3,724 research outputs found
Spin Alignment of Heavy Meson Revisited
Using heavy quark effective theory a factorized form for inclusive production
rate of a heavy meson can be obtained, in which the nonperturbative effect
related to the heavy meson can be characterized by matrix elements defined in
the heavy quark effective theory. Using this factorization, predictions for the
full spin density matrix of a spin-1 and spin-2 meson can be obtained and they
are characterized only by one coefficient representing the nonperturbative
effect. Predictions for spin-1 heavy meson are compared with experiment
performed at colliders in the energy range from GeV to
GeV, a complete agreement is found for - and -meson.
There are distinct differences from the existing approach and they are
discussed.Comment: 6 pages, Talk given at 3rd Circum-Pan-Pacific Symposium on "High
Energy Spin Physics", Beijing, China, 8-13, 200
Determination of biaxial creep strength of T-111 tantalum alloy
Biaxial creep strength of T-111 tantalum alloy tubing in high temperature, high vacuum environmen
Spin properties of top quark pairs produced at hadron colliders
We discuss the spin properties of top quark pairs produced at hadron
colliders at next-to-leading order in the coupling constant alpha_s of the
strong interaction. Specifically we present, for some decay channels, results
for differential angular distributions that are sensitive to t tbar spin
correlations.Comment: Invited talk given by A. Brandenburg at the Cracow epiphany
conference on heavy flavours, 3 - 6 January 2003, Cracow, Polan
Investigation of Top quark spin correlations at hadron collider
We report on our results about hadronic production at NLO QCD
including spin effects, especially on spin correlations.Comment: talk given at the 32nd International Conference on High Energy
Physics (ICHEP'04), Beijing, China, 16-22 Aug. 200
Turbulent transport in hydromagnetic flows
The predictive power of mean-field theory is emphasized by comparing theory
with simulations under controlled conditions. The recently developed test-field
method is used to extract turbulent transport coefficients both in kinematic as
well as nonlinear and quasi-kinematic cases. A striking example of the
quasi-kinematic method is provided by magnetic buoyancy-driven flows that
produce an alpha effect and turbulent diffusion.Comment: 17 pages, 6 figures, topical issue of Physica Scripta on turbulent
mixing and beyon
Magnetic helicity fluxes in interface and flux transport dynamos
Dynamos in the Sun and other bodies tend to produce magnetic fields that
possess magnetic helicity of opposite sign at large and small scales,
respectively. The build-up of magnetic helicity at small scales provides an
important saturation mechanism. In order to understand the nature of the solar
dynamo we need to understand the details of the saturation mechanism in
spherical geometry. In particular, we want to understand the effects of
magnetic helicity fluxes from turbulence and meridional circulation. We
consider a model with just radial shear confined to a thin layer (tachocline)
at the bottom of the convection zone. The kinetic alpha owing to helical
turbulence is assumed to be localized in a region above the convection zone.
The dynamical quenching formalism is used to describe the build-up of mean
magnetic helicity in the model, which results in a magnetic alpha effect that
feeds back on the kinetic alpha effect. In some cases we compare with results
obtained using a simple algebraic alpha quenching formula. In agreement with
earlier findings, the magnetic alpha effect in the dynamical alpha quenching
formalism has the opposite sign compared with the kinetic alpha effect and
leads to a catastrophic decrease of the saturation field strength with
increasing magnetic Reynolds numbers. However, at high latitudes this quenching
effect can lead to secondary dynamo waves that propagate poleward due to the
opposite sign of alpha. Magnetic helicity fluxes both from turbulent mixing and
from meridional circulation alleviate catastrophic quenching.Comment: 9 pages, 14 figures, submitted to A &
The inverse cascade and nonlinear alpha-effect in simulations of isotropic helical hydromagnetic turbulence
A numerical model of isotropic homogeneous turbulence with helical forcing is
investigated. The resulting flow, which is essentially the prototype of the
alpha^2 dynamo of mean-field dynamo theory, produces strong dynamo action with
an additional large scale field on the scale of the box (at wavenumber k=1;
forcing is at k=5). This large scale field is nearly force-free and exceeds the
equipartition value. As the magnetic Reynolds number R_m increases, the
saturation field strength and the growth rate of the dynamo increase. However,
the time it takes to built up the large scale field from equipartition to its
final super-equipartition value increases with magnetic Reynolds number. The
large scale field generation can be identified as being due to nonlocal
interactions originating from the forcing scale, which is characteristic of the
alpha-effect. Both alpha and turbulent magnetic diffusivity eta_t are
determined simultaneously using numerical experiments where the mean-field is
modified artificially. Both quantities are quenched in a R_m-dependent fashion.
The evolution of the energy of the mean field matches that predicted by an
alpha^2 dynamo model with similar alpha and eta_t quenchings. For this model an
analytic solution is given which matches the results of the simulations. The
simulations are numerically robust in that the shape of the spectrum at large
scales is unchanged when changing the resolution from 30^3 to 120^3 meshpoints,
or when increasing the magnetic Prandtl number (viscosity/magnetic diffusivity)
from 1 to 100. Increasing the forcing wavenumber to 30 (i.e. increasing the
scale separation) makes the inverse cascade effect more pronounced, although it
remains otherwise qualitatively unchanged.Comment: 21 pages, 26 figures, ApJ (accepted
Numerical simulations of the decay of primordial magnetic turbulence
We perform direct numerical simulations of forced and freely decaying 3D
magnetohydrodynamic turbulence in order to model magnetic field evolution
during cosmological phase transitions in the early Universe. Our approach
assumes the existence of a magnetic field generated either by a process during
inflation or shortly thereafter, or by bubble collisions during a phase
transition. We show that the final configuration of the magnetic field depends
on the initial conditions, while the velocity field is nearly independent of
initial conditions.Comment: 10 pages, 6 figures, references added, PRD accepte
Shear-driven and diffusive helicity fluxes in alpha-Omega dynamos
We present nonlinear mean-field alpha-Omega dynamo simulations in spherical
geometry with simplified profiles of kinematic alpha effect and shear. We take
magnetic helicity evolution into account by solving a dynamical equation for
the magnetic alpha effect. This gives a consistent description of the quenching
mechanism in mean-field dynamo models. The main goal of this work is to explore
the effects of this quenching mechanism in solar-like geometry, and in
particular to investigate the role of magnetic helicity fluxes, specifically
diffusive and Vishniac-Cho (VC) fluxes, at large magnetic Reynolds numbers
(Rm). For models with negative radial shear or positive latitudinal shear, the
magnetic alpha effect has predominantly negative (positive) sign in the
northern (southern) hemisphere. In the absence of fluxes, we find that the
magnetic energy follows an Rm^-1 dependence, as found in previous works. This
catastrophic quenching is alleviated in models with diffusive magnetic helicity
fluxes resulting in magnetic fields comparable to the equipartition value even
for Rm=10^7. On the other hand, models with a shear-driven Vishniac-Cho flux
show an increase of the amplitude of the magnetic field with respect to models
without fluxes, but only for Rm<10^4. This is mainly a consequence of assuming
a vacuum outside the Sun which cannot support a significant VC flux across the
boundary. However, in contrast with the diffusive flux, the VC flux modifies
the distribution of the magnetic field. In addition, if an ill-determined
scaling factor in the expression for the VC flux is large enough, subcritical
dynamo action is possible that is driven by the action of shear and the
divergence of current helicity flux.Comment: 12 pages, 12 figures. Submitted to MNRA
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