WIMPless Dark Matter from an AMSB Hidden Sector with No New Mass Parameters

We present a model with dark matter in an anomaly-mediated supersymmetry breaking hidden sector with a U(1)xU(1) gauge symmetry. The symmetries of the model stabilize the dark matter and forbid the introduction of new mass parameters. As a result, the thermal relic density is completely determined by the gravitino mass and dimensionless couplings. Assuming non-hierarchical couplings, the thermal relic density is ~ 0.1, independent of the dark matter's mass and interaction strength, realizing the WIMPless miracle. The model has several striking features. For particle physics, stability of the dark matter is completely consistent with R-parity violation in the visible sector, with implications for superpartner collider signatures; also the thermal relic's mass may be ~ 10 GeV or lighter, which is of interest given recent direct detection results. Interesting astrophysical signatures are dark matter self-interactions through a long-range force, and massless hidden photons and fermions that contribute to the number of relativistic degrees of freedom at BBN and CMB. The latter are particularly interesting, given current indications for extra degrees of freedom and near future results from the Planck observatory.Comment: 18 pages, pdflate

WIMPless Dark Matter in Anomaly-Mediated Supersymmetry Breaking with Hidden QED

In anomaly-mediated supersymmetry breaking, superpartners in a hidden sector have masses that are proportional to couplings squared, and so naturally freeze out with the desired dark matter relic density for a large range of masses. We present an extremely simple realization of this possibility, with WIMPless dark matter arising from a hidden sector that is supersymmetric QED with N_F flavors. Dark matter is multi-component, composed of hidden leptons and sleptons with masses anywhere from 10 GeV to 10 TeV, and hidden photons provide the thermal bath. The dark matter self-interacts through hidden sector Coulomb scatterings that are potentially observable. In addition, the hidden photon contribution to the number of relativistic degrees of freedom is in the range \Delta N_eff ~ 0 - 2, and, if the hidden and visible sectors were initially in thermal contact, the model predicts \Delta N_eff ~ 0.2 - 0.4. Data already taken by Planck may provide evidence of such deviations.Comment: 17 page

Localized Stress Fluctuations Drive Shear Thickening in Dense Suspensions

The mechanical response of solid particles dispersed in a Newtonian fluid exhibits a wide range of nonlinear phenomena including a dramatic increase in the viscosity \cite{1-3} with increasing stress. If the volume fraction of the solid phase is moderately high, the suspension will undergo continuous shear thickening (CST), where the suspension viscosity increases smoothly with applied shear stress; at still higher volume fractions the suspension can display discontinuous shear thickening (DST), where the viscosity changes abruptly over several orders of magnitude upon increasing applied stress. Proposed models to explain this phenomenon are based in two distinct types of particle interactions, hydrodynamic\cite{2,4,5} and frictional\cite{6-10}. In both cases, the increase in the bulk viscosity is attributed to some form of localized clustering\cite{11,12}. However, the physical properties and dynamical behavior of these heterogeneities remains unclear. Here we show that continuous shear thickening originates from dynamic localized well defined regions of particles with a high viscosity that increases rapidly with concentration. Furthermore, we find that the spatial extent of these regions is largely determined by the distance between the shearing surfaces. Our results demonstrate that continuous shear thickening arises from increasingly frequent localized discontinuous transitions between coexisting low and high viscosity Newtonian fluid phases. Our results provide a critical physical link between the microscopic dynamical processes that determine particle interactions and bulk rheological response of shear thickened fluids

Finite Doubly Transitive Affine Planes

AbstractIn this article, we classify the finite affine planes admitting doubly transitive collineation groups

Infinite hyper-regulus Sperner spaces

New constructions of infinite hyper-reguli are given, which produce a variety of new translation planes and Sperner spaces

Translation planes of order q2q2 admitting collineation groups of order q3uq3u preserving a parabolic unital

The set of translation planes of order $q2$ that admit collineation groups of order $q3u$, where u is a prime p-primitive divisor of $q2-1$, consists of exactly the Desarguesian plane, assuming that the group does not contain a translation subgroup of order a multiple of $q2$. This applies to show that if the group preserves a parabolic unital then the plane is forced to be Desarguesian