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, $\alpha$ 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 $t(\uparrow)\to b l^+\nu_l$. As a byproduct, we reinvestigate the SUSY-QCD corrections to the total decay width $\Gamma(t\to W^+b)$ and resolve a discrepancy between two previous results in the literature.Comment: 11 pages, 6 ps figure