17,674 research outputs found
Chiral phase transition and meson spectrum in improved soft-wall AdS/QCD
We investigate in detail the chiral thermal transition of QCD in an improved
soft-wall AdS/QCD model with a simply modified 5D conformal mass of the bulk
scalar field. We also present a calculation in this model for the light meson
spectra and other low-energy characteristic quantities including the pion form
factor, the pi-rho coupling constant and the decay constants of pi, rho, a_1,
which are shown to result in a good agreement with experimental data except for
the pion decay constant. The thermal behavior of chiral condensate is studied.
It is found that such a simply improved soft-wall model incorporates the
crossover behavior of chiral thermal transition indicated by lattice
simulations. The expected chiral transition temperature can be obtained
Lifshitz Scaling Effects on Holographic Superconductors
Via numerical and analytical methods, the effects of the Lifshitz dynamical
exponent on holographic superconductors are studied in some detail,
including wave and wave models. Working in the probe limit, we find
that the behaviors of holographic models indeed depend on concrete value of
. We obtain the condensation and conductivity in both Lifshitz black hole
and soliton backgrounds with general . For both wave and wave models
in the black hole backgrounds, as increases, the phase transition becomes
more difficult and the growth of conductivity is suppressed. For the Lifshitz
soliton backgrounds, when increases (), the critical chemical
potential decreases in the wave cases but increases in the wave cases.
For wave models in both Lifshitz black hole and soliton backgrounds, the
anisotropy between the AC conductivity in different spatial directions is
suppressed when increases. The analytical results uphold the numerical
results.Comment: Typos corrected; Footnote added; References added; To be published in
Nuclear Physics
A Finite Time Analysis of Two Time-Scale Actor Critic Methods
Actor-critic (AC) methods have exhibited great empirical success compared
with other reinforcement learning algorithms, where the actor uses the policy
gradient to improve the learning policy and the critic uses temporal difference
learning to estimate the policy gradient. Under the two time-scale learning
rate schedule, the asymptotic convergence of AC has been well studied in the
literature. However, the non-asymptotic convergence and finite sample
complexity of actor-critic methods are largely open. In this work, we provide a
non-asymptotic analysis for two time-scale actor-critic methods under
non-i.i.d. setting. We prove that the actor-critic method is guaranteed to find
a first-order stationary point (i.e., ) of the non-concave performance function
, with sample
complexity. To the best of our knowledge, this is the first work providing
finite-time analysis and sample complexity bound for two time-scale
actor-critic methods.Comment: 45 page
Five-dimensional generalized gravity with curvature-matter coupling
The generalized gravity with curvature-matter coupling in
five-dimensional (5D) spacetime can be established by assuming a
hypersurface-orthogonal spacelike Killing vector field of 5D spacetime, and it
can be reduced to the 4D formulism of FRW universe. This theory is quite
general and can give the corresponding results to the Einstein gravity,
gravity with both no-coupling and non-minimal coupling in 5D spacetime as
special cases, that is, we would give the some new results besides previous
ones given by Ref.\cite{60}. Furthermore, in order to get some insight into the
effects of this theory on the 4D spacetime, by considering a specific type of
models with and , we not
only discuss the constraints on the model parameters , , but also
illustrate the evolutionary trajectories of the scale factor , the
deceleration parameter and the scalar field , in
the reduced 4D spacetime. The research results show that this type of
gravity models given by us could explain the current accelerated expansion of
our universe without introducing dark energy.Comment: arXiv admin note: text overlap with arXiv:0912.4581,
arXiv:gr-qc/0411066 by other author
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