6,030 research outputs found
Predictions for top quark spin correlations at the Tevatron and the LHC at next-to-leading order in alpha_s
Predictions for angular distributions of top quark decay products that are
sensitive to t tbar spin correlations are presented at next-to-leading order in
alpha_s for the Tevatron and the LHC.Comment: 3 pages, 1 figure. Contribution to the HEP2003 Europhysics Conference
in Aachen, German
Kinematic alpha effect in isotropic turbulence simulations
Using numerical simulations at moderate magnetic Reynolds numbers up to 220
it is shown that in the kinematic regime, isotropic helical turbulence leads to
an alpha effect and a turbulent diffusivity whose values are independent of the
magnetic Reynolds number, \Rm, provided \Rm exceeds unity. These turbulent
coefficients are also consistent with expectations from the first order
smoothing approximation. For small values of \Rm, alpha and turbulent
diffusivity are proportional to \Rm. Over finite time intervals meaningful
values of alpha and turbulent diffusivity can be obtained even when there is
small-scale dynamo action that produces strong magnetic fluctuations. This
suggests that small-scale dynamo-generated fields do not make a correlated
contribution to the mean electromotive force.Comment: Accepted for publication in MNRAS Letter
Test-field method for mean-field coefficients with MHD background
Aims: The test-field method for computing turbulent transport coefficients
from simulations of hydromagnetic flows is extended to the regime with a
magnetohydrodynamic (MHD) background. Methods: A generalized set of test
equations is derived using both the induction equation and a modified momentum
equation. By employing an additional set of auxiliary equations, we derive
linear equations describing the response of the system to a set of prescribed
test fields. Purely magnetic and MHD backgrounds are emulated by applying an
electromotive force in the induction equation analogously to the ponderomotive
force in the momentum equation. Both forces are chosen to have Roberts
flow-like geometry. Results: Examples with an MHD background are studied where
the previously used quasi-kinematic test-field method breaks down. In cases
with homogeneous mean fields it is shown that the generalized test-field method
produces the same results as the imposed-field method, where the field-aligned
component of the actual electromotive force from the simulation is used.
Furthermore, results for the turbulent diffusivity tensor are given, which are
inaccessible to the imposed-field method. For MHD backgrounds, new mean-field
effects are found that depend on the occurrence of cross-correlations between
magnetic and velocity fluctuations. For strong imposed fields, is
found to be quenched proportional to the fourth power of the field strength,
regardless of the type of background studied.Comment: 17 pages, 10 figures, submitted to Astronomy & Astrophysic
Relaxation of writhe and twist of a bi-helical magnetic field
In the past few years suggestions have emerged that the solar magnetic field
might have a bi-helical component with oppositely polarized magnetic fields at
large and small scales, and that the shedding of such fields may be crucial for
the operation of the dynamo. It is shown that, if a bi-helical field is shed
into the solar wind, positive and negative contributions of the magnetic
helicity spectrum tend to mix and decay. Even in the absence of turbulence,
mixing and decay can occur on a time scale faster than the resistive one
provided the two signs of magnetic helicity originate from a single tube. In
the presence of turbulence, positively and negatively polarized contributions
mix rapidly in such a way that the ratio of magnetic helicity to magnetic
energy is largest both at the largest scale and in the dissipation range. In
absolute units the small scale excess of helical fields is however negligible.Comment: 6 pages, 9 figures, to be published in Astron. Astrophy
Impact of SUSY-QCD corrections on top quark decay distributions
We compute the supersymmetric QCD corrections to the decay distribution of
polarized top quarks for the semileptonic decay mode . As a byproduct, we reinvestigate the SUSY-QCD corrections to the
total decay width and resolve a discrepancy between two
previous results in the literature.Comment: 11 pages, 6 ps figure
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