Viscosity and scale invariance in the unitary Fermi gas

We compute the shear viscosity of the unitary Fermi gas above the superfluid transition temperature, using a diagrammatic technique that starts from the exact Kubo formula. The formalism obeys a Ward identity associated with scale invariance which guarantees that the bulk viscosity vanishes identically. For the shear viscosity, vertex corrections and the associated Aslamazov-Larkin contributions are shown to be crucial to reproduce the full Boltzmann equation result in the high-temperature, low fugacity limit. The frequency dependent shear viscosity $\eta(\omega)$ exhibits a Drude-like transport peak and a power-law tail at large frequencies which is proportional to the Tan contact. The weight in the transport peak is given by the equilibrium pressure, in agreement with a sum rule due to Taylor and Randeria. Near the superfluid transition the peak width is of the order of $0.5 T_F$, thus invalidating a quasiparticle description. The ratio $\eta/s$ between the static shear viscosity and the entropy density exhibits a minimum near the superfluid transition temperature whose value is larger than the string theory bound $\hbar/(4\pi k_B)$ by a factor of about seven.Comment: 34 pages, 9 figures; final form (contains new derivation of sum rule), accepted for publication in Annals of Physic

Re)Design considerations for scalable large-file content distribution

The CoBlitz system was designed to provide efficient large file transfer in a managed infrastructure environment. It uses a content distribution network (CDN) coupled with a swarm-style chunk distribution system to reduce the bandwidth required at origin servers. With 6 months of operation, we have been able to observe its behavior in typical usage, and glean information on how it could be redesigned to better suit its target audience. At its heart, this paper describes what happens when a plausible conceptual design meets the harsh realities of life on the Internet. We describe our experiences improving CoBlitz’s performance via a range of techniques, including measurement-based feedback, heuristic changes, and new algorithms. In the process, we triple CoBlitz’s performance, and we reduce the load it places on origin servers by a factor of five. In addition to improving performance for CoBlitz’s users, we believe that our experiences will also be beneficial to other researchers working on large-file transfer and content distribution networks.

High Bone Density in Young Hutterite children.

We previously reported greater than average aBMD in adult Hutterites; however, it is unknown whether higher aBMD occurs at younger ages. We examined Hutterite children to test the hypotheses that aBMD Z-scores in younger (\u3c15 years) Hutterite children would be similar to reference data; but greater in older children after they enter the adult workforce at age 15. A secondary aim was to determine lifestyle factors associated with bone measures among Hutterite children. Hip, femoral neck, and spine BMC and aBMD were measured in 323 Hutterite children aged 8 through 19 years: 186 (108 girls) were \u3c15 years (younger) and 137 (87 girls) were \u3eor=15 years (older). Anthropometric measurements and activity and dietary recalls were obtained. Overall, children were lighter (Z=-0.29+/-0.72 [mean+/-SD]), shorter (Z=-0.15+/-0.86, and had lower BMI\u27s (Z=-0.27+/-0.70) than other South Dakota children residing in the same counties (all,