13,261 research outputs found
The partially averaged field approach to cosmic ray diffusion
The kinetic equation for particles interacting with turbulent fluctuations is derived by a new nonlinear technique which successfully corrects the difficulties associated with quasilinear theory. In this new method the effects of the fluctuations are evaluated along particle orbits which themselves include the effects of a statistically averaged subset of the possible configurations of the turbulence. The new method is illustrated by calculating the pitch angle diffusion coefficient D sub Mu Mu for particles interacting with slab model magnetic turbulence, i.e., magnetic fluctuations linearly polarized transverse to a mean magnetic field. Results are compared with those of quasilinear theory and also with those of Monte Carlo calculations. The major effect of the nonlinear treatment in this illustration is the determination of D sub Mu Mu in the vicinity of 90 deg pitch angles where quasilinear theory breaks down. The spatial diffusion coefficient parallel to a mean magnetic field is evaluated using D sub Mu Mu as calculated by this technique. It is argued that the partially averaged field method is not limited to small amplitude fluctuating fields and is hence not a perturbation theory
A new approach to cosmic ray diffusion theory
An approach is presented for deriving a diffusion equation for charged particles in a static, random magnetic field. The approach differs from the usual, quasi-linear one, in that particle orbits in the average field are replaced by particle orbits in a partially averaged field. In this way the fluctuating component of the field significantly modifies the particle orbits in those cases where the orbits in the average field are unrealistic. The method permits the calculation of a finite value for the pitch angle diffusion coefficient for particles with a pitch angle of 90 rather than the divergent or ambiguous results obtained by quasi-linear theories. Results of the approach are compared with results of computer simulations using Monte Carlo techniques
Chiral 3-exchange NN-potentials: Results for dominant next-to-leading order contributions
We calculate in (two-loop) chiral perturbation theory the local NN-potentials
generated by the three-pion exchange diagrams with one insertion from the
second order chiral effective pion-nucleon Lagrangian proportional to the
low-energy constants . The resulting isoscalar central potential
vanishes identically. In most cases these -exchange potentials are larger
than the ones generated by the diagrams involving only leading order vertices
due to the large values of (which mainly represent virtual
-excitation). A similar feature has been observed for the chiral
-exchange. We also give suitable (double-integral) representations for
the spin-spin and tensor potentials generated by the leading-order diagrams
proportional to involving four nucleon propagators. In these cases the
Cutkosky rule cannot be used to calculate the spectral-functions in the
infinite nucleon mass limit since the corresponding mass-spectra start with a
non-vanishing value at the -threshold. Altogether, one finds that chiral
-exchange leads to small corrections in the region fm where
- and chiral -exchange alone provide a very good strong NN-force as
shown in a recent analysis of the low-energy pp-scattering data-base.Comment: 11 pages, 7 figures, to be published in The Physical Review
Coherent light transport in a cold Strontium cloud
We study light coherent transport in the weak localization regime using
magneto-optically cooled strontium atoms. The coherent backscattering cone is
measured in the four polarization channels using light resonant with a J=0 to
J=1 transition of the Strontium atom. We find an enhancement factor close to 2
in the helicity preserving channel, in agreement with theoretical predictions.
This observation confirms the effect of internal structure as the key mechanism
for the contrast reduction observed with an Rubidium cold cloud (see: Labeyrie
et al., PRL 83, 5266 (1999)). Experimental results are in good agreement with
Monte-Carlo simulations taking into account geometry effects.Comment: 4 pages, 2 figure
Redshift-Independent Distances to Type Ia Supernovae
We describe a procedure for accurately determining luminosity distances to
Type Ia supernovae (SNe Ia) without knowledge of redshift. This procedure,
which may be used as an extension of any of the various distance determination
methods currently in use, is based on marginalizing over redshift, removing the
requirement of knowing a priori. We demonstrate that the Hubble diagram
scatter of distances measured with this technique is approximately equal to
that of distances derived from conventional redshift-specific methods for a set
of 60 nearby SNe Ia. This indicates that accurate distances for cosmological
SNe Ia may be determined without the requirement of spectroscopic redshifts,
which are typically the limiting factor for the number of SNe that modern
surveys can collect. Removing this limitation would greatly increase the number
of SNe for which current and future SN surveys will be able to accurately
measure distance. The method may also be able to be used for high- SNe Ia to
determine cosmological density parameters without redshift information.Comment: 12 pages, 3 figures, accepted for publication in Astrophysical
Journal Letter
Surface Shape and Local Critical Behaviour in Two-Dimensional Directed Percolation
Two-dimensional directed site percolation is studied in systems directed
along the x-axis and limited by a free surface at y=\pm Cx^k. Scaling
considerations show that the surface is a relevant perturbation to the local
critical behaviour when k<1/z where z=\nu_\parallel/\nu is the dynamical
exponent. The tip-to-bulk order parameter correlation function is calculated in
the mean-field approximation. The tip percolation probability and the fractal
dimensions of critical clusters are obtained through Monte-Carlo simulations.
The tip order parameter has a nonuniversal, C-dependent, scaling dimension in
the marginal case, k=1/z, and displays a stretched exponential behaviour when
the perturbation is relevant. The k-dependence of the fractal dimensions in the
relevant case is in agreement with the results of a blob picture approach.Comment: 13 pages, Plain TeX file, epsf, 6 postscript-figures, minor
correction
Saturation induced coherence loss in coherent backscattering of light
We use coherent backscattering (CBS) of light by cold Strontium atoms to
study the mutual coherence of light waves in the multiple scattering regime. As
the probe light intensity is increased, the atomic optical transition starts to
be saturated. Nonlinearities and inelastic scattering then occur. In our
experiment, we observe a strongly reduced enhancement factor of the coherent
backscattering cone when the intensity of the probe laser is increased,
indicating a partial loss of coherence in multiple scattering
Direct measurements of the polarization of terrestrial kilometric radiation from Voyagers 1 and 2
Terrestrial radiation measurements obtained with planetary radio astronomy experiments on Voyager-1 and 2 during the early portions of each flight show the signals to be predominantly left-hand circularly polarized. Since these emissions were most probably generated above the Northern Hemisphere auroral zone, it is concluded that the radiation is emitted primarily in the extraordinary mode
- …