10,647 research outputs found
Total-pressure measurement in pulsating flows
Pneumatic-type probe was used as comparison instrument with total pressure tubes to determine true average pressure and, thus, to determine if nonlinear averaging effects were significant. Since pneumatic probe is more complicated to use than a total-pressure tube, it is used only as a comparison instrument to determine extent of averaging effects
Mach-Zehnder optical configuration with Brewster window and two quarter-wave plates
Configuration is improvement because of the following: It provides higher efficiency. It reduces or eliminates feedthrough of untranslated local oscillator, which would produce a beat signal at shifted frequency of translator. When used without translator and with low-power detector, telescope secondary mirror reflects portion of output to local oscillator
Axiomatization and Models of Scientific Theories
In this paper we discuss two approaches to the axiomatization of scien- tific theories in the context of the so called semantic approach, according to which (roughly) a theory can be seen as a class of models. The two approaches are associated respectively to Suppes’ and to da Costa and Chuaqui’s works. We argue that theories can be developed both in a way more akin to the usual mathematical practice (Suppes), in an informal set theoretical environment, writing the set theoretical predicate in the language of set theory itself or, more rigorously (da Costa and Chuaqui), by employing formal languages that help us in writing the postulates to define a class of structures. Both approaches are called internal, for we work within a mathematical framework, here taken to be first-order ZFC. We contrast these approaches with an external one, here discussed briefly. We argue that each one has its strong and weak points, whose discussion is relevant for the philosophical foundations of science
The relation between magnetic and material arms in models for spiral galaxies
Context. Observations of polarized radio emission show that large-scale
(regular) magnetic fields in spiral galaxies are not axisymmetric, but
generally stronger in interarm regions. In some nearby galaxies such as NGC
6946 they are organized in narrow magnetic arms situated between the material
spiral arms. Aims. The phenomenon of magnetic arms and their relation to the
optical spiral arms (the material arms) call for an explanation in the
framework of galactic dynamo theory. Several possibilities have been suggested
but are not completely satisfactory; here we attempt a consistent
investigation. Methods. We use a 2D mean-field dynamo model in the no-z
approximation and add injections of small-scale magnetic field, taken to result
from supernova explosions, to represent the effects of dynamo action on smaller
scales. This injection of small scale field is situated along the spiral arms,
where star-formation mostly occurs. Results. A straightforward explanation of
magnetic arms as a result of modulation of the dynamo mechanism by material
arms struggles to produce pronounced magnetic arms, at least with realistic
parameters, without introducing new effects such as a time lag between Coriolis
force and {\alpha}-effect. In contrast, by taking into account explicitly the
small-scale magnetic field that is injected into the arms by the action of the
star forming regions that are concentrated there, we can obtain dynamo models
with magnetic structures of various forms that can be compared with magnetic
arms. (abbrev). Conclusions. We conclude that magnetic arms can be considered
as coherent magnetic structures generated by large-scale dynamo action, and
associated with spatially modulated small-scale magnetic fluctuations, caused
by enhanced star formation rates within the material arms.Comment: 13 pages, 8 figures, accepted for publication to A&
Two-Loop Corrections to the Neutral Higgs Boson Masses in the CP-Violating NMSSM
We present our calculation of the two-loop corrections of to the neutral Higgs boson masses of the CP-violating
Next-to-Minimal Supersymmetric extension of the Standard Model (NMSSM). The
calculation is performed in the Feynman diagrammatic approach in the gaugeless
limit at vanishing external momentum. We apply a mixed
-on-shell (OS) renormalization scheme for the NMSSM
input parameters. Furthermore, we exploit a as well as
an OS renormalization in the top/stop sector. The corrections are implemented
in the Fortran code NMSSMCALC for the calculation of the Higgs spectrum both in
the CP-conserving and CP-violating NMSSM. The code also provides the Higgs
boson decays including the state-of-the-art higher-order corrections. The
corrections computed in this work improve the already available corrections in
NMSSMCALC which are the full one-loop corrections without any approximation and
the two-loop corrections in the gaugeless limit
and at vanishing external momentum. Depending on the chosen parameter point, we
find that the corrections add about
4-7% to the one-loop mass of the SM-like Higgs boson for
renormalization in the top/stop sector and they reduce
the mass by about 6-9% if OS renormalization is applied. For an estimate of the
theoretical uncertainty we vary the renormalization scale and change the
renormalization scheme and show that care has to be taken in the corresponding
interpretation
Breakdown of Angular Momentum Selection Rules in High Pressure Optical Pumping Experiments
We present measurements, using two complementary methods, of the breakdown of
atomic angular momentum selection rules in He-broadened Rb vapor. Atomic dark
states are rendered weakly absorbing due to fine-structure mixing during Rb-He
collisions. The effect substantially increases the photon demand for optical
pumping of dense vapors
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