6,637 research outputs found
Hypernuclei and in-medium chiral dynamics
A recently introduced relativistic nuclear energy density functional,
constrained by features of low-energy QCD, is extended to describe the
structure of hypernuclei. The density-dependent mean field and the spin-orbit
potential of a -hyperon in a nucleus, are consistently calculated
using the SU(3) extension of in-medium chiral perturbation theory. The leading
long-range interaction arises from kaon-exchange and
-exchange with a -hyperon in the intermediate state. Scalar and
vector mean fields, originating from in-medium changes of the quark
condensates, produce a sizeable {\em short-range} spin-orbit interaction. The
model, when applied to oxygen as a test case, provides a natural explanation
for the smallness of the effective spin-orbit potential: an almost
complete cancellation between the background contributions (scalar and vector)
and the long-range terms generated by two-pion exchange.Comment: 8 pages and 6 figures. To appear in the proceedings of the
Theoretical nuclear physics school "Exotic Nuclei: New Challenges", Les
Houches (France) May 7-18, 200
On the Spectrum of Fluctuations in an Effective Field Theory of the Ekpyrotic Universe
We consider the four-dimensional effective field theory which has been used
in previous studies of perturbations in the Ekpyrotic Universe, and discuss the
spectrum of cosmological fluctuations induced on large scales by quantum
fluctuations of the bulk brane. By matching cosmological fluctuations on a
constant energy density hypersurface we show that the growing mode during the
very slow collapsing pre-impact phase couples only to the decaying mode in the
expanding post-impact phase, and that hence no scale-invariant spectrum of
adiabatic fluctuations is generated. Note that our conclusions may not apply to
improved toy models for the Ekpyrotic scenario.Comment: 8 pages, few sentences added. Conclusions unchanged. Added
references. Missing name added to Ref. 5
CMB Constraints on a Stochastic Background of Primordial Magnetic Fields
We constrain a stochastic background of primordial magnetic field (PMF) by
its contribution to the angular power spectrum of cosmic microwave background
anisotropies. We parametrize such stochastic background by a power-law spectrum
with index and by its Gaussian smoothed amplitude on a
comoving length . We give an approximation for the spectra of the
relevant correlators of the energy-momentum of the stochastic background of PMF
for any . By using the WMAP 7 year data in combination with ACBAR, BICEP
and QUAD we obtain the constraint nG at 95% confidence
level for a stochastic background of non-helical PMF. We discuss the relative
importance of the scalar and vector contribution to CMB anisotropies in
obtaining these constraints. We then forecast {\sc Planck} capabilities in
constraining .Comment: 13 pages, 7 figures, 3 tables. Revised version accepted for
publication in Phys. Rev. D, conclusions unchange
On the dipole straylight contamination in spinning space missions dedicated to CMB anisotropy
We present an analysis of the dipole straylight contamination (DSC) for
spinning space-missions designed to measure CMB anisotropies. Although this
work is mainly devoted to the {\sc Planck} project, it is relatively general
and allows to focus on the most relevant DSC implications. We first study a
simple analytical model for the DSC in which the pointing direction of the main
spillover can be assumed parallel or not to the spacecraft spin axis direction
and compute the time ordered data and map. The map is then analysed paying
particular attention to the DSC of the low multipole coefficients of the map.
Through dedicated numerical simulations we verify the analytical results and
extend the analysis to higher multipoles and to more complex (and realistic)
cases by relaxing some of the simple assumptions adopted in the analytical
approach. We find that the systematic effect averages out in an even number of
surveys, except for a contamination of the dipole itself that survives when
spin axis and spillover directions are not parallel and for a contamination of
the other multipoles in the case of complex scanning strategies. In particular,
the observed quadrupole can be affected by the DSC in an odd number of surveys
or in the presence of survey uncompleteness or over-completeness. Various
aspects relevant in CMB space projects (such as implications for calibration,
impact on polarization measurements, accuracy requirement in the far beam
knowledge for data analysis applications, scanning strategy dependence) are
discussed.Comment: 21 pages, 13 Figures, 1 Table. To appear in MNRAS. Accepted 2006 July
13. Received 2006 July 13; in original form 2006 June 7. This work has been
done in the framework of the Planck LFI activitie
Primordial density perturbations with running spectral index: impact on non-linear cosmic structures
(abridged) We explore the statistical properties of non-linear cosmic
structures in a flat CDM cosmology in which the index of the
primordial power spectrum for scalar perturbations is allowed to depend on the
scale. Within the inflationary paradigm, the running of the scalar spectral
index can be related to the properties of the inflaton potential, and it is
hence of critical importance to test it with all kinds of observations, which
cover the linear and non-linear regime of gravitational instability. We focus
on the amount of running allowed by an updated
combination of CMB anisotropy data and the 2dF Galaxy Redshift Survey. Our
analysis constrains
at 95% Confidence Level when (not) taking into
account primordial gravitational waves in a ratio as predicted by canonical
single field inflation, in agreement with other works. For the cosmological
models best fitting the data both with and without running we studied the
abundance of galaxy clusters and of rare objects, the halo bias, the
concentration of dark matter halos, the Baryon Acoustic Oscillation, the power
spectrum of cosmic shear, and the Integrated Sachs-Wolfe effect. We find that
counting galaxy clusters in future X-ray and Sunyaev-Zel'dovich surveys could
discriminate between the two models, more so if broad redshift information
about the cluster samples will be available. Likewise, measurements of the
power spectrum of cosmological weak lensing as performed by planned all-sky
optical surveys such as EUCLID could detect a running of the primordial
spectral index, provided the uncertainties about the source redshift
distribution and the underlying matter power spectrum are well under control.Comment: 17 pages, 14 figures, 4 tables. Accepted for publication on MNRA
- …