1,564 research outputs found
Strong- vs. weak-coupling pictures of jet quenching: a dry run using QED
High-energy partons () traveling through a quark-gluon plasma lose
energy by splitting via bremsstrahlung and pair production. Regardless of
whether or not the quark-gluon plasma itself is strongly coupled, an important
question lying at the heart of philosophically different approaches to energy
loss is whether the high-energy partons of an in-medium shower can be thought
of as a collection of individual particles, or whether their coupling to each
other is also so strong that a description as high-energy `particles' is
inappropriate. We discuss some possible theorists' tests of this question for
simple situations (e.g. an infinite, non-expanding plasma) using thought
experiments and first-principles quantum field theory calculations (with some
simplifying approximations). The physics of in-medium showers is substantially
affected by the Landau-Pomeranchuk-Midgal (LPM) effect, and our proposed tests
require use of what might be called `next-to-leading order' LPM results, which
account for quantum interference between consecutive splittings. The complete
set of such results is not yet available for QCD but is already available for
the theory of large- QED. We therefore use large- QED as an example,
presenting numerical results as a function of , where is
the strength of the coupling at the relevant high-energy scale characterizing
splittings of the high-energy particles.Comment: 31 pages + appendices for 48 pages total, 21 figures. [Difference
from version 2: Main change was to eliminate some summary formulas of NLO
rates in section III.B, made unnecessary by a clear summary of formulas
having been added to ref. [13].
Fantasies of the Library
Anne Royston, Assistant Professor, Rochester Institute of Technology, reviews:Anna-Sophie Springer and Etienne Turpin, eds. FANTASIES OF THE LIBRARY (2016)andCraig Dworkin, DEF (2018).Â
A linear systems analysis of the yaw dynamics of a dynamically scaled insect model
Recent studies suggest that fruit flies use subtle changes to their wing motion to actively generate forces during aerial maneuvers. In addition, it has been estimated that the passive rotational damping caused by the flapping wings of an insect is around two orders of magnitude greater than that for the body alone. At present, however, the relationships between the active regulation of wing kinematics, passive damping produced by the flapping wings and the overall trajectory of the animal are still poorly understood. In this study, we use a dynamically scaled robotic model equipped with a torque feedback mechanism to study the dynamics of yaw turns in the fruit fly Drosophila melanogaster. Four plausible mechanisms for the active generation of yaw torque are examined. The mechanisms deform the wing kinematics of hovering in order to introduce asymmetry that results in the active production of yaw torque by the flapping wings. The results demonstrate that the stroke-averaged yaw torque is well approximated by a model that is linear with respect to both the yaw velocity and the magnitude of the kinematic deformations. Dynamic measurements, in which the yaw torque produced by the flapping wings was used in real-time to determine the rotation of the robot, suggest that a first-order linear model with stroke-average coefficients accurately captures the yaw dynamics of the system. Finally, an analysis of the stroke-average dynamics suggests that both damping and inertia will be important factors during rapid body saccades of a fruit fly
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