2,888 research outputs found
Absence of Embedded Mass Shells: Cerenkov Radiation and Quantum Friction
We show that, in a model where a non-relativistic particle is coupled to a
quantized relativistic scalar Bose field, the embedded mass shell of the
particle dissolves in the continuum when the interaction is turned on, provided
the coupling constant is sufficiently small. More precisely, under the
assumption that the fiber eigenvectors corresponding to the putative mass shell
are differentiable as functions of the total momentum of the system, we show
that a mass shell could exist only at a strictly positive distance from the
unperturbed embedded mass shell near the boundary of the energy-momentum
spectrum.Comment: Revised version: a remark added at the end of Section
Magnetism and the Weiss Exchange Field - A Theoretical Analysis Inspired by Recent Experiments
The huge spin precession frequency observed in recent experiments with
spin-polarized beams of hot electrons shot through magnetized films is
interpreted as being caused by Zeeman coupling of the electron spins to the
so-called Weiss exchange field in the film. A "Stern-Gerlach experiment" for
electrons moving through an inhomogeneous exchange field is proposed. The
microscopic origin of exchange interactions and of large mean exchange fields,
leading to different types of magnetic order, is elucidated. A microscopic
derivation of the equations of motion of the Weiss exchange field is presented.
Novel proofs of the existence of phase transitions in quantum XY-models and
antiferromagnets, based on an analysis of the statistical distribution of the
exchange field, are outlined.Comment: 36 pages, 3 figure
Long-range magnetic fields in the ground state of the Standard Model plasma
In thermal equilibrium the ground state of the plasma of Standard Model
particles is determined by temperature and exactly conserved combinations of
baryon and lepton numbers. We show that at non-zero values of the global
charges a translation invariant and homogeneous state of the plasma becomes
unstable and the system transits into a new state, containing a large-scale
magnetic field. The origin of this effect is the parity-breaking character of
weak interactions and chiral anomaly. This situation can occur in the early
Universe and may play an important role in its subsequent evolution.Comment: 6 pages. Comments are welcom
Non-demolition measurements of observables with general spectra
It has recently been established that, in a non-demolition measurement of an
observable with a finite point spectrum, the density matrix of
the system approaches an eigenstate of , i.e., it "purifies" over
the spectrum of . We extend this result to observables with
general spectra. It is shown that the spectral density of the state of the
system converges to a delta function exponentially fast, in an appropriate
sense. Furthermore, for observables with absolutely continuous spectra, we show
that the spectral density approaches a Gaussian distribution over the spectrum
of . Our methods highlight the connection between the theory of
non-demolition measurements and classical estimation theory.Comment: 22 page
Some Hamiltonian Models of Friction
Mathematical results on some models describing the motion of a tracer
particle through a Bose-Einstein condensate are described. In the limit of a
very dense, very weakly interacting Bose gas and for a very large particle
mass, the dynamics of the coupled system is determined by classical non-linear
Hamiltonian equations of motion. The particle's motion exhibits deceleration
corresponding to friction (with memory) caused by the emission of Cerenkov
radiation of gapless modes into the gas.
Precise results are stated and outlines of proofs are presented. Some
technical details are deferred to forthcoming papers.Comment: 19 Pages, 1 figur
On Long-Range Order in Low-Dimensional Lattice-Gas Models of Nematic Liquid Crystals
The problem of the orientational ordering transition for lattice-gas models
of liquid crystals is discussed in the low-dimensional case . For
isotropic short-range interactions, orientational long-range order at finite
temperature is excluded for any packing of molecules on the lattice ; on
the other hand, for reflection-positive long-range isotropic interactions, we
prove existence of an orientational ordering transition for high packing () and low temperatures ().Comment: 11 page
Absence of spontaneous magnetic order at non-zero temperature in one- and two-dimensional Heisenberg and XY systems with long-range interactions
The Mermin-Wagner theorem is strengthened so as to rule out magnetic
long-range order at T>0 in one- or two-dimensional Heisenberg and XY systems
with long-range interactions decreasing as R^{-alpha} with a sufficiently large
exponent alpha. For oscillatory interactions, ferromagnetic long-range order at
T>0 is ruled out if alpha >= 1 (D=1) or alpha > 5/2 (D=2). For systems with
monotonically decreasing interactions ferro- or antiferromagnetic long-range
order at T>0 is ruled out if alpha >= 2D.Comment: RevTeX, 4 pages. Further (p)reprints available from
http://www.mpi-halle.de/~theory ; v2: revised versio
Laminar and turbulent dynamos in chiral magnetohydrodynamics-I: Theory
The magnetohydrodynamic (MHD) description of plasmas with relativistic
particles necessarily includes an additional new field, the chiral chemical
potential associated with the axial charge (i.e., the number difference between
right- and left-handed relativistic fermions). This chiral chemical potential
gives rise to a contribution to the electric current density of the plasma
(\emph{chiral magnetic effect}). We present a self-consistent treatment of the
\emph{chiral MHD equations}, which include the back-reaction of the magnetic
field on a chiral chemical potential and its interaction with the plasma
velocity field. A number of novel phenomena are exhibited. First, we show that
the chiral magnetic effect decreases the frequency of the Alfv\'{e}n wave for
incompressible flows, increases the frequencies of the Alfv\'{e}n wave and of
the fast magnetosonic wave for compressible flows, and decreases the frequency
of the slow magnetosonic wave. Second, we show that, in addition to the
well-known laminar chiral dynamo effect, which is not related to fluid motions,
there is a dynamo caused by the joint action of velocity shear and chiral
magnetic effect. In the presence of turbulence with vanishing mean kinetic
helicity, the derived mean-field chiral MHD equations describe turbulent
large-scale dynamos caused by the chiral alpha effect, which is dominant for
large fluid and magnetic Reynolds numbers. The chiral alpha effect is due to an
interaction of the chiral magnetic effect and fluctuations of the small-scale
current produced by tangling magnetic fluctuations (which are generated by
tangling of the large-scale magnetic field by sheared velocity fluctuations).
These dynamo effects may have interesting consequences in the dynamics of the
early universe, neutron stars, and the quark--gluon plasma.Comment: 23 pages, 4 figure
Magnetic activity, differential rotation and dynamo action in the pulsating F9IV star KIC 5955122
We present photometric spot modeling of the nearly four-year long light-curve
of the Kepler target KIC 5955122 in terms of persisting dark circular surface
features. With a Bayesian technique, we produced a plausible surface map that
shows dozens of small spots. After some artifacts are removed, the residuals
are at \,mmag. The shortest rotational period found is days. The equator-to-pole extrapolated differential rotation is rad/d. The spots are roughly half as bright as the unperturbed stellar
photosphere. Spot latitudes are restricted to the zone latitude.
There is no indication for any near-pole spots. In addition, the p-mode
pulsations enabled us to determine the evolutionary status of the star, the
extension of the convective zone, and its radius and mass. We discuss the
possibility that the clear signature of active regions in the light curve of
the F9IV star KIC 5955122 is produced by a flux-transport dynamo action at the
base of the convection zone. In particular, we argue that this star has evolved
from an active to a quiet status during the Q0--Q16 period of observation, and
we predict, according to our dynamo model, that the characteristic activity
cycle is of the order of the solar one.Comment: 9 pages, 12 figures, to be published on A&
Reconstructed and measured total solar irradiance: Is there a secular trend between 1978 and 2003?
Total solar irradiance reconstructed between 1978 and 2003 using solar
surface magnetic field distributions is compared with three composites of total
solar irradiance measurements. A good correspondence is found with the total
solar irradiance composite from PMOD/WRC, with no bias between the three
cycles. The agreement with the other composites (the ACRIM composite, mainly
based on the Active Cavity Radiometer Irradiance Monitors I, II & III, and the
IRMB composite from the Institut Royal Meteorologique Belgique) is
significantly poorer. In particular, a secular increase in the irradiance
exhibited by these composites is not present in the reconstructions. Hence any
secular trend in total solar irradiance between 1978 and 2003 is not due to
magnetic fields at the solar surface
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