6,733 research outputs found
Ionizing radiation fluctuations and large-scale structure in the Lyman-alpha forest
We investigate the large-scale inhomogeneities of the hydrogen ionizing
radiation field in the Universe at redshift z=3. Using a raytracing algorithm,
we simulate a model in which quasars are the dominant sources of radiation. We
make use of large scale N-body simulations of a LambdaCDM universe, and include
such effects as finite quasar lifetimes and output on the lightcone, which
affects the shape of quasar light echoes. We create Lya forest spectra that
would be generated in the presence of such a fluctuating radiation field,
finding that the power spectrum of the Lya forest can be suppressed by as much
as 15 % for modes with k=0.05-1 Mpc/h. This relatively small effect may have
consequences for high precision measurements of the Lya power spectrum on
larger scales than have yet been published. We also investigate another
radiation field probe, the cross-correlation of quasar positions and the Lya
forest. For both quasar lifetimes which we simulate (10^7 yr and 10^8 yr), we
expect to see a strong decrease in the Lya absorption close to other quasars
(the ``foreground'' proximity effect). We then use data from the Sloan Digital
Sky Survey First Data Release to make an observational determination of this
statistic. We find no sign of our predicted lack of absorption, but instead
increased absorption close to quasars. If the bursts of radiation from quasars
last on average < 10^6 yr, then we would not expect to be able to see the
foreground effect. However, the strength of the absorption itself seems to be
indicative of rare objects, and hence much longer total times of emission per
quasar. Variability of quasars in bursts with timescales > 10^4yr and < 10^6 yr
could reconcile these two facts.Comment: Submitted to ApJ, 21 pages, 17 postscript figures, emulateapj.st
Heavy-to-light transition form factors and their relations in light-cone QCD sum rules
The improved light-cone QCD sum rules by using chiral current correlator is
systematically reviewed and applied to the calculation of all the
heavy-to-light form factors, including all the semileptonic and penguin ones.
By choosing suitable chiral currents, the light-cone sum rules for all the form
factors are greatly simplified and depend mainly on one leading twist
distribution amplitude of the light meson. As a result, relations between these
form factors arise naturally. At the considered accuracy these relations
reproduce the results obtained in the literature. Moreover, since the explicit
dependence on the leading twist distribution amplitudes is preserved, these
relations may be more useful to simulate the experimental data and extract the
information on the distribution amplitude.Comment: 1+16 pages, no figure
Hyperon polarization in e^-p --> e^-HK with polarized electron beams
We apply the picture proposed in a recent Letter for transverse hyperon
polarization in unpolarized hadron-hadron collisions to the exclusive process
e^-p --> e^-HK such as e^-p-->e^-\Lambda K^+, e^-p --> e^-\Sigma^+ K^0, or
e^-p--> e^-\Sigma^0 K^+, or the similar process e^-p\to e^-n\pi^+ with
longitudinally polarized electron beams. We present the predictions for the
longitudinal polarizations of the hyperons or neutron in these reactions, which
can be used as further tests of the picture.Comment: 15 pages, 2 figures. submitted to Phys. Rev.
A method to find unstable periodic orbits for the diamagnetic Kepler Problem
A method to determine the admissibility of symbolic sequences and to find the
unstable periodic orbits corresponding to allowed symbolic sequences for the
diamagnetic Kepler problem is proposed by using the ordering of stable and
unstable manifolds. By investigating the unstable periodic orbits up to length
6, a one to one correspondence between the unstable periodic orbits and their
corresponding symbolic sequences is shown under the system symmetry
decomposition
Correlation effects in the ground state charge density of Mott-insulating NiO: a comparison of ab-initio calculations and high-energy electron diffraction measurements
Accurate high-energy electron diffraction measurements of structure factors
of NiO have been carried out to investigate how strong correlations in the Ni
3d shell affect electron charge density in the interior area of nickel ions and
whether the new ab-initio approaches to the electronic structure of strongly
correlated metal oxides are in accord with experimental observations. The
generalized gradient approximation (GGA) and the local spin density
approximation corrected by the Hubbard U term (LSDA+U) are found to provide the
closest match to experimental measurements. The comparison of calculated and
observed electron charge densities shows that correlations in the Ni 3d shell
suppress covalent bonding between the oxygen and nickel sublattices.Comment: 6 pages, LaTeX and 5 figures in the postscript forma
Self-energy Effects in the Superfluidity of Neutron Matter
The superfluidity of neutron matter in the channel is studied by
taking into account the effect of the ground-state correlations in the
self-energy. To this purpose the gap equation has been solved within the
generalized Gorkov approach. A sizeable suppression of the energy gap is driven
by the quasi-particle strength around the Fermi surface.Comment: 8 pages and 3 figure
Probing nuclear symmetry energy with the sub-threshold pion production
Within the framework of semiclassical Boltzmann-Uehling-Uhlenbeck (BUU)
transport model, we investigated the effects of symmetry energy on the
sub-threshold pion using the isospin MDI interaction with the stiff and soft
symmetry energies in the central collision of Ca + Ca at the
incident beam energies of 100, 150, 200, 250 and 300 MeV/nucleon, respectively.
We find that the ratio of of sub-threshold charged pion
production is greatly sensitive to the symmetry energy, particularly around 100
MeV/nucleon energies. Large sensitivity of sub-threshold charged pion
production to nuclear symmetry energy may reduce uncertainties of probing
nuclear symmetry energy via heavy-ion collision.Comment: 5 pages, 5 figures, typo corrections, submitted to Chinese Physics
Letter
Isospin splitting of the nucleon mean field
The isospin splitting of the nucleon mean field is derived from the Brueckner
theory extended to asymmetric nuclear matter. The Argonne V18 has been adopted
as bare interaction in combination with a microscopic three body force. The
isospin splitting of the effective mass is determined from the
Brueckner-Hartree-Fock self-energy: It is linear acording to the Lane ansatz
and such that for neutron-rich matter. The symmetry potential
is also determined and a comparison is made with the predictions of the
Dirac-Brueckner approach and the phenomenological interactions. The theoretical
predictions are also compared with the empirical parametrizations of neutron
and proton optical-model potentials based on the experimental nucleon-nucleus
scattering and the phenomenological ones adopted in transport-model simulations
of heavy-ion collisions. The direct contribution of the rearrangement term due
to three-body forces to the single particle potential and symmetry potential is
discussed.Comment: 8 pages, 10 figure
Transition Form Factors and Decay Rates with Extraction of the CKM parameters , ,
A systematic calculation for the transition form factors of heavy to light
mesons () is carried out
by using light-cone sum rules in the framework of heavy quark effective field
theory. The heavy quark symmetry at the leading order of expansion
enables us to reduce the independent wave functions and establish interesting
relations among form factors. Some relations hold for the whole region of
momentum transfer. The meson distribution amplitudes up to twist-4 including
the contributions from higher conformal spin partial waves and light meson mass
corrections are considered. The CKM matrix elements , and
are extracted from some relatively well-measured decay channels. A
detailed prediction for the branching ratios of heavy to light meson decays is
then presented. The resulting predictions for the semileptonic and radiative
decay rates of heavy to light mesons () are found to be compatible with the current experimental data
and can be tested by more precise experiments at B-factory, LHCb, BEPCII and
CLEOc.Comment: 23 pages, 32 figures, 25 tables,published version, minor corrections
and references adde
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