1,775 research outputs found
Optimal synchronization of Kuramoto oscillators: a dimensional reduction approach
A recently proposed dimensional reduction approach for studying
synchronization in the Kuramoto model is employed to build optimal network
topologies to favor or to suppress synchronization. The approach is based in
the introduction of a collective coordinate for the time evolution of the phase
locked oscillators, in the spirit of the Ott-Antonsen ansatz. We show that the
optimal synchronization of a Kuramoto network demands the maximization of the
quadratic function , where stands for the vector of
the natural frequencies of the oscillators, and for the network Laplacian
matrix. Many recently obtained numerical results can be re-obtained
analytically and in a simpler way from our maximization condition. A
computationally efficient {hill climb} rewiring algorithm is proposed to
generate networks with optimal synchronization properties. Our approach can be
easily adapted to the case of the Kuramoto models with both attractive and
repulsive interactions, and again many recent numerical results can be
rederived in a simpler and clearer analytical manner.Comment: 6 pages, 6 figures, final version to appear in PR
Explosive synchronization with partial degree-frequency correlation
Networks of Kuramoto oscillators with a positive correlation between the
oscillators frequencies and the degree of the their corresponding vertices
exhibits the so-called explosive synchronization behavior, which is now under
intensive investigation. Here, we study and report explosive synchronization in
a situation that has not yet been considered, namely when only a part,
typically small, of the vertices is subjected to a degree frequency
correlation. Our results show that in order to have explosive synchronization,
it suffices to have degree-frequency correlations only for the hubs, the
vertices with the highest degrees. Moreover, we show that a partial
degree-frequency correlation does not only promotes but also allows explosive
synchronization to happen in networks for which a full degree-frequency
correlation would not allow it. We perform exhaustive numerical experiments for
synthetic networks and also for the undirected and unweighted version of the
neural network of the worm Caenorhabditis elegans. The latter is an explicit
example where partial degree-frequency correlation leads to explosive
synchronization with hysteresis, in contrast with the fully correlated case,
for which no explosive synchronization is observed.Comment: 10 pages, 6 figures, final version to appear in PR
Optimal global synchronization of partially forced Kuramoto oscillators
We consider the problem of global synchronization in a large random network
of Kuramoto oscillators where some of them are subject to an external
periodically driven force. We explore a recently proposed dimensional reduction
approach and introduce an effective two-dimensional description for the
problem. From the dimensionally reduced model, we obtain analytical predictions
for some critical parameters necessary for the onset of a globally synchronized
state in the system. Moreover, the low dimensional model also allows us to
introduce an optimization scheme for the problem. Our main conclusion, which
has been corroborated by exhaustive numerical simulations, is that for a given
large random network of Kuramoto oscillators, with random natural frequencies
, such that a fraction of them is subject to an external periodic
force with frequency , the best global synchronization properties
correspond to the case where the fraction of the forced oscillators is chosen
to be those ones such that is maximal. Our results might
shed some light on the structure and evolution of natural systems for which the
presence or the absence of global synchronization are desired properties. Some
properties of the optimal forced networks and its relation to recent results in
the literature are also discussed.Comment: 8 pages, 3 figures. Final version accepted for publication in Chaos.
After it is published, it will be found at
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Regular rotating black holes and the weak energy condition
We revisit here a recent work on regular rotating black holes. We introduce a
new mass function generalizing the commonly used Bardeen and Hayward mass
functions and extend the recently proposed solutions in order to accommodate a
cosmological constant . We discuss some aspects of the causal
structure (horizons) and the ergospheres of the new proposed solutions. We also
show that, in contrast with the spherically symmetrical case, the black hole
rotation will unavoidably lead to the violation of the weak energy condition
for any physically reasonable choice of the mass function, reinforcing the idea
the description of the interior region of a Kerr black hole is much more
challenging than in the Schwarzschild case.Comment: 8 pages, 3 figure
Vertical stability of circular orbits in relativistic razor-thin disks
During the last few decades, there has been a growing interest in exact
solutions of Einstein equations describing razor-thin disks. Despite the
progress in the area, the analytical study of geodesic motion crossing the disk
plane in these systems is not yet so developed. In the present work, we propose
a definite vertical stability criterion for circular equatorial timelike
geodesics in static, axially symmetric thin disks, possibly surrounded by other
structures preserving axial symmetry. It turns out that the strong energy
condition for the disk stress-energy content is sufficient for vertical
stability of these orbits. Moreover, adiabatic invariance of the vertical
action variable gives us an approximate third integral of motion for oblique
orbits which deviate slightly from the equatorial plane. Such new approximate
third integral certainly points to a better understanding of the analytical
properties of these orbits. The results presented here, derived for static
spacetimes, may be a starting point to study the motion around rotating,
stationary razor-thin disks. Our results also allow us to conjecture that the
strong energy condition should be sufficient to assure transversal stability of
periodic orbits for any singular timelike hypersurface, provided it is
invariant under the geodesic flow.Comment: 13 pages, 4 figures; Accepted for publication in Physical Review
The Axion and the Goldstone Higgs
We consider the renormalizable -model, in which the
Higgs particle has a pseudo-Nambu-Goldstone boson character, and explore what
the minimal field extension required to implement the Peccei-Quinn symmetry
(PQ) is, within the partial compositeness scenario. It turns out that the
minimal model does not require the enlargement of the exotic fermionic sector,
but only the addition of a singlet scalar: it is sufficient that the exotic
fermions involved in partial compositeness and the singlet scalar become
charged under Peccei-Quinn transformations. We explore the phenomenological
predictions for photonic signals in axion searches for all models discussed.
Because of the constraints imposed on the exotic fermion sector by the Standard
Model fermion masses, the expected range of allowed axion-photon couplings
turns out to be generically narrowed with respect to that of standard invisible
axion models, impacting the experimental quest.Comment: 31 pages, 2 Figures. Description improved, results unchange
Probing the Majorana neutrinos and their CP violation in decays of charged scalar mesons
Some of the outstanding questions of particle physics today concern the
neutrino sector, in particular whether there are more neutrinos than those
already known and whether they are Dirac or Majorana particles.There are
different ways to explore these issues. In this article we describe
neutrino-mediated decays of charged pseudoscalar mesons such as ,
and , in scenarios where extra neutrinos are heavy and can
be on their mass shell. We discuss semileptonic and leptonic decays of such
kinds. We investigate possible ways of using these decays in order to
distinguish between the Dirac and Majorana character of neutrinos. Further, we
argue that there are significant possibilities of detecting CP violation in
such decays when there are at least two almost degenerate Majorana neutrinos
involved. This latter type of scenario fits well into the known neutrino
minimal standard model (MSM) which could simultaneously explain the Dark
Matter and Baryon Asymmetry of the Universe.Comment: v3: 37 pages, 14 figures; minor typographical errors corrected;
published in Symmetr
On the renormalization of the electroweak chiral Lagrangian with a Higgs
We consider the scalar sector of the effective non-linear electroweak
Lagrangian with a light "Higgs" particle, up to four derivatives in the chiral
expansion. The complete off-shell renormalization procedure is implemented,
including one loop corrections stemming from the leading two-derivative terms,
for finite Higgs mass. This determines the complete set of independent chiral
invariant scalar counterterms required for consistency; these include bosonic
operators often disregarded. Furthermore, new counterterms involving the Higgs
particle which are apparently chiral non-invariant are identified in the
perturbative analysis. A novel general parametrization of the pseudoescalar
field redefinitions is proposed, which reduces to the various usual ones for
specific values of its parameter; the non-local field redefinitions reabsorbing
all chiral non-invariant counterterms are then explicitly determined. The
physical results translate into renormalization group equations which may be
useful when comparing future Higgs data at different energies
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