11,468 research outputs found
Transition Form Factors in the PQCD approach
Under two different scenarios for the light scalar mesons, we investigate the
transition form factors of mesons decay into a scalar meson in the
perturbative QCD approach. In the large recoiling region, the form factors are
dominated by the short-distance dynamics and can be calculated using
perturbation theory. We adopt the dipole parametrization to recast the
dependence of the form factors. Since the decay constants defined by the scalar
current are large, our predictions on the form factors are much larger
than the transitions, especially in the second scenario. Contributions
from various light-cone distribution amplitudes (LCDAs) are elaborated and we
find that the twist-3 LCDAs provide more than a half contributions to the form
factors. The two terms of the twist-2 LCDAs give destructive contributions in
the first scenario while they give constructive contributions in the second
scenario. With the form factors, we also predict the decay width and branching
ratios of the semileptonic and decays. The
branching ratios of channels are found to have the order of
while those of have the order of . These
predictions can be tested by the future experiments.Comment: 20 pages, 31 figure
Violating Electrodynamics and Chern-Simons Modified Gravity
The electrodynamics with a Chern-Simons term
violates Lorentz and symmetries
with a non-vanishing . For a fixed vector , in this paper we
point out that the energy-momentum tensor of this theory coupled to the gravity
minimally is symmetric but not divergence free, which consequently makes the
gravitational field equation inconsistent. To preserve the consistency, we
introduce a Chern-Simons term in the gravity sector with the coefficient
determined by the Lorentz and violating term in the electromagnetic
field. Further we study the phenomenologies of the model on the cosmic
microwave background radiation and the relic gravitational waves.Comment: 11 pages, 1 figure, the version to appear in Physics Letters
Dark Energy Perturbations Revisited
In this paper we study the evolution of cosmological perturbations in the
presence of dynamical dark energy, and revisit the issue of dark energy
perturbations. For a generally parameterized equation of state (EoS) such as
w_D(z) = w_0+w_1\frac{z}{1+z}, (for a single fluid or a single scalar field )
the dark energy perturbation diverges when its EoS crosses the cosmological
constant boundary w_D=-1. In this paper we present a method of treating the
dark energy perturbations during the crossing of the surface by
imposing matching conditions which require the induced 3-metric on the
hypersurface of w_D=-1 and its extrinsic curvature to be continuous. These
matching conditions have been used widely in the literature to study
perturbations in various models of early universe physics, such as Inflation,
the Pre-Big-Bang and Ekpyrotic scenarios, and bouncing cosmologies. In all of
these cases the EoS undergoes a sudden change. Through a detailed analysis of
the matching conditions, we show that \delta_D and \theta_D are continuous on
the matching hypersurface. This justifies the method used[1-4] in the numerical
calculation and data fitting for the determination of cosmological parameters.
We discuss the conditions under which our analysis is applicable.Comment: 10 pages and 1 figure
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