7,768 research outputs found
Nematic order by elastic interactions and cellular rigidity sensing
We predict spontaneous nematic order in an ensemble of active force
generators with elastic interactions as a minimal model for early nematic
alignment of short stress fibers in non-motile, adhered cells. Mean-field
theory is formally equivalent to Maier-Saupe theory for a nematic liquid.
However, the elastic interactions are long-ranged (and thus depend on cell
shape and matrix elasticity) and originate in cell activity. Depending on the
density of force generators, we find two regimes of cellular rigidity sensing
for which orientational, nematic order of stress fibers depends on matrix
rigidity either in a step-like manner or with a maximum at an optimal rigidity.Comment: 12 pages, 4 figure
The ultra-wideband pulse
Since the birth of mode-locking the temporal duration of optical pulses has radically
diminished. In parallel to this, bandwidths have grown so large that almost entire
frequency octaves are present in today’s few-cycle pulses.
This thesis investigates the character of ultra-wideband pulses in nonlinear environments.
Because of the growth in optical bandwidths, traditional definitions and propagation
models break down, requiring newer more accurate numerical techniques. A
novel approach capturing the uni-directionality of pulses is presented in the form of Gvariables
by combining the electric and magnetic field descriptions. These G-variables
have the advantage of both an accurate spectral representation and a reduced computational
overhead, making them significantly more efficient than existing direct Maxwell
solvers. Such approaches are particularly important where large propagation distances
and/or transverse dimensions are concerned.
Pseudo-spectral techniques play a key role in the success of these wideband models
enabling sub-cycle dynamics to be studied. One such phenomenon is Carrier Wave
Shocking (CWS), where the optical carrier undergoes self-steepening in the presence of
third-order nonlinearity. This process is carefully studied, focussing on the effect of dispersion
and the feasibility of its physical realisation. The process is then generalised to
arbitrary nonlinear order, where the quadratic form finds potential applications in High
Harmonic Generation (HHG). Shock detection schemes are also developed, and agree
with analytical solutions in the dispersionless regime.
To fully characterise few-cycle pulses, the absolute Carrier Envelope Phase (CEP)
must be known. A novel 0 − f self-referencing scheme relying on wideband interference
is investigated. By applying robust frequency domain definitions a proposal is made to
convert this scheme into one that determines absolute CEP. The scheme maps the level
of spectral interference to absolute CEP using numerical simulations
Dark-Matter Harmonics Beyond Annual Modulation
The count rate at dark-matter direct-detection experiments should modulate
annually due to the motion of the Earth around the Sun. We show that
higher-frequency modulations, including daily modulation, are also present and
in some cases are nearly as strong as the annual modulation. These higher-order
modes are particularly relevant if (i) the dark matter is light, O(10) GeV,
(ii) the scattering is inelastic, or (iii) velocity substructure is present;
for these cases, the higher-frequency modes are potentially observable at
current and ton-scale detectors. We derive simple expressions for the harmonic
modes as functions of the astrophysical and geophysical parameters describing
the Earth's orbit, using an updated expression for the Earth's velocity that
corrects a common error in the literature. For an isotropic halo velocity
distribution, certain ratios of the modes are approximately constant as a
function of nuclear recoil energy. Anisotropic distributions can also leave
observable features in the harmonic spectrum. Consequently, the higher-order
harmonic modes are a powerful tool for identifying a potential signal from
interactions with the Galactic dark-matter halo.Comment: 40 pages, 10 figures; v2 refs added, minor improvements; v3 refs
added, minor improvements, JCAP versio
Distinguishing Dark Matter from Unresolved Point Sources in the Inner Galaxy with Photon Statistics
Data from the Fermi Large Area Telescope suggests that there is an extended
excess of GeV gamma-ray photons in the Inner Galaxy. Identifying potential
astrophysical sources that contribute to this excess is an important step in
verifying whether the signal originates from annihilating dark matter. In this
paper, we focus on the potential contribution of unresolved point sources, such
as millisecond pulsars (MSPs). We propose that the statistics of the
photons---in particular, the flux probability density function (PDF) of the
photon counts below the point-source detection threshold---can potentially
distinguish between the dark-matter and point-source interpretations. We
calculate the flux PDF via the method of generating functions for these two
models of the excess. Working in the framework of Bayesian model comparison, we
then demonstrate that the flux PDF can potentially provide evidence for an
unresolved MSP-like point-source population.Comment: 27 pages, 8 figures; v2, reference added and other minor change
Searching for sgluons in multitop events at a center-of-mass energy of 8 TeV
Large classes of new physics theories predict the existence of new scalar
states, commonly dubbed sgluons, lying in the adjoint representation of the QCD
gauge group. Since these new fields are expected to decay into colored Standard
Model particles, and in particular into one or two top quarks, these theories
predict a possible enhancement of the hadroproduction rate associated with
multitop final states. We therefore investigate multitop events produced at the
Large Hadron Collider, running at a center-of-mass energy of 8 TeV, and employ
those events to probe the possible existence of color adjoint scalar particles.
We first construct a simplified effective field theory motivated by R-symmetric
supersymmetric models where sgluon fields decay dominantly into top quarks. We
then use this model to analyze the sensitivity of the Large Hadron Collider in
both a multilepton plus jets and a single lepton plus jets channel. After
having based our event selection strategy on the possible presence of two,
three and four top quarks in the final state, we find that sgluon-induced new
physics contributions to multitop cross sections as low as 10-100 fb can be
excluded at the 95% confidence level, assuming an integrated luminosity of 20
inverse fb. Equivalently, sgluon masses of about 500-700 GeV can be reached for
several classes of benchmark scenarios.Comment: 26 pages; 8 figures, 6 tables; version accepted by JHE
Finding Spanning Trees in Strongly Connected Graphs with Per-Vertex Degree Constraints
In this project, I sought to develop and prove new algorithms to create spanning trees on general graphs with per-vertex degree constraints. This means that each vertex in the graph would have some additional value, a degree constraint d. For a spanning tree to be correct, every vertex vi in the spanning tree must have a degree exactly equal to a degree constraint di. This poses an additional constraint on what would otherwise be a trivial spanning tree problem. In this paper, two proofs related to my studies will be discussed and analyzed, leading to my algorithm for determining a spanning tree on strongly connected graphs. It is my hope that in the future this algorithm can be modified to apply to the general case
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