1,658 research outputs found
Experimental study of quasi-elastic neutrino interactions on Ar with a liquid Ar TPC exposed to the WANF neutrino beam
We present results from the first exposure of a liquid Ar time projection
chamber to a neutrino beam. The data have been collected in 1997 with a 50
liter ICARUS-like chamber located between the CHORUS and NOMAD experiment at
the CERN West Area Neutrino Facility. We focus on the analysis of quasi-elastic
interactions; despite the limited size of the detector, nuclear effects beyond
Fermi motion and Pauli blocking have been observed as perturbations to the pure
quasi-elastic kinematics.Comment: 6 pages, 4 figures, prepared for the proceedings of NuInt0
Evidence for self-interaction of charge distribution in charge-coupled devices
Charge-coupled devices (CCDs) are widely used in astronomy to carry out a
variety of measurements, such as for flux or shape of astrophysical objects.
The data reduction procedures almost always assume that ther esponse of a given
pixel to illumination is independent of the content of the neighboring pixels.
We show evidence that this simple picture is not exact for several CCD sensors.
Namely, we provide evidence that localized distributions of charges (resulting
from star illumination or laboratory luminous spots) tend to broaden linearly
with increasing brightness by up to a few percent over the whole dynamic range.
We propose a physical explanation for this "brighter-fatter" effect, which
implies that flatfields do not exactly follow Poisson statistics: the variance
of flatfields grows less rapidly than their average, and neighboring pixels
show covariances, which increase similarly to the square of the flatfield
average. These covariances decay rapidly with pixel separation. We observe the
expected departure from Poisson statistics of flatfields on CCD devices and
show that the observed effects are compatible with Coulomb forces induced by
stored charges that deflect forthcoming charges. We extract the strength of the
deflections from the correlations of flatfield images and derive the evolution
of star shapes with increasing flux. We show for three types of sensors that
within statistical uncertainties,our proposed method properly bridges
statistical properties of flatfields and the brighter-fatter effect
High-spin states in the five-valence-particle nucleus 213Po
Excited states in 213Po have been populated using the 18O + 208Pb reaction at 85 MeV beam energy and studied with the Euroball IV -multidetector array. The level scheme has been built up to 2.0 MeV excitation energy and spin I 25/2 ~ from the triple -coincidence data. Spin and parity values of several yrast states have been assigned from the -angular properties. The configurations of the yrast states are discussed using results of empirical shell-model calculations and by analogy with the neighbouring nuclei. The spin and parity values of several low-spin states of 213Po previously identified from the -decay of 213Bi are revised
First observation of high-spin states in Po: Probing the valence space beyond ^{208}Pb
Excited states in Po-214 have been populated using the O-18 + Pb-208 reaction at 85-MeV beam energy and studied with the Euroball IV gamma-multidetector array. The level scheme has been built up to similar to 2.7-MeV excitation energy and spin I = 12 (h) over bar from the triple-gamma coincidence data. Spin and parity values of most of the observed states have been assigned from the gamma-angular properties. The configurations of the yrast states are discussed using results of empirical shell-model calculations and by analogy with the neighboring nuclei. The Po-214 level scheme established in this work constitutes an important step for the determination of the effective nucleon-nucleon interactions beyond N = 126
Constraining the CDM and Galileon models with recent cosmological data
The Galileon theory belongs to the class of modified gravity models that can
explain the late-time accelerated expansion of the Universe. In previous works,
cosmological constraints on the Galileon model were derived, both in the
uncoupled case and with a disformal coupling of the Galileon field to matter.
There, we showed that these models agree with the most recent cosmological
data. In this work, we used updated cosmological data sets to derive new
constraints on Galileon models, including the case of a constant conformal
Galileon coupling to matter. We also explored the tracker solution of the
uncoupled Galileon model. After updating our data sets, especially with the
latest \textit{Planck} data and BAO measurements, we fitted the cosmological
parameters of the CDM and Galileon models. The same analysis framework
as in our previous papers was used to derive cosmological constraints, using
precise measurements of cosmological distances and of the cosmic structure
growth rate. We showed that all tested Galileon models are as compatible with
cosmological data as the CDM model. This means that present
cosmological data are not accurate enough to distinguish clearly between both
theories. Among the different Galileon models, we found that a conformal
coupling is not favoured, contrary to the disformal coupling which is preferred
at the level over the uncoupled case. The tracker solution of the
uncoupled Galileon model is also highly disfavoured due to large tensions with
supernovae and \textit{Planck}+BAO data. However, outside of the tracker
solution, the general uncoupled Galileon model, as well as the general
disformally coupled Galileon model, remain the most promising Galileon
scenarios to confront with future cosmological data. Finally, we also discuss
constraints coming from Lunar Laser Ranging experiment and gravitational wave
speed of propagation.Comment: 22 pages, 17 figures, published version in A&
Experimental constraints on the uncoupled Galileon model from SNLS3 data and other cosmological probes
The Galileon model is a modified gravity theory that may provide an
explanation for the accelerated expansion of the Universe. This model does not
suffer from instabilities or ghost problems (normally associated with
higher-order derivative theories), restores local General Relativity -- thanks
to the Vainshtein screening effect -- and predicts late time acceleration of
the expansion. In this paper, we derive a new definition of the Galileon
parameters that allows us to avoid having to choose initial conditions for the
Galileon field, and then test this model against precise measurements of the
cosmological distances and the rate of growth of cosmic structures. We observe
a small tension between the constraints set by growth data and those from
distances. However, we find that the Galileon model remains consistent with
current observations and is still competitive with the \Lambda CDM model,
contrary to what was concluded in recent publications.Comment: 19 pages, 15 figures, accepted to Astronomy and Astrophysic
First experimental constraints on the disformally coupled Galileon model
The Galileon model is a modified gravity model that can explain the late-time
accelerated expansion of the Universe. In a previous work, we derived
experimental constraints on the Galileon model with no explicit coupling to
matter and showed that this model agrees with the most recent cosmological
data. In the context of braneworld constructions or massive gravity, the
Galileon model exhibits a disformal coupling to matter, which we study in this
paper. After comparing our constraints on the uncoupled model with recent
studies, we extend the analysis framework to the disformally coupled Galileon
model and derive the first experimental constraints on that coupling, using
precise measurements of cosmological distances and the growth rate of cosmic
structures. In the uncoupled case, with updated data, we still observe a low
tension between the constraints set by growth data and those from distances. In
the disformally coupled Galileon model, we obtain better agreement with data
and favour a non-zero disformal coupling to matter at the level.
This gives an interesting hint of the possible braneworld origin of Galileon
theory.Comment: 9 pages, 6 figures, updated versio
Supernovae and the Nature of the Dark Energy
The use of Type Ia supernovae as calibrated standard candles is one of the
most powerful tools to study the expansion history of the universe and thereby
its energy components. While the analysis of some ~50 supernovae at redshifts
around z~0.5 have provided strong evidence for an energy component with
negative pressure, ``dark energy'', more data is needed to enable an accurate
estimate of the amount and nature of this energy. This might be accomplished by
a dedicated space telescope, the SuperNova / Acceleration Probe (2000; SNAP),
which aims at collecting a large number of supernovae with z<2.
In this paper we assess the ability of the SNAP mission to determine various
properties of the ``dark energy.'' To exemplify, we expect SNAP, if operated
for three years to study Type Ia supernovae, to be able to determine the
parameters in a linear equation of state w(z)=w0 + w1 z to within a statistical
uncertainty of +-0.04 for w0 and +0.15,-0.17 for w1 assuming that the universe
is known to be flat and an independent high precision (sigma_{Omega_m}=0.015)
measurement of the mass density Omega_m, is used to constrain the fit. An
additional improvement can be obtained if a large number of low-z, as well as
high-z, supernovae are included in the sample.Comment: 13 pages, submitted to A&
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