130,348 research outputs found
News on PHOTOS Monte Carlo: gamma^* -> pi^+ pi^-(gamma) and K^\pm -> pi^+ pi^- e^\pm nu (gamma)
PHOTOS Monte Carlo is widely used for simulating QED effects in decay of
intermediate particles and resonances. It can be easily connected to other main
process generators. In this paper we consider decaying processes gamma^* ->
pi^+ pi^-(gamma) and K^\pm -> pi^+ pi^- e^\pm nu (gamma) in the framework of
Scalar QED. These two processes are interesting not only for the technical
aspect of PHOTOS Monte Carlo, but also for precision measurement of
alpha_{QED}(M_Z), g-2, as well as pi pi scattering lengths.Comment: 6 pages, 11 figures, proceedings of the PhiPsi09, Oct. 13-16, 2009,
Beijing, Chin
Fluence dependent femtosecond quasi-particle and Eu^{2+} -spin relaxation dynamics in EuFe_{2}(As,P)_{2}
We investigated temperature and fluence dependent dynamics of the time
resolved optical reflectivity in undoped spin-density-wave (SDW) and doped
superconducting (SC) EuFe(As,P) with emphasis on the ordered
Eu-spin temperature region. The data indicate that the SDW order
coexists at low temperature with the SC and Eu-ferromagnetic order.
Increasing the excitation fluence leads to a thermal suppression of the
Eu-spin order due to the crystal-lattice heating while the SDW order is
suppressed nonthermally at a higher fluence
Evidence for anisotropic polar nanoregions in relaxor PMN: A neutron study of the elastic constants and anomalous TA phonon damping
We use neutron scattering to characterize the acoustic phonons in the relaxor
PMN and demonstrate the presence of an anisotropic damping mechanism directly
related to short-range, polar correlations. For a large range of temperatures
above Tc ~ 210, K, where dynamic polar correlations exist, acoustic phonons
propagating along [1\bar{1}0] and polarized along [110] (TA2 phonons) are
overdamped and softened across most of the Brillouin zone. By contrast,
acoustic phonons propagating along [100] and polarized along [001] (TA1
phonons) are overdamped and softened for only a limited range of wavevectors.
The anisotropy and temperature dependence of the acoustic phonon energy
linewidth are directly correlated with the elastic diffuse scattering,
indicating that polar nanoregions are the cause of the anomalous behavior. The
damping and softening vanish for q -> 0, i.e. for long-wavelength acoustic
phonons, which supports the notion that the anomalous damping is a result of
the coupling between the relaxational component of the diffuse scattering and
the harmonic TA phonons. Therefore, these effects are not due to large changes
in the elastic constants with temperature because the elastic constants
correspond to the long-wavelength limit. We compare the elastic constants we
measure to those from Brillouin scattering and to values reported for pure PT.
We show that while the values of C44 are quite similar, those for C11 and C12
are significantly less in PMN and result in a softening of (C11-C12) over PT.
There is also an increased elastic anisotropy (2C44/(C11-C12)) versus that in
PT. These results suggest an instability to TA2 acoustic fluctuations in
relaxors. We discuss our results in the context of the debate over the
"waterfall" effect and show that they are inconsistent with TA-TO phonon
coupling or other models that invoke the presence of a second optic mode.Comment: (21 pages, 16 figures, to be published in Physical Review B
A Unifying Perspective: Solitary Traveling Waves As Discrete Breathers And Energy Criteria For Their Stability
In this work, we provide two complementary perspectives for the (spectral)
stability of solitary traveling waves in Hamiltonian nonlinear dynamical
lattices, of which the Fermi-Pasta-Ulam and the Toda lattice are prototypical
examples. One is as an eigenvalue problem for a stationary solution in a
co-traveling frame, while the other is as a periodic orbit modulo shifts. We
connect the eigenvalues of the former with the Floquet multipliers of the
latter and based on this formulation derive an energy-based spectral stability
criterion. It states that a sufficient (but not necessary) condition for a
change in the wave stability occurs when the functional dependence of the
energy (Hamiltonian) of the model on the wave velocity changes its
monotonicity. Moreover, near the critical velocity where the change of
stability occurs, we provide explicit leading-order computation of the unstable
eigenvalues, based on the second derivative of the Hamiltonian
evaluated at the critical velocity . We corroborate this conclusion with a
series of analytically and numerically tractable examples and discuss its
parallels with a recent energy-based criterion for the stability of discrete
breathers
Reconstruction from Radon projections and orthogonal expansion on a ball
The relation between Radon transform and orthogonal expansions of a function
on the unit ball in \RR^d is exploited. A compact formula for the partial
sums of the expansion is given in terms of the Radon transform, which leads to
algorithms for image reconstruction from Radon data. The relation between
orthogonal expansion and the singular value decomposition of the Radon
transform is also exploited.Comment: 15 page
Simulating liquid-vapor phase separation under shear with lattice Boltzmann method
We study liquid-vapor phase separation under shear via the Shan-Chen lattice
Boltzmann model. Besides the rheological characteristics, we analyze the
Kelvin-Helmholtz(K-H) instability resulting from the tangential velocity
difference of the fluids on two sides of the interface. We discuss also the
growth behavior of droplets. The domains being close to the walls are
lamellar-ordered, where the hydrodynamic effects dominate. The patterns in the
bulk of the system are nearly isotropic, where the domain growth results mainly
from the diffusion mechanism. Both the interfacial tension and the K-H
instability make the liquid-bands near the walls tend to rupture. When the
shear rate increases, the inequivalence of evaporation in the upstream and
coagulation in the downstream of the flow as well as the role of surface
tension makes the droplets elongate obliquely. Stronger convection makes easier
the transferring of material particles so that droplets become larger.Comment: Science in China (Series G) (in press
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