Saffman-Taylor instability in a non-Brownian suspension: finger selection and destabilization

We study the Saffman-Taylor instability in a non-Brownian suspension by injection of air. We find that flow structuration in the Hele-Shaw cell can be described by an effective viscosity depending on the volume fraction. When this viscosity is used to define the control parameter of the instability, the classical finger selection for Newtonian fluids is recovered. However, this picture breaks down when the cell thickness is decreased below approximatively 10 grain sizes. The discrete nature of the grains plays also a determinant role in the the early destabilization of the fingers observed. The grains produce a perturbation at the interface proportional to the grain size and can thus be considered as a "controlled noise". The finite amplitude instability mechanism proposed earlier by Bensimon et al. allows to link this perturbation to the actual values of the destabilization threshold.Comment: 4 pages, 4 figures, submitted to PR

Optically Generated 2-Dimensional Photonic Cluster State from Coupled Quantum Dots

We propose a method to generate a two-dimensional cluster state of polarization encoded photonic qubits from two coupled quantum dot emitters. We combine the proposal for generating one-dimensional cluster state strings from a single dot, with a new proposal for an induced conditional phase gate between the two quantum dots. The entanglement between the two dots translates to entanglement between the two photonic cluster state strings. Further interpair coupling of the quantum dots using cavities and waveguides can lead to a two-dimensional cluster sheet, the importance of which stems from the fact that it is a universal resource for quantum computation. Analysis of errors indicates that our proposal is feasible with current technology. Crucially, the emitted photons need not have identical frequencies, and so there are no constraints on the resonance energies for the quantum dots

Metastable π Junction between an s±-Wave and an s-Wave Superconductor

We examine a contact between a superconductor whose order parameter changes sign across the Brillioun zone, and an ordinary, uniform-sign superconductor. Within a Ginzburg-Landau-type model, we find that if the barrier between the two superconductors is not too high, the frustration of the Josephson coupling between different portions of the Fermi surface across the contact can lead to surprising consequences. These include time-reversal symmetry breaking at the interface and unusual energy-phase relations with multiple local minima. We propose this mechanism as a possible explanation for the half-integer flux quantum transitions in composite niobium-iron pnictide superconducting loops, which were discovered in recent experiments [C.-T. Chen et al., Nature Phys. 6, 260 (2010).]

Fermionic WIMPs and Vacuum Stability in the Scotogenic Model

We demonstrate that the condition of vacuum stability severely restricts scenarios with fermionic WIMP dark matter in the scotogenic model. The sizable Yukawa couplings that are required to satisfy the dark matter constraint via thermal freeze-out in these scenarios tend to destabilise the vacuum at scales below that of the heaviest singlet fermion, rendering the model inconsistent from a theoretical point of view. By means of a scan over the parameter space, we study the impact of these renormalisation group effects on the viable regions of this model. Our analysis shows that a fraction of more than 90% of the points compatible with all known experimental constraints - including neutrino masses, the dark matter density, and lepton flavour violation - is actually inconsistent.Comment: 8 pages, 6 figures; content matches published versio

On the Consistency of Perturbativity and Gauge Coupling Unification

We investigate constraints that the requirements of perturbativity and gauge coupling unification impose on extensions of the Standard Model and of the MSSM. In particular, we discuss the renormalization group running in several SUSY left-right symmetric and Pati-Salam models and show how the various scales appearing in these models have to be chosen in order to achieve unification. We find that unification in the considered models occurs typically at scales below M^{min}_{B violation} = 10^16 GeV, implying potential conflicts with the non-observation of proton decay. We emphasize that extending the particle content of a model in order to push the GUT scale higher or to achieve unification in the first place will very often lead to non-perturbative evolution. We generalize this observation to arbitrary extensions of the Standard Model and of the MSSM and show that the requirement of perturbativity up to M^{min}_{B violation}, if considered a valid guideline for model building, severely limits the particle content of any such model, especially in the supersymmetric case. However, we also discuss several mechanisms to circumvent perturbativity and proton decay issues, for example in certain classes of extra dimensional models.Comment: LaTeX, 20 pages, 8 figures, 1 tabl

Left-Right Symmetry and Lepton Number Violation at the Large Hadron Electron Collider

We show that the proposed Large Hadron electron Collider (LHeC) will provide an opportunity to search for left-right symmetry and establish lepton number violation, complementing current and planned searches based on LHC data and neutrinoless double beta decay. We consider several plausible configurations for the LHeC -- including different electron energies and polarizations, as well as distinct values for the charge misidentification rate. Within left-right symmetric theories we determine the values of right-handed neutrino and gauge boson masses that could be tested at the LHeC after one, five and ten years of operation. Our results indicate that this collider might probe, via the $\Delta L =2$ signal $e^-p\to e^+jjj$, Majorana neutrino masses up to 1 TeV and $W_R$ masses up to 6.5 TeV. Interestingly, part of this parameter space is beyond the expected reach of the LHC and of future neutrinoless double beta decay experiments.Comment: 22 pages, 6 figures. Matches version published in JHE