808 research outputs found

### Electromagnetic Form Factors of Nucleons in a Light-cone Diquark Model

We investigate the electromagnetic form factors of nucleons within a simple
relativistic quark spectator-diquark model using the light-cone formalism.
Melosh rotations are applied to both quark and vector diquark. It is shown that
the difference between vector and scalar spectator diquarks reproduces the
right electric form factor of neutrons, and both the form factors $G_E(Q^2)$
and $G_M(Q^2)$ of the proton and neutron agree with experimental data well up
to $Q^2=2 ~\rm{GeV}^2$ in this simple model.Comment: 16 pages, Revtex4, minor changes, to appear in Phys. Rev.

### Utilizing the Updated Gamma-Ray Bursts and Type Ia Supernovae to Constrain the Cardassian Expansion Model and Dark Energy

We update gamma-ray burst (GRB) luminosity relations among certain spectral
and light-curve features with 139 GRBs. The distance modulus of 82 GRBs at
$z>1.4$ can be calibrated with the sample at $z\leq1.4$ by using the cubic
spline interpolation method from the Union2.1 Type Ia supernovae (SNe Ia) set.
We investigate the joint constraints on the Cardassian expansion model and dark
energy with 580 Union2.1 SNe Ia sample ($z<1.4$) and 82 calibrated GRBs data
($1.4<z\leq8.2$). In $\Lambda$CDM, we find that adding 82 high-\emph{z} GRBs to
580 SNe Ia significantly improves the constrain on
$\Omega_{m}-\Omega_{\Lambda}$ plane. In the Cardassian expansion model, the
best fit is $\Omega_{m}= 0.24_{-0.15}^{+0.15}$ and $n=0.16_{-0.52}^{+0.30}$
$(1\sigma)$, which is consistent with the $\Lambda$CDM cosmology $(n=0)$ in the
$1\sigma$ confidence region. We also discuss two dark energy models in which
the equation of state $w(z)$ is parametrized as $w(z)=w_{0}$ and
$w(z)=w_{0}+w_{1}z/(1+z)$, respectively. Based on our analysis, we see that our
Universe at higher redshift up to $z=8.2$ is consistent with the concordance
model within $1\sigma$ confidence level.Comment: 17 pages, 6 figures, 2 tables; accepted for publication in Advances
in Astronomy, special issue on Gamma-Ray Burst in Swift and Fermi Era. arXiv
admin note: text overlap with arXiv:0802.4262, arXiv:0706.0938 by other
author

### Fragmentation function of $g\to Q\bar{Q}(^3S_1^{[8]})$ in soft gluon factorization and threshold resummation

We study the fragmentation function of the gluon to color-octet $^3S_1$ heavy
quark-antiquark pair using the soft gluon factorization (SGF) approach, which
expresses the fragmentation function in a form of perturbative short-distance
hard part convoluted with one-dimensional color-octet $^3S_1$ soft gluon
distribution (SGD). The short distance hard part is calculated to
next-to-leading order in $\alpha_s$ and a renormalization group equation for
the SGD is derived. By solving the renormalization group equation, threshold
logarithms are resummed to all orders in perturbation theory. The comparison
with gluon fragmentation function calculated in NRQCD factorization approach
indicates that the SGF formula resums a series of velocity corrections in NRQCD
which are important for phenomenological study.Comment: 38 pages, 8 figure

### A CRASH simulation of the contribution of binary stars to the epoch of reionization

We use a set of 3D radiative transfer simulations to study the effect that a
large fraction of binary stars in galaxies during the epoch of reionization has
on the physical properties of the intergalactic medium (i.e. the gas
temperature and the ionization state of hydrogen and helium), on the topology
of the ionized bubbles and on the 21 cm power spectra. Consistently to previous
literature, we find that the inclusion of binary stars can speed up the
reionization process of HI and HeI, while HeII reionization is still dominated
by more energetic sources, especially accreting black holes. The earlier
ionization attained with binary stars allows for more time for cooling and
recombination, so that gas fully ionized by binary stars is typically colder
than that ionized by single stars at any given redshift. With the same volume
averaged ionization fraction, the inclusion of binary stars results in fewer
small ionized bubbles and more large ones, with visible effects also on the
large scales of the 21 cm power spectrum.Comment: 14 pages, 11 figures, MNRAS accepte

### A New Method to Calculate Electromagnetic Impedance Matching Degree in One-Layer Microwave Absorbers

A delta-function method was proposed to quantitatively evaluate the
electromagnetic impedance matching degree. Measured electromagnetic parameters
of {\alpha}-Fe/Fe3B/Y2O3 nanocomposites are applied to calculate the matching
degree by the method. Compared with reflection loss and quarter-wave principle
theory, the method accurately reveals the intrinsic mechanism of microwave
transmission and reflection properties. A possible honeycomb structure with
promising high-performance microwave absorption according to the method is also
proposed.Comment: 13 pages, 3 figure

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