49,482 research outputs found

### Tracking Simulations Near Half-Integer Resonance at PEP-II

Beam-beam simulations predict that PEP-II luminosity can be increased by
operating the horizontal betatron tune near and above a half-integer resonance.
However, effects of the resonance and its synchrotron sidebands significantly
enhance betatron and chromatic perturbations which tend to reduce dynamic
aperture. In the study, chromatic variation of horizontal tune near the
resonance was minimized by optimizing local sextupoles in the Interaction
Region. Dynamic aperture was calculated using tracking simulations in LEGO
code. Dependence of dynamic aperture on the residual orbit, dispersion and
distortion of beta function after correction was investigated.Comment: 4 page

### Cosmology with minimal length uncertainty relations

We study the effects of the existence of a minimal observable length in the
phase space of classical and quantum de Sitter (dS) and Anti de Sitter (AdS)
cosmology. Since this length has been suggested in quantum gravity and string
theory, its effects in the early universe might be expected. Adopting the
existence of such a minimum length results in the Generalized Uncertainty
Principle (GUP), which is a deformed Heisenberg algebra between minisuperspace
variables and their momenta operators. We extend these deformed commutating
relations to the corresponding deformed Poisson algebra in the classical limit.
Using the resulting Poisson and Heisenberg relations, we then construct the
classical and quantum cosmology of dS and Ads models in a canonical framework.
We show that in classical dS cosmology this effect yields an inflationary
universe in which the rate of expansion is larger than the usual dS universe.
Also, for the AdS model it is shown that GUP might change the oscillatory
nature of the corresponding cosmology. We also study the effects of GUP in
quantized models through approximate analytical solutions of the Wheeler-DeWitt
(WD) equation, in the limit of small scale factor for the universe, and compare
the results with the ordinary quantum cosmology in each case.Comment: 11 pages, 4 figures, to appear in IJMP

### Cyclic cosmology from Lagrange-multiplier modified gravity

We investigate cyclic and singularity-free evolutions in a universe governed
by Lagrange-multiplier modified gravity, either in scalar-field cosmology, as
well as in $f(R)$ one. In the scalar case, cyclicity can be induced by a
suitably reconstructed simple potential, and the matter content of the universe
can be successfully incorporated. In the case of $f(R)$-gravity, cyclicity can
be induced by a suitable reconstructed second function $f_2(R)$ of a very
simple form, however the matter evolution cannot be analytically handled.
Furthermore, we study the evolution of cosmological perturbations for the two
scenarios. For the scalar case the system possesses no wavelike modes due to a
dust-like sound speed, while for the $f(R)$ case there exist an oscillation
mode of perturbations which indicates a dynamical degree of freedom. Both
scenarios allow for stable parameter spaces of cosmological perturbations
through the bouncing point.Comment: 8 pages, 3 figures, references added, accepted for publicatio

### A Lambda CDM bounce scenario

We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio

### Dynamical Effects Due to Fringe Field of the Magnet in Circular Accelerators

The leading Lie generators, including the chromatic effects, due to hard-edge
fringe field of single multipole and solenoid are derived from the vector
potentials within a Hamiltonian system. These nonlinear generators are applied
to the interaction region of PEP-II to analyze the linear errors due to the
feed-down from the off-centered quadrupoles and solenoid. The nonlinear effects
of tune shifts at large amplitude, the synchro-betatron sidebands near half
integer and their impacts on the dynamic aperture are studied in the paper.Comment: 3 pages, 2 figure

### Bounce and cyclic cosmology in extended nonlinear massive gravity

We investigate non-singular bounce and cyclic cosmological evolutions in a
universe governed by the extended nonlinear massive gravity, in which the
graviton mass is promoted to a scalar-field potential. The extra freedom of the
theory can lead to certain energy conditions violations and drive cyclicity
with two different mechanisms: either with a suitably chosen scalar-field
potential under a given Stuckelberg-scalar function, or with a suitably chosen
Stuckelberg-scalar function under a given scalar-field potential. Our analysis
shows that extended nonlinear massive gravity can alter significantly the
evolution of the universe at both early and late times.Comment: 20 pages, 5 figures, version published at JCA

### Testing the Lorentz and CPT Symmetry with CMB polarizations and a non-relativistic Maxwell Theory

We present a model for a system involving a photon gauge field and a scalar
field at quantum criticality in the frame of a Lifthitz-type non-relativistic
Maxwell theory. We will show this model gives rise to Lorentz and CPT violation
which leads to a frequency-dependent rotation of polarization plane of
radiations, and so leaves potential signals on the cosmic microwave background
temperature and polarization anisotropies.Comment: 7 pages, 2 figures, accepted on JCAP, a few references adde

### Exact Outage Performance Analysis of Multiuser Multi-relay Spectrum Sharing Cognitive Networks

In this paper, we investigate the outage performance of dual-hop multiuser multi-relay cognitive radio networks under spectrum sharing constraints. Using an efficient relay-destination selection scheme, the exact and asymptotic closed-form expressions for the outage probability are derived. From these expressions it is indicated that the achieved diversity order is only determined by the number of secondary user (SU) relays and destinations, and equals to M+N (where M and N are the number of destination nodes and relay nodes, respectively). Further, we find that the coding gain of the SU network will be affected by the interference threshold $bar I$ at the primary user (PU) receiver. Specifically, as the increases of the interference threshold, the coding gain of the considered network approaches to that of the multiuser multi-relay system in the non-cognitive network. Finally, our study is corroborated by representative numerical examples

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