24,663 research outputs found
Coarse Brownian Dynamics for Nematic Liquid Crystals: Bifurcation Diagrams via Stochastic Simulation
We demonstrate how time-integration of stochastic differential equations
(i.e. Brownian dynamics simulations) can be combined with continuum numerical
bifurcation analysis techniques to analyze the dynamics of liquid crystalline
polymers (LCPs). Sidestepping the necessity of obtaining explicit closures, the
approach analyzes the (unavailable in closed form) coarse macroscopic
equations, estimating the necessary quantities through appropriately
initialized, short bursts of Brownian dynamics simulation. Through this
approach, both stable and unstable branches of the equilibrium bifurcation
diagram are obtained for the Doi model of LCPs and their coarse stability is
estimated. Additional macroscopic computational tasks enabled through this
approach, such as coarse projective integration and coarse stabilizing
controller design, are also demonstrated
Supersoft Supersymmetry is Super-Safe
We show that supersymmetric models with a large Dirac gluino mass can evade
much of the jets plus missing energy searches at LHC. Dirac gaugino masses
arise from "supersoft" operators that lead to finite one-loop suppressed
contributions to the scalar masses. A little hierarchy between the Dirac gluino
mass 5 - 10 times heavier than the squark masses is automatic and technically
natural, in stark contrast to supersymmetric models with Majorana gaugino
masses. At the LHC, colored sparticle production is suppressed not only by the
absence of gluino pair (or associated) production, but also because several of
the largest squark pair production channels are suppressed or absent. We recast
the null results from the present jets plus missing energy searches at LHC for
supersymmetry onto a supersoft supersymmetric simplified model (SSSM). Assuming
a massless LSP, we find the strongest bounds are: 748 GeV from a 2j + MET
search at ATLAS (4.7 fb^{-1}), and 684 GeV from a combined jets plus missing
energy search using at CMS (1.1 fb^{-1}). In the absence of a future
observation, we estimate the bounds on the squark masses to improve only
modestly with increased luminosity. We also briefly consider the further
weakening in the bounds as the LSP mass is increased.Comment: 13 pages, 8 figure
Generation of Coherent Structures After Cosmic Inflation
We investigate the nonlinear dynamics of hybrid inflation models, which are
characterized by two real scalar fields interacting quadratically. We start by
solving numerically the coupled Klein-Gordon equations in static Minkowski
spacetime, searching for possible coherent structures. We find long-lived,
localized configurations, which we identify as a new kind of oscillon. We
demonstrate that these two-field oscillons allow for "excited" states with much
longer lifetimes than those found in previous studies of single-field
oscillons. We then solve the coupled field equations in an expanding
Friedmann-Robertson-Walker spacetime, finding that as the field responsible for
inflating the Universe rolls down to oscillate about its minimum, it triggers
the formation of long-lived two-field oscillons, which can contribute up to 20%
of the total energy density of the Universe. We show that these oscillons
emerge for a wide range of parameters consistent with WMAP 7-year data. These
objects contain total energy of about 25*10^20 GeV, localized in a region of
approximate radius 6*10^-26 cm. We argue that these structures could have
played a key role during the reheating of the Universe.Comment: 12 pages, 10 .pdf figures, uses RevTex4; v2: expanded discussion in
section IV, accepted for publication in Phys.Rev. D. Results remain the sam
Interpreting Dark Matter Direct Detection Independently of the Local Velocity and Density Distribution
We demonstrate precisely what particle physics information can be extracted
from a single direct detection observation of dark matter while making
absolutely no assumptions about the local velocity distribution and local
density of dark matter. Our central conclusions follow from a very simple
observation: the velocity distribution of dark matter is positive definite,
f(v) >= 0. We demonstrate the utility of this result in several ways. First, we
show a falling deconvoluted recoil spectrum (deconvoluted of the nuclear form
factor), such as from ordinary elastic scattering, can be "mocked up" by any
mass of dark matter above a kinematic minimum. As an example, we show that dark
matter much heavier than previously considered can explain the CoGeNT excess.
Specifically, m_chi < m_Ge} can be in just as good agreement as light dark
matter, while m_\chi > m_Ge depends on understanding the sensitivity of Xenon
to dark matter at very low recoil energies, E_R ~ 6 keVnr. Second, we show that
any rise in the deconvoluted recoil spectrum represents distinct particle
physics information that cannot be faked by an arbitrary f(v). As examples of
resulting non-trivial particle physics, we show that inelastic dark matter and
dark matter with a form factor can both yield such a rise
Large rare fluctuations in systems with delayed dissipation
We study the probability distribution and the escape rate in systems with
delayed dissipation that comes from the coupling to a thermal bath. To
logarithmic accuracy in the fluctuation intensity, the problem is reduced to a
variational problem. It describes the most probable fluctuational paths, which
are given by acausal equations due to the delay. In thermal equilibrium, the
most probable path passing through a remote state has time reversal symmetry,
even though one cannot uniquely define a path that starts from a state with
given system coordinate and momentum. The corrections to the distribution and
the escape activation energy for small delay and small noise correlation time
are obtained in the explicit form.Comment: 9 page
Dynamics of a two-level system strongly coupled to a high-frequency quantum oscillator
Recent experiments on quantum behavior in microfabricated solid-state systems
suggest tantalizing connections to quantum optics. Several of these experiments
address the prototypical problem of cavity quantum electrodynamics: a two-level
system coupled to a quantum harmonic oscillator. Such devices may allow the
exploration of parameter regimes outside the near-resonance and weak-coupling
assumptions of the ubiquitous rotating-wave approximation (RWA), necessitating
other theoretical approaches. One such approach is an adiabatic approximation
in the limit that the oscillator frequency is much larger than the
characteristic frequency of the two-level system. A derivation of the
approximation is presented and the time evolution of the two-level-system
occupation probability is calculated using both thermal- and coherent-state
initial conditions for the oscillator. Closed-form evaluation of the time
evolution in the weak-coupling limit provides insight into the differences
between the thermal- and coherent-state models. Finally, potential experimental
observations in solid-state systems, particularly the Cooper-pair
box--nanomechanical resonator system, are discussed and found to be promising.Comment: 16 pages, 11 figures; revised abstract; some text revisions; added
two figures and combined others; added references. Submitted to Phys. Rev.
Diurnal variation in harbour porpoise detection – potential implications for management
Peer reviewedPublisher PD
The possibility of a metal insulator transition in antidot arrays induced by an external driving
It is shown that a family of models associated with the kicked Harper model
is relevant for cyclotron resonance experiments in an antidot array. For this
purpose a simplified model for electronic motion in a related model system in
presence of a magnetic field and an AC electric field is developed. In the
limit of strong magnetic field it reduces to a model similar to the kicked
Harper model. This model is studied numerically and is found to be extremely
sensitive to the strength of the electric field. In particular, as the strength
of the electric field is varied a metal -- insulator transition may be found.
The experimental conditions required for this transition are discussed.Comment: 6 files: kharp.tex, fig1.ps fig2.ps fi3.ps fig4.ps fig5.p
Long-Lived Time-Dependent Remnants During Cosmological Symmetry Breaking: From Inflation to the Electroweak Scale
Through a detailed numerical investigation in three spatial dimensions, we
demonstrate that long-lived time-dependent field configurations emerge
dynamically during symmetry breaking in an expanding de Sitter spacetime. We
investigate two situations: a single scalar field with a double-well potential
and the bosonic sector of an SU(2) non-Abelian Higgs model. For the single
scalar, we show that large-amplitude oscillon configurations emerge
spontaneously and persist to contribute about 1.2% of the energy density of the
universe. We also show that for a range of parameters, oscillon lifetimes are
enhanced by the expansion and that this effect is a result of parametric
resonance. For the SU(2) case, we see about 4% of the final energy density in
oscillons.Comment: 10 pages, RevTex4, 6 figures; v2: expanded SU(2) model section, added
2 figures, added one section, improved overall presentation and updated
references, accepted for publication in Phys. Rev. D. Results remain the sam
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