1,032 research outputs found
Monocyte:T cell interaction regulates human T cell activation through a CD28/CD46 crosstalk
International audienc
Cosmological constraints on unparticle dark matter
In unparticle dark matter (unmatter) models the equation of state of the
unmatter is given by , where is the scaling factor.
Unmatter with such equations of state would have a significant impact on the
expansion history of the universe. Using type Ia supernovae (SNIa), the baryon
acoustic oscillation (BAO) measurements and the shift parameter of the cosmic
microwave background (CMB) to place constraints on such unmatter models we find
that if only the SNIa data is used the constraints are weak. However, with the
BAO and CMB shift parameter data added strong constraints can be obtained. For
the UDM model, in which unmatter is the sole dark matter, we find that
at 95% C.L. For comparison, in most unparticle physics models it is
assumed . For the CUDM model, in which unmatter co-exists with
cold dark matter, we found that the unmatter can at most make up a few percent
of the total cosmic density if , thus it can not be the major component
of dark matter.Comment: Replaced with revised version. BAO data is added to make a tighter
constraint. Version accepted for publication on Euro.Phys.J.
The DICE calibration project: design, characterization, and first results
We describe the design, operation, and first results of a photometric
calibration project, called DICE (Direct Illumination Calibration Experiment),
aiming at achieving precise instrumental calibration of optical telescopes. The
heart of DICE is an illumination device composed of 24 narrow-spectrum,
high-intensity, light-emitting diodes (LED) chosen to cover the
ultraviolet-to-near-infrared spectral range. It implements a point-like source
placed at a finite distance from the telescope entrance pupil, yielding a flat
field illumination that covers the entire field of view of the imager. The
purpose of this system is to perform a lightweight routine monitoring of the
imager passbands with a precision better than 5 per-mil on the relative
passband normalisations and about 3{\AA} on the filter cutoff positions. The
light source is calibrated on a spectrophotometric bench. As our fundamental
metrology standard, we use a photodiode calibrated at NIST. The radiant
intensity of each beam is mapped, and spectra are measured for each LED. All
measurements are conducted at temperatures ranging from 0{\deg}C to 25{\deg}C
in order to study the temperature dependence of the system. The photometric and
spectroscopic measurements are combined into a model that predicts the spectral
intensity of the source as a function of temperature. We find that the
calibration beams are stable at the level -- after taking the slight
temperature dependence of the LED emission properties into account. We show
that the spectral intensity of the source can be characterised with a precision
of 3{\AA} in wavelength. In flux, we reach an accuracy of about 0.2-0.5%
depending on how we understand the off-diagonal terms of the error budget
affecting the calibration of the NIST photodiode. With a routine 60-mn
calibration program, the apparatus is able to constrain the passbands at the
targeted precision levels.Comment: 25 pages, 27 figures, accepted for publication in A&
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.
Sterile neutrinos: direct mixing effects versus induced mass matrix of active neutrinos
Mixing of active neutrinos with sterile ones generate ``induced''
contributions to the mass matrix of active neutrinos , where is the Majorana mass of the sterile neutrino
and is the active-sterile mixing angle. We study possible effects
of the induced matrix which can modify substantially the implications of
neutrino oscillation results. We have identified the regions of and
where the induced matrix (i) provides the dominant
structures, (ii) gives the sub-dominant effects and (iii) where its effects can
be neglected. The induced matrix can be responsible for peculiar properties of
the lepton mixing and neutrino mass spectrum, in particular, it can generate
the tri-bimaximal mixing. We update and discuss bounds on the induced masses
from laboratory measurements, astrophysics and cosmology. We find that
substantial impact of the induced matrix is possible if eV and
or MeV and
. The bounds can be relaxed in cosmological
scenarios with low reheating temperature, if sterile neutrinos decay
sufficiently fast, or their masses change with time.Comment: Figures updated, version to be published in Phys. Rev.
Evolution of shell structure in neutron-rich calcium isotopes
We employ interactions from chiral effective field theory and compute the
binding energies and low-lying excitations of calcium isotopes with the
coupled-cluster method. Effects of three-nucleon forces are included
phenomenologically as in-medium two-nucleon interactions, and the coupling to
the particle continuum is taken into account using a Berggren basis. The
computed ground-state energies and the low-lying 2+ states for the isotopes
42,48,50,52Ca are in good agreement with data, and we predict the excitation
energy of the first 2+ state in 54Ca at 1.9 MeV, displaying only a weak
sub-shell closure. In the odd-mass nuclei 53,55,61Ca we find that the positive
parity states deviate strongly from the naive shell model.Comment: 5 pages, 4 figures; small correction of effective 3NF and slight
change of the corresponding parameters; updated figures and tables; main
results and conclusions unchange
Cosmic Microwave Background Polarization
Cosmic microwave background (CMB) anisotropy is our richest source of
cosmological information; the standard cosmological model was largely
established thanks to study of the temperature anisotropies. By the end of the
decade, the Planck satellite will close this important chapter and move us
deeper into the new frontier of polarization measurements. Numerous
ground--based and balloon--borne experiments are already forging into this new
territory. Besides providing new and independent information on the primordial
density perturbations and cosmological parameters, polarization measurements
offer the potential to detect primordial gravity waves, constrain dark energy
and measure the neutrino mass scale. A vigorous experimental program is
underway worldwide and heading towards a new satellite mission dedicated to CMB
polarization.Comment: Review given at TAUP 2005; References added; Additional reference
Dark energy constraints and correlations with systematics from CFHTLS weak lensing, SNLS supernovae Ia and WMAP5
We combine measurements of weak gravitational lensing from the CFHTLS-Wide
survey, supernovae Ia from CFHT SNLS and CMB anisotropies from WMAP5 to obtain
joint constraints on cosmological parameters, in particular, the dark energy
equation of state parameter w. We assess the influence of systematics in the
data on the results and look for possible correlations with cosmological
parameters.
We implement an MCMC algorithm to sample the parameter space of a flat CDM
model with a dark-energy component of constant w. Systematics in the data are
parametrised and included in the analysis. We determine the influence of
photometric calibration of SNIa data on cosmological results by calculating the
response of the distance modulus to photometric zero-point variations. The weak
lensing data set is tested for anomalous field-to-field variations and a
systematic shape measurement bias for high-z galaxies.
Ignoring photometric uncertainties for SNLS biases cosmological parameters by
at most 20% of the statistical errors, using supernovae only; the parameter
uncertainties are underestimated by 10%. The weak lensing field-to-field
variance pointings is 5%-15% higher than that predicted from N-body
simulations. We find no bias of the lensing signal at high redshift, within the
framework of a simple model. Assuming a systematic underestimation of the
lensing signal at high redshift, the normalisation sigma_8 increases by up to
8%. Combining all three probes we obtain -0.10<1+w<0.06 at 68% confidence
(-0.18<1+w<0.12 at 95%), including systematic errors. Systematics in the data
increase the error bars by up to 35%; the best-fit values change by less than
0.15sigma. [Abridged]Comment: 14 pages, 10 figures. Revised version, matches the one to be
published in A&A. Modifications have been made corresponding to the referee's
suggestions, including reordering of some section
Constraints on Cosmological Models and Reconstructing the Acceleration History of the Universe with Gamma-Ray Burst Distance Indicators
Gamma-ray bursts (GRBs) have been regarded as standard candles at very high
redshift for cosmology research. We have proposed a new method to calibrate GRB
distance indicators with Type Ia supernova (SNe Ia) data in a completely
cosmology-independent way to avoid the circularity problem that had limited the
direct use of GRBs to probe cosmology [N. Liang, W. K. Xiao, Y. Liu, and S. N.
Zhang, Astrophys. J. 685, 354 (2008).]. In this paper, a simple method is
provided to combine GRB data into the joint observational data analysis to
constrain cosmological models; in this method those SNe Ia data points used for
calibrating the GRB data are not used to avoid any correlation between them. We
find that the CDM model is consistent with the joint data in the
1- confidence region, using the GRB data at high redshift calibrated
with the interpolating method, the Constitution set of SNe Ia, the cosmic
microwave background radiation from Wilkinson Microwave Anisotropy Probe five
year observation, the baryonic acoustic oscillation from the spectroscopic
Sloan Digital Sky Survey Data Release 7 galaxy sample, the x-ray baryon mass
fraction in clusters of galaxies, and the observational Hubble parameter versus
redshift data. Comparing to the joint constraints with GRBs and without GRBs,
we find that the contribution of GRBs to the joint cosmological constraints is
a slight shift in the confidence regions of cosmological parameters to better
enclose the CDM model. Finally, we reconstruct the acceleration
history of the Universe up to with the distance moduli of SNe Ia and GRBs
and find some features that deviate from the CDM model and seem to
favor oscillatory cosmology models; however further investigations are needed
to better understand the situation.Comment: 14 pages, 9 figures, 2 tables; v3: the revised version, fig. 6 and
some discussions added, accepted for for publication in Phys. Rev. D; v4: the
published version (Phys. Rev. D 81, 083518, 2010
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