53,195 research outputs found
Small clique number graphs with three trivial critical ideals
The critical ideals of a graph are the determinantal ideals of the
generalized Laplacian matrix associated to a graph. In this article we provide
a set of minimal forbidden graphs for the set of graphs with at most three
trivial critical ideals. Then we use these forbidden graphs to characterize the
graphs with at most three trivial critical ideals and clique number equal to 2
and 3.Comment: 33 pages, 3 figure
Computing the differential Galois group of a parameterized second-order linear differential equation
We develop algorithms to compute the differential Galois group associated
to a parameterized second-order homogeneous linear differential equation of the
form where the coefficients are rational
functions in with coefficients in a partial differential field of
characteristic zero. Our work relies on the procedure developed by Dreyfus to
compute under the assumption that . We show how to complete this
procedure to cover the cases where , by reinterpreting a classical
change of variables procedure in Galois-theoretic terms.Comment: 14 page
Singlet-Doublet Dirac Dark Matter
We analyze a simple extension of the Standard Model where the dark matter
particle is a Dirac fermion that is mixture of a singlet and an SU(2) doublet.
The model contains only four free parameters: the singlet and the doublet
masses and two new Yukawa couplings. Direct detection bounds in this model are
very strong and require the dark matter particle to be singlet-like. As a
result, its relic density has to be obtained via coannihilations with the
doublet. We find that the dark matter mass should be below 750 GeV, that the
singlet-doublet mass difference cannot exceed 9%, and that direct detection
experiments offer the best chance to probe this scenario. Finally, we also show
that this model can effectively arise in well-motivated extensions of the
Standard Model.Comment: 14 page
Computation of the unipotent radical of the differential Galois group for a parameterized second-order linear differential equation
We propose a new method to compute the unipotent radical of the
differential Galois group associated to a parameterized second-order
homogeneous linear differential equation of the form
where is a rational
function in with coefficients in a -field of characteristic zero,
and is a commuting set of parametric derivations. The procedure developed
by Dreyfus reduces the computation of to solving a creative
telescoping problem, whose effective solution requires the assumption that the
maximal reductive quotient is a -constant linear differential
algebraic group. When this condition is not satisfied, we compute a new set of
parametric derivations such that the associated differential Galois
group has the property that is -constant, and such
that is defined by the same differential equations as . Thus
the computation of is reduced to the effective computation of
. We expect that an elaboration of this method will be successful in
extending the applicability of some recent algorithms developed by Minchenko,
Ovchinnikov, and Singer to compute unipotent radicals for higher order
equations.Comment: 12 page
Inverse decays and the relic density of the sterile sneutrino
We consider a weak scale supersymmetric seesaw model where the Higgsino is
the next-to-lightest supersymmetric particle and the right-handed sneutrino is
the dark matter candidate. It is shown that, in this model, inverse decays,
which had been previously neglected, may suppress the sneutrino relic density
by several orders of magnitude. After including such processes and numerically
solving the appropriate Boltzmann equation, we study the dependence of the
relic density on the mu parameter, the sneutrino mass, and the neutrino Yukawa
coupling. We find that, even though much smaller than in earlier calculations,
the sneutrino relic density is still larger than the observed dark matter
density.Comment: 15 pages, 4 figure
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