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