7,749 research outputs found
Nonequilibrium quantum fluctuations of a dispersive medium: Spontaneous emission, photon statistics, entropy generation, and stochastic motion
We study the implications of quantum fluctuations of a dispersive medium,
under steady rotation, either in or out of thermal equilibrium with its
environment. A rotating object exhibits a quantum instability by dissipating
its mechanical motion via spontaneous emission of photons, as well as internal
heat generation. Universal relations are derived for the radiated energy and
angular momentum as trace formulas involving the object's scattering matrix. We
also compute the quantum noise by deriving the full statistics of the radiated
photons out of thermal and/or dynamic equilibrium. The (entanglement) entropy
generation is quantified, and the total entropy is shown to be always
increasing. Furthermore, we derive a Fokker-Planck equation governing the
stochastic angular motion resulting from the fluctuating back-reaction
frictional torque. As a result, we find a quantum limit on the uncertainty of
the object's angular velocity in steady rotation. Finally, we show in some
detail that a rotating object drags nearby objects, making them spin parallel
to its axis of rotation. A scalar toy model is introduced in the first part to
simplify the technicalities and ease the conceptual complexities; a detailed
discussion of quantum electrodynamics is presented in the second part
On the relation between nuclear and nucleon Structure Functions and their moments
Calculations of nuclear Structure Functions (SF) F_k^A(x,Q^2) routinely
exploit a generalized convolution, involving the SF for nucleons F_k^N and the
linking SF f^{PN,A} of a fictitious nucleus, composed of point-particles, with
the latter usually expressed in terms of hadronic degrees of freedom. For
finite Q^2 the approach seemed to be lacking a solid justification and the same
is the case for recently proposed, effective nuclear parton distribution
functions (pdf), which exactly reproduce the above-mentioned hadronically
computed F_k^A. Many years ago Jaffe and West proved the above convolution in
the Plane Wave Impulse Approximation (PWIA) for the nuclear components in the
convolution. In the present note we extend the above proof to include classes
of nuclear Final State Interactions (FSI). One and the same function appears to
relate parton distribution functions (pdf) in nuclei and nucleons, and SF for
nuclear targets and for nucleons. That relation is the previously conjectured
one,with an entirely different interpretation of f^{PN,A}. We conclude with an
extensive analysis of moments of nuclear SF based on the generalized
convolution. Characteristics of those moments are shown to be quite similar to
the same for a nucleon. We conclude that the above evidences asymptotic freedom
of a nucleon in a medium and not of a composite nucleus.Comment: 18 pages, 9 figure
Bound States and Threshold Resonances in Quantum Wires with Circular Bends
We study the solutions to the wave equation in a two-dimensional tube of unit
width comprised of two straight regions connected by a region of constant
curvature. We introduce a numerical method which permits high accuracy at high
curvature. We determine the bound state energies as well as the transmission
and reflection matrices, and and focus on the nature of
the resonances which occur in the vicinity of channel thresholds. We explore
the dependence of these solutions on the curvature of the tube and angle of the
bend and discuss several limiting cases where our numerical results confirm
analytic predictions.Comment: 24 pages, revtex file, one style file and 17 PostScript figures
include
Implications of the Babinet Principle for Casimir Interactions
We formulate the Babinet Principle (BP) as a relation between the scattering
amplitudes for electromagnetic waves, and combine it with multiple scattering
techniques to derive new properties of Casimir forces. We show that the Casimir
force exerted by a planar conductor or dielectric on a self- complementary
perforated planar mirror is approximately half that on a uniform mirror
independent of the distance between them. The BP suggests that Casimir edge
effects are anomalously small, supporting results obtained earlier in special
cases. Finally, we illustrate how the BP can be used to estimate Casimir forces
between perforated planar mirrors
On Distribution Functions for Partons in Nuclei
We suggest that a previously conjectured relation between Structure Functions
(SF) for nuclei and nucleons also links distribution functions (df) for partons
in a nucleus and in nucleons. The above suggestion ensures in principle
identical results for SF , whether computed with hadronic or partonic
degrees of freedom. In practice there are differences, due to different
input. We show that the thus defined nuclear parton distribution functions
(pdf) respect standard sumrules. In addition we numerically compare some
moments of nuclear SF, and find agreement between results, using hadronic and
partonic descriptions. We present computations of EMC ratios for both, and
compare those with hadronic predictions and data. In spite of substantial
differences in the participating SF, the two representations produce
approximately the same EMC ratios. The apparent correlation between the above
deviations is ascribed to a sumrule for . We conclude with a discussion
of alternative approaches to nuclear pdf.Comment: 14 pages, 4 figure
Tortuous ways to the extraction of neutron observables from inclusive lepton scattering
We analyze new JLAB data for inclusive electron scattering on various
targets. Computed and measured total inclusive cross sections in the range
show on a logarithmic scale reasonable agreement
for all targets. However, closer inspection of the Quasi-Elastic components
bares serious discrepancies. EMC ratios which may contain less systematic
errors fare the same. The above observations for the new data do not enable the
extraction of the magnetic form factor (FF) and the Structure Function
(SFs) of the neutron, although the application of exactly the same
analysis to older data had been successful. We add to the above analysis older
CLAS collaboration on . Removing some scattered points, it appears
possible to obtain the above mentioned neutron information. We compare our
results with others from alternative sources. Particular attention is paid to
the A=3 iso-doublet. Present data exist only for He, but the available
input and charge symmetry also enable computations for H. Their average is
the computed iso-scalar part and is compared with the empirical modification of
He towards a fictitious A=3 iso-singlet.Comment: 27 pages, 30 figure
Quark Masses: An Environmental Impact Statement
We investigate worlds that lie on a slice through the parameter space of the
Standard Model over which quark masses vary. We allow as many as three quarks
to participate in nuclei, while fixing the mass of the electron and the average
mass of the lightest baryon flavor multiplet. We classify as "congenial" worlds
that satisfy the environmental constraint that the quark masses allow for
stable nuclei with charges one, six, and eight, making organic chemistry
possible. Whether a congenial world actually produces observers depends on a
multitude of historical contingencies, beginning with primordial
nucleosynthesis, which we do not explore. Such constraints may be independently
superimposed on our results. Environmental constraints such as the ones we
study may be combined with information about the a priori distribution of quark
masses over the landscape of possible universes to determine whether the
measured values of the quark masses are determined environmentally, but our
analysis is independent of such an anthropic approach.
We estimate baryon masses as functions of quark masses and nuclear masses as
functions of baryon masses. We check for the stability of nuclei against
fission, strong particle emission, and weak nucleon emission. For two light
quarks with charges 2/3 and -1/3, we find a band of congeniality roughly 29 MeV
wide in their mass difference. We also find another, less robust region of
congeniality with one light, charge -1/3 quark, and two heavier, approximately
degenerate charge -1/3 and 2/3 quarks. No other assignment of light quark
charges yields congenial worlds with two baryons participating in nuclei. We
identify and discuss the region in quark-mass space where nuclei would be made
from three or more baryon species.Comment: 40 pages, 16 figures (in color), 4 tables. See paper for a more
detailed abstract. v4: Cleaning up minor typo
The dusty torus in the Circinus galaxy: a dense disk and the torus funnel
(Abridged) With infrared interferometry it is possible to resolve the nuclear
dust distributions that are commonly associated with the dusty torus in active
galactic nuclei (AGN). The Circinus galaxy hosts the closest Seyfert 2 nucleus
and previous interferometric observations have shown that its nuclear dust
emission is well resolved.
To better constrain the dust morphology in this active nucleus, extensive new
observations were carried out with MIDI at the Very Large Telescope
Interferometer.
The emission is distributed in two distinct components: a disk-like emission
component with a size of ~ 0.2 1.1 pc and an extended component with a
size of ~ 0.8 1.9 pc. The disk-like component is elongated along PA ~
46{\deg} and oriented perpendicular to the ionisation cone and outflow. The
extended component is elongated along PA ~ 107{\deg}, roughly perpendicular to
the disk component and thus in polar direction. It is interpreted as emission
from the inner funnel of an extended dust distribution and shows a strong
increase in the extinction towards the south-east. We find no evidence of an
increase in the temperature of the dust towards the centre. From this we infer
that most of the near-infrared emission probably comes from parsec scales as
well. We further argue that the disk component alone is not sufficient to
provide the necessary obscuration and collimation of the ionising radiation and
outflow. The material responsible for this must instead be located on scales of
~ 1 pc, surrounding the disk.
The clear separation of the dust emission into a disk-like emitter and a
polar elongated source will require an adaptation of our current understanding
of the dust emission in AGN. The lack of any evidence of an increase in the
dust temperature towards the centre poses a challenge for the picture of a
centrally heated dust distribution.Comment: 30 pages, 12 figures; A&A in pres
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