7,707 research outputs found
Quantization of scalar perturbations in brane-world inflation
We consider a quantization of scalar perturbations about a de Sitter brane in
a 5-dimensional anti-de Sitter (AdS) bulk spacetime. We first derive the second
order action for a master variable for 5-dimensional gravitational
perturbations. For a vacuum brane, there is a continuum of normalizable
Kaluza-Klein (KK) modes with . There is also a light radion mode with
which satisfies the junction conditions for two branes, but is
non-normalizable for a single brane model. We perform the quantization of these
bulk perturbations and calculate the effective energy density of the projected
Weyl tensor on the barne. If there is a test scalar field perturbation on the
brane, the mode together with the zero-mode and an infinite ladder
of discrete tachyonic modes become normalizable in a single brane model. This
infinite ladder of discrete modes as well as the continuum of KK modes with
introduce corrections to the scalar field perturbations at first-order
in a slow-roll expansion. We derive the second order action for the
Mukhanov-Sasaki variable coupled to the bulk perturbations which is needed to
perform the quantization and determine the amplitude of scalar perturbations
generated during inflation on the brane.Comment: 14 page
Resolution of puzzles from the LSND, KARMEN, and MiniBooNE experiments
This work has attempted to reconcile puzzling neutrino oscillation results
from the LSND, KARMEN and MiniBooNE experiments. We show that the LSND evidence
for oscillations, its long-standing
disagreement with the results from KARMEN, and the anomalous event excess
observed by MiniBooNE in and data could all be
explained by the existence of a heavy sterile neutrino (). All these
results are found to be consistent with each other assuming that the is
created in neutral-current interactions and decays radiatively into a
photon and a light neutrino. Assuming the is produced through mixing
with , the combined analysis of the LSND and MiniBooNe excess events
suggests that the mass is in the range from 40 to 80 MeV, the mixing
strength is , and the lifetime is
s. Surprisingly, this LSND-MiniBooNE parameters
window is found to be unconstrained by the results from the most sensitive
experiments searching for heavy neutrino. We set new limits on
for the LSND-MiniBooNE favorable mass region from the precision measurements of
the Michel spectrum by the TWIST experiment. The results obtained provide a
strong motivation for a sensitive search for the in a near future
decay or neutrino experiments, which fit well in the existing/planned
experimental programs at CERN or FNAL. The question of whether the heavy
neutrino is Dirac or Majorana particle is briefly discussed.Comment: 24 pages, 28 figures, version to appear in PR
Muon content of ultra-high-energy air showers: Yakutsk data versus simulations
We analyse a sample of 33 extensive air showers (EAS) with estimated primary
energies above 2\cdot 10^{19} eV and high-quality muon data recorded by the
Yakutsk EAS array. We compare, event-by-event, the observed muon density to
that expected from CORSIKA simulations for primary protons and iron, using
SIBYLL and EPOS hadronic interaction models. The study suggests the presence of
two distinct hadronic components, ``light'' and ``heavy''. Simulations with
EPOS are in a good agreement with the expected composition in which the light
component corresponds to protons and the heavy component to iron-like nuclei.
With SYBILL, simulated muon densities for iron primaries are a factor of \sim
1.5 less than those observed for the heavy component, for the same
electromagnetic signal. Assuming two-component proton-iron composition and the
EPOS model, the fraction of protons with energies E>10^{19} eV is
0.52^{+0.19}_{-0.20} at 95% confidence level.Comment: 8 pages, 3 figures; v2: replaced with journal versio
Exciton correlations in coupled quantum wells and their luminescence blue shift
In this paper we present a study of an exciton system where electrons and
holes are confined in double quantum well structures. The dominating
interaction between excitons in such systems is a dipole - dipole repulsion. We
show that the tail of this interaction leads to a strong correlation between
excitons and substantially affects the behavior of the system. Making use of
qualitative arguments and estimates we develop a picture of the exciton -
exciton correlations in the whole region of temperature and concentration where
excitons exist. It appears that at low concentration degeneracy of the excitons
is accompanied with strong multi-particle correlation so that the system cannot
be considered as a gas. At high concentration the repulsion suppresses the
quantum degeneracy down to temperatures that could be much lower than in a Bose
gas with contact interaction. We calculate the blue shift of the exciton
luminescence line which is a sensitive tool to observe the exciton - exciton
correlations.Comment: 27 pages in PDF and DVI format, 8 figure
The Lorentz Integral Transform (LIT) method and its applications to perturbation induced reactions
The LIT method has allowed ab initio calculations of electroweak cross
sections in light nuclear systems. This review presents a description of the
method from both a general and a more technical point of view, as well as a
summary of the results obtained by its application. The remarkable features of
the LIT approach, which make it particularly efficient in dealing with a
general reaction involving continuum states, are underlined. Emphasis is given
on the results obtained for electroweak cross sections of few--nucleon systems.
Their implications for the present understanding of microscopic nuclear
dynamics are discussed.Comment: 83 pages, 31 figures. Topical review. Corrected typo
Magnetic interactions and spin dynamics in the bond-disordered pyrochlore fluoride NaCaCoF
We report high-frequency/high-field electron spin resonance (ESR) and
high-field magnetization studies on single crystals of the bond-disordered
pyrochlore NaCaCoF. Frequency- and temperature-dependent ESR
investigations above the freezing temperature K reveal the
coexistence of two distinct magnetic phases. A cooperative paramagnetic phase,
evidenced by a gapless excitation mode, is found as well as a spin-glass phase
developing below 20 K which is associated with a gapped low-energy excitation.
Effective -factors close to 2 are obtained for both modes in line with
pulsed high-field magnetization measurements which show an unsaturated
isotropic behavior up to 58 T at 2 K. In order to describe the field-dependent
magnetization in high magnetic fields, we propose an empirical model accounting
for highly anisotropic ionic -tensors expected for this material and taking
into account the strongly competing interactions between the spins which lead
to a frustrated ground state. As a detailed quantitative relation between
effective -factors as determined from ESR and the local -tensors obtained
by neutron scattering [Ross et al., Phys. Rev. B 93, 014433 (2016)] is still
sought after, our work motivates further theoretical investigations of the
low-energy excitations in bond-disordered pyrochlores.Comment: 9 pages, 6 figure
Proposal to Search for Heavy Neutral Leptons at the SPS
A new fixed-target experiment at the CERN SPS accelerator is proposed that
will use decays of charm mesons to search for Heavy Neutral Leptons (HNLs),
which are right-handed partners of the Standard Model neutrinos. The existence
of such particles is strongly motivated by theory, as they can simultaneously
explain the baryon asymmetry of the Universe, account for the pattern of
neutrino masses and oscillations and provide a Dark Matter candidate.
Cosmological constraints on the properties of HNLs now indicate that the
majority of the interesting parameter space for such particles was beyond the
reach of the previous searches at the PS191, BEBC, CHARM, CCFR and NuTeV
experiments. For HNLs with mass below 2 GeV, the proposed experiment will
improve on the sensitivity of previous searches by four orders of magnitude and
will cover a major fraction of the parameter space favoured by theoretical
models.
The experiment requires a 400 GeV proton beam from the SPS with a total of
2x10^20 protons on target, achievable within five years of data taking. The
proposed detector will reconstruct exclusive HNL decays and measure the HNL
mass. The apparatus is based on existing technologies and consists of a target,
a hadron absorber, a muon shield, a decay volume and two magnetic
spectrometers, each of which has a 0.5 Tm magnet, a calorimeter and a muon
detector. The detector has a total length of about 100 m with a 5 m diameter.
The complete experimental set-up could be accommodated in CERN's North Area.
The discovery of a HNL would have a great impact on our understanding of
nature and open a new area for future research
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