gino \ra g \neut{1} can be
dominant and used as a sensitive probe of small mass splittings. We examine the
sensitivity of constraints of some Run I experimental searches on this decay
after recasting them within the \texttt{MadAnalysis5} framework. The recasted
searches are now part of the \texttt{MadAnalysis5} Public Analysis Database. We
also design a dedicated search strategy and investigate its prospects to
uncover this decay mode of the gluino at run II of the LHC. We emphasize that a
multijet search strategy may be more sensitive than a monojet one, even in the
case of very small mass differences.Comment: 34 pages , 6 figures. Version accepted for publication for JHE
Boosting Higgs decays into gamma and a Z in the NMSSM
In this work we present the computation of the Higgs decay into a photon and
a boson at one-loop level in the framework of the Next-to-Minimal
Supersymmetric Standard Model (NMSSM). The numerical evaluation of this decay
width was performed within the framework of the SloopS code, orginally
developped for the Minimal Supersymmetric Standard Model (MSSM) but which was
recently extended to deal with the NMSSM. Thanks to the high level of
automation of SloopS all contributions from the various sector of the NMSSM are
consistently taken into account, in particular the non-diagonal chargino and
sfermion contributions. We then explored the NMSSM parameter space, using
HiggsBounds and HiggsSignals, to investigate to which extent these signal can
be enhanced.Comment: 14 pages, 3 figures, typos fixed, matches the public version, results
and conclusions unchange
Coannihilation with a chargino and gauge boson pair production at one-loop
We present a complete calculation of the electroweak one-loop corrections to
the relic density within the MSSM framework. In the context of the neutralino
as dark matter candidate, we review different scenarios of annihilation and
coannihilation with a chargino. In particular we investigate predictions for
the annihilation into gauge boson pairs for different kinds of neutralino:
bino-, wino- and higgsino-like. We present some interesting effects which are
not present at tree-level and show up at one-loop. To deal with the large
number of diagrams occuring in the calculations, we have developed an automatic
tool for the computation at one-loop of any process in the MSSM. We have
implemented a complete on-shell gauge invariant renormalization scheme, with
the possibility of switching to other schemes. We emphasize the variations due
to the choice of the renormalization scheme, in particular the one-loop
definition of the parameter tan(beta).Comment: 4 pages, 5 figures, to appear in the proceedings of the 17th
International Conference on Supersymmetry and the Unification of Fundamental
Interactions (SUSY09), Boston, USA, 5-10 June, 200
Beyond pressureless gas dynamics: Quadrature-based velocity moment models
Following the seminal work of F. Bouchut on zero pressure gas dynamics which
has been extensively used for gas particle-flows, the present contribution
investigates quadrature-based velocity moments models for kinetic equations in
the framework of the infinite Knudsen number limit, that is, for dilute clouds
of small particles where the collision or coalescence probability
asymptotically approaches zero. Such models define a hierarchy based on the
number of moments and associated quadrature nodes, the first level of which
leads to pressureless gas dynamics. We focus in particular on the four moment
model where the flux closure is provided by a two-node quadrature in the
velocity phase space and provide the right framework for studying both smooth
and singular solutions. The link with both the kinetic underlying equation as
well as with zero pressure gas dynamics is provided and we define the notion of
measure solutions as well as the mathematical structure of the resulting system
of four PDEs. We exhibit a family of entropies and entropy fluxes and define
the notion of entropic solution. We study the Riemann problem and provide a
series of entropic solutions in particular cases. This leads to a rigorous link
with the possibility of the system of macroscopic PDEs to allow particle
trajectory crossing (PTC) in the framework of smooth solutions. Generalized
-choc solutions resulting from Riemann problem are also investigated.
Finally, using a kinetic scheme proposed in the literature without mathematical
background in several areas, we validate such a numerical approach in the
framework of both smooth and singular solutions.Comment: Submitted to Communication in Mathematical Science
On the Eulerian Large Eddy Simulation of disperse phase flows: an asymptotic preserving scheme for small Stokes number flows
In the present work, the Eulerian Large Eddy Simulation of dilute disperse
phase flows is investigated. By highlighting the main advantages and drawbacks
of the available approaches in the literature, a choice is made in terms of
modelling: a Fokker-Planck-like filtered kinetic equation proposed by Zaichik
et al. 2009 and a Kinetic-Based Moment Method (KBMM) based on a Gaussian
closure for the NDF proposed by Vie et al. 2014. The resulting Euler-like
system of equations is able to reproduce the dynamics of particles for small to
moderate Stokes number flows, given a LES model for the gaseous phase, and is
representative of the generic difficulties of such models. Indeed, it
encounters strong constraints in terms of numerics in the small Stokes number
limit, which can lead to a degeneracy of the accuracy of standard numerical
methods. These constraints are: 1/as the resulting sound speed is inversely
proportional to the Stokes number, it is highly CFL-constraining, and 2/the
system tends to an advection-diffusion limit equation on the number density
that has to be properly approximated by the designed scheme used for the whole
range of Stokes numbers. Then, the present work proposes a numerical scheme
that is able to handle both. Relying on the ideas introduced in a different
context by Chalons et al. 2013: a Lagrange-Projection, a relaxation formulation
and a HLLC scheme with source terms, we extend the approach to a singular flux
as well as properly handle the energy equation. The final scheme is proven to
be Asymptotic-Preserving on 1D cases comparing to either converged or
analytical solutions and can easily be extended to multidimensional
configurations, thus setting the path for realistic applications
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