26,932 research outputs found
Compton Polarimetry at a TEV Collider
An electron beam polarization of 80% or greater will be a key feature of a 1
TeV Linear Collider. Accurate measurements of the beam polarization will
therefore be needed. We discuss design considerations and capabilities for a
Compton-scattering polarimeter located in the extraction line from the
Interaction Point. Polarization measurements with 1% accuracy taken parasitic
to collision data look feasible, but detailed simulations are needed.
Polarimeter design issues are similar for both electron-positron and
electron-electron collider modes, though beam disruption creates more
difficulties for the electron-electron mode.Comment: 7 pages, 5 figure
Interaction of a Modulated Electron Beam with a Plasma
The results of a theoretical and experimental investigation of the high-frequency interaction of an electron beam with a plasma are reported. An electron beam, modulated at a microwave frequency, passes through a uniform region of a mercury arc discharge after which it is demodulated. Exponentially growing wave amplification along the electron beam was experimentally observed for the first time at a microwave frequency equal to the plasma frequency. Approximate theories of the effects of 1) plasma-electron collision frequencies, 2) plasma-electron thermal velocities and 3) finite beam diameter, are given. In a second experiment the interaction between a modulated electron beam and a slow electrostatic wave on a plasma column has been studied. A strong interaction occurs when the velocity of the electron beam is approximately equal to the velocity of the wave and the interaction is essentially the same as that which occurs in traveling-wave amplifiers, except that here the plasma colum replaces the usual helical slow-wave circuit. The theory predicting rates of growth is presented and compared with the experimental results
The nature of turbulence in OMC1 at the star forming scale: observations and simulations
Aim: To study turbulence in the Orion Molecular Cloud (OMC1) by comparing
observed and simulated characteristics of the gas motions.
Method: Using a dataset of vibrationally excited H2 emission in OMC1
containing radial velocity and brightness which covers scales from 70AU to
30000AU, we present the transversal structure functions and the scaling of the
structure functions with their order. These are compared with the predictions
of two-dimensional projections of simulations of supersonic hydrodynamic
turbulence.
Results: The structure functions of OMC1 are not well represented by power
laws, but show clear deviations below 2000AU. However, using the technique of
extended self-similarity, power laws are recovered at scales down to 160AU. The
scaling of the higher order structure functions with order deviates from the
standard scaling for supersonic turbulence. This is explained as a selection
effect of preferentially observing the shocked part of the gas and the scaling
can be reproduced using line-of-sight integrated velocity data from subsets of
supersonic turbulence simulations. These subsets select regions of strong flow
convergence and high density associated with shock structure. Deviations of the
structure functions in OMC1 from power laws cannot however be reproduced in
simulations and remains an outstanding issue.Comment: 12 pages, 8 figures, accepted A&A. Revised in response to referee.
For higher resolution, see http://www.astro.phys.au.dk/~maikeng/sim_paper
Observations of spatial and velocity structure in the Orion Molecular Cloud
Observations are reported of H2 IR emission in the S(1) v=1-0 line at 2.121
microns in the Orion Molecular Cloud, OMC1, using the GriF instrument on the
Canada-France-Hawaii Telescope. GriF is a combination of adaptive optics and
Fabry-Perot interferometry, yielding a spatial resolution of 0.15" to 0.18" and
a velocity discrimination as high as 1 km/s. Thanks to the high spatial and
velocity resolution of the GriF data, 193 bright H2 emission regions can be
identified in OMC1. The general characteristics of these features are described
in terms of radial velocities, brightness and spatial displacement of maxima of
velocity and brightness, the latter to yield the orientation of flows in the
plane of the sky. Strong spatial correlation between velocity and bright H2
emission is found and serves to identify many features as shocks. Important
results are: (i) velocities of the excited gas illustrate the presence of a
zone to the south of BN-IRc2 and Peak 1, and the west of Peak 2, where there is
a powerful blue-shifted outflow with an average velocity of -18 km/s. This is
shown to be the NIR counterpart of an outflow identified in the radio from
source I, a very young O-star. (ii) There is a band of weak velocity features
(<5 km/s) in Peak 1 which may share a common origin through an explosive event,
in the BN-IRc2 region, with the fast-moving fingers (or bullets) to the NW of
OMC1. (iii) A proportion of the flows are likely to represent sites of low mass
star formation and several regions show multiple outflows, probably indicative
of multiple star formation within OMC1. The high spatial and velocity
resolution of the GriF data show these and other features in more detail than
has previously been possible.Comment: 27 pages, 19 figures, submitted to A&A Version 2: Several additions,
including a section on protostellar candidates in OMC1, have been made based
on the referee's suggestions v3: corrected typograph
Recommended from our members
Neural processing of imminent collision in humans
Detecting a looming object and its imminent collision is imperative to survival. For most humans, it is a fundamental aspect of daily activities such as driving, road crossing and participating in sport, yet little is known about how the brain both detects and responds to such stimuli. Here we use functional magnetic resonance imaging to assess neural response to looming stimuli in comparison with receding stimuli and motion-controlled static stimuli. We demonstrate for the first time that, in the human, the superior colliculus and the pulvinar nucleus of the thalamus respond to looming in addition to cortical regions associated with motor preparation. We also implicate the anterior insula in making timing computations for collision events
Persistent Challenges of Quantum Chromodynamics
Unlike some models whose relevance to Nature is still a big question mark,
Quantum Chromodynamics will stay with us forever. Quantum Chromodynamics (QCD),
born in 1973, is a very rich theory supposed to describe the widest range of
strong interaction phenomena: from nuclear physics to Regge behavior at large
E, from color confinement to quark-gluon matter at high densities/temperatures
(neutron stars); the vast horizons of the hadronic world: chiral dynamics,
glueballs, exotics, light and heavy quarkonia and mixtures thereof, exclusive
and inclusive phenomena, interplay between strong forces and weak interactions,
etc. Efforts aimed at solving the underlying theory, QCD, continue. In a
remarkable entanglement, theoretical constructions of the 1970s and 1990s
combine with today's ideas based on holographic description and strong-weak
coupling duality, to provide new insights and a deeper understanding.Comment: Julius Edgar Lilienfeld Prize Lecture at the April Meeting of APS,
Dallas, TX, April 22-25, 2006; v.2: reference added; v.3: reference adde
Individual Control of Risk: Seat Belt Use, Subjective Norms and the Theory of Reasoned Action
When faced with a risk for which an inexpensive solution is available, individuals often choose the risk rather than the solution. Protection from certain kinds of risks, e.g., using seat belts or condoms or insulating against radon, is largely under personal control, but individuals often choose not to comply with behaviors which would reduce the risk. The Theory of Reasoned Action (TRA) has been used to predict when individuals will comply. The authors attempted to validate aspects of the TRA by the use of scenarios. Factor analysis of their data supports the theory that intention is a major determinate of behavior but fails to establish the influence of scenarios on subjects\u27 intention to wear seat belts
Phenomenology of the Flavor-Asymmetry in the Light-Quark Sea of the Nucleon
A phenomenological ansatz for the flavor-asymmetry of the light sea
distributions of the nucleon, based on the Pauli exclusion principle, is
proposed. This ansatz is compatible with the measured flavor-asymmetry of the
unpolarized sea distributions, , of the nucleon. A prediction
for the corresponding polarized flavor-asymmetry is presented and shown to
agree with predictions of (chiral quark--soliton) models which successfully
reproduced the flavor-asymmetry of the unpolarized sea.Comment: 5 pages, LaTeX, 2 figures, uses epsfi
Polarization of Astronomical Maser Radiation. IV. Circular Polarization Profiles
Profile comparison of the Stokes parameters and is a powerful tool
for maser data analysis, providing the first direct methods for unambiguous
determination of (1) the maser saturation stage, (2) the amplification optical
depth and intrinsic Doppler width of unsaturated masers, and (3) the
comparative magnitudes of Zeeman splitting and Doppler linewidth. Circular
polarization recently detected in OH 1720 MHz emission from the Galactic center
appears to provide the first direct evidence for maser saturation.Comment: 14 pages, 1 Postscript figures (included), uses aaspp4.sty. To appear
in Astrophysical Journa
Derivation of the Lorentz Force Law, the Magnetic Field Concept and the Faraday-Lenz Law using an Invariant Formulation of the Lorentz Transformation
It is demonstrated how the right hand sides of the Lorentz Transformation
equations may be written, in a Lorentz invariant manner, as 4--vector scalar
products. This implies the existence of invariant length intervals analogous to
invariant proper time intervals. This formalism, making essential use of the
4-vector electromagnetic potential concept, provides a short derivation of the
Lorentz force law of classical electrodynamics, the conventional definition of
the magnetic field, in terms of spatial derivatives of the 4--vector potential
and the Faraday-Lenz Law. An important distinction between the physical
meanings of the space-time and energy-momentum 4--vectors is pointed out.Comment: 15 pages, no tables 1 figure. Revised and extended version of
physics/0307133 Some typos removed and minor text improvements in this
versio
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