561 research outputs found
Standard Model Neutrinos as Warm Dark Matter
Standard Model neutrinos are not usually considered plausible dark matter
candidates because the usual treatment of their decoupling in the early
universe implies that their mass must be sufficiently small to make them
``hot'' dark matter. In this paper we show that decoupling of Standard Model
neutrinos in low reheat models may result in neutrino densities very much less
than usually assumed, and thus their mass may be in the keV range. Standard
Model neutrinos may therefore be warm dark matter candidates.Comment: 5 pages, 5 figures, LaTeX file uses revtex packag
Exact Cross Sections for the Neutralino-Slepton Coannihilation
Coannihilation processes provide an important additional mechanism for
reducing the density of stable relics in the Universe. In the case of the
stable lightest neutralino of the MSSM, and in particular the Constrained MSSM
(CMSSM), the coannihilation with sleptons plays a major role in opening up
otherwise cosmologically excluded ranges of supersymmetric parameters. In this
paper, we derive a full set of exact, analytic expressions for the
coannihilation of the lightest neutralino with the sleptons into all two--body
tree--level final states in the framework of minimal supersymmetry. We make no
simplifying assumptions about the neutralino nor about sfermion masses and
mixings other than the absence of explicit CP--violating terms and
inter--family mixings. The expressions should be particularly useful in
computing the neutralino WIMP relic abundance without the approximation of
partial wave expansion. We illustrate the effect of our analytic results with
numerical examples and demonstrate a sizeable difference with approximate
expressions available in the literature.Comment: LaTeX, 46 pages, 8 eps figure
Dynamical Dark Energy model parameters with or without massive neutrinos
We use WMAP5 and other cosmological data to constrain model parameters in
quintessence cosmologies, focusing also on their shift when we allow for
non-vanishing neutrino masses. The Ratra-Peebles (RP) and SUGRA potentials are
used here, as examples of slowly or fastly varying state parameter w(a). Both
potentials depend on an energy scale \Lambda. Here we confirm the results of
previous analysis with WMAP3 data on the upper limits on \Lambda, which turn
out to be rather small (down to ~10^{-9} in RP cosmologies and ~10^{-5} for
SUGRA). Our constraints on \Lambda are not heavily affected by the inclusion of
neutrino mass as a free parameter. On the contrary, when the neutrino mass
degree of freedom is opened, significant shifts in the best-fit values of other
parameters occur.Comment: 9 pages, 3 figures, submitted to JCA
On the reheating stage after inflation
We point out that inflaton decay products acquire plasma masses during the
reheating phase following inflation. The plasma masses may render inflaton
decay kinematicaly forbidden, causing the temperature to remain frozen for a
period at a plateau value. We show that the final reheating temperature may be
uniquely determined by the inflaton mass, and may not depend on its coupling.
Our findings have important implications for the thermal production of
dangerous relics during reheating (e.g., gravitinos), for extracting bounds on
particle physics models of inflation from Cosmic Microwave Background
anisotropy data, for the production of massive dark matter candidates during
reheating, and for models of baryogenesis or leptogensis where massive
particles are produced during reheating.Comment: 8 pages, 2 figures. Submitted for publication in Phys. Rev.
Modern cosmologies from empty Kaluza-Klein solutions in 5D
We show that the empty five-dimensional solutions of
Davidson-Sonnenschtein-Vozmediano, {\em Phys. Rev.} {\bf D32} (1985)1330, in
the "old" Kaluza-Klein gravity, under appropriate interpretation can generate
an ample variety of cosmological models in 4D, which include the
higher-dimensional modifications to general relativity predicted by "modern"
versions of noncompactified 5D gravity as, e.g., induced-matter and braneworld
theories. This is the first time that these solutions are investigated in a
systematic way as embeddings for cosmological models in 4D. They provide a
different formulation, which is complementary to the approaches used in current
versions of 5D relativity.Comment: Accepted for publication in JHE
Dynamics of a string coupled to gravitational waves - Gravitational wave scattering by a Nambu-Goto straight string
We study the perturbative dynamics of an infinite gravitating Nambu-Goto
string within the general-relativistic perturbation framework. We develop the
gauge invariant metric perturbation on a spacetime containing a
self-gravitating straight string with a finite thickness and solve the
linearized Einstein equation. In the thin string case, we show that the string
does not emit gravitational waves by its free oscillation in the first order
with respect to its oscillation amplitude, nevertheless the string actually
bends when the incidental gravitational waves go through it.Comment: Published in Physical Review D. Some explanations are changed to
clarify our point
Bronchus-associated lymphoid tissue in kabuki syndrome with associated hyper-IgM syndrome/common variable immunodeficiency
A 28-year-old woman with a medical history significant for Kabuki syndrome with associated hyper-IgM syndrome/common variable immunodeficiency presented with a 3-month history of dyspnea, pleuritic pain, and nonproductive cough. Imaging demonstrated nodular infiltrates increasing toward the lung bases (Figure 1A). Bronchoscopy with lavage and transbronchial biopsies was performed to evaluate for an infectious or inflammatory etiology; however, results were unrevealing, and no infectious etiology was identified. Therefore, she underwent a video-assisted thoracic surgery biopsy that demonstrated nodular lymphoid hyperplasia with follicles centered on small airways and areas of organizing pneumonia (Figure 1B). Immunostains demonstrated follicles composed of CD31 T cells and CD201 B cells (Figure 1B) and no morphologic evidence of lymphoma or plasma cell neoplasm. Results were consistent with bronchus-associated lymphoid tissue (BALT). High-dose steroids were administered without resolution of symptoms or radiographic findings. A report of effective responses with rituximab and azathioprine in patients with common variable immunodeficiency who had a similar inflammatory lung process containing tertiary lymphoid structures, granulomas, and organizing pneumonia has been published (1). After four doses of rituximab (weekly), together with the initiation of azathioprine, her symptoms and radiographic findings improved (Figure 1C)
Reconciling inflation with openness
It is already understood that the increasing observational evidence for an
open Universe can be reconciled with inflation if our horizon is contained
inside one single huge bubble nucleated during the inflationary phase
transition. In this frame of ideas, we show here that the probability of living
in a bubble with the right (now the observations require ) can be comparable with unity, rather than infinitesimally small.
For this purpose we modify both quantitatively and qualitatively an intuitive
toy model based upon fourth order gravity. As this scheme can be implemented in
canonical General Relativity as well (although then the inflation driving
potential must be designed entirely ad hoc), inferring from the observations
that not only does not conflict with the inflationary paradigm,
but rather supports therein the occurrence of a primordial phase transition.Comment: 4 pages, one postscript figure, to be published on Physical Review D
PACS: 98.80. C
Maximising Social Interactions and Effectiveness within Distance Learning Courses: Cases from Construction
Advanced Internet technologies have revolutionised the delivery of distance learning education. As a result, the physical proximity between learners and the learning providers has become less important. However, whilst the pervasiveness of these technological developments has reached unprecedented levels, critics argue that the student learning experience is still not as effective as conventional face-to-face delivery. In this regard, surveys of distance learning courses reveal that there is often a lack of social interaction attributed to this method of delivery, which tends to leave learners feeling isolated due to a lack of engagement, direction, guidance and support by the tutor. This paper defines and conceptualises this phenomenon by investigating the extent to which distance-learning programmes provide the social interactions of an equivalent traditional classroom setting. In this respect, two distance learning case studies were investigated, covering the UK and Slovenian markets respectively. Research findings identified that delivery success is strongly dependent on the particular context to which the specific distance learning course is
designed, structured and augmented. It is therefore recommended that designers of distance learning courses should balance the tensions and nuances associated with commercial viability and pedagogic effectiveness
On the Spontaneous CP Breaking at Finite Temperature in a Nonminimal Supersymmetric Standard Model
We study the spontaneous CP breaking at finite temperature in the Higgs
sector in the Minimal Supersymmetric Standard Model with a gauge singlet. We
consider the contribution of the standard model particles and that of stops,
charginos, neutralinos, charged and neutral Higgs boson to the one-loop
effective potential. Plasma effects for all bosons are also included. Assuming
CP conservation at zero temperature, so that experimental constraints coming
from, {\it e.g.}, the electric dipole moment of the neutron are avoided, and
the electroweak phase transition to be of the first order and proceeding via
bubble nucleation, we show that spontaneous CP breaking cannot occur inside the
bubble mainly due to large effects coming from the Higgs sector. However,
spontaneous CP breaking can be present in the region of interest for the
generation of the baryon asymmetry, namely inside the bubble wall. The
important presence of very tiny explicit CP violating phases is also commented.Comment: 28 pages, 4 figures available upon request, DFPD 94/TH/38 and SISSA
94/81-A preprint
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