501 research outputs found
Towards elimination of Hepatitis C in Vietnam
Vietnam has a high burden of viral hepatitis. This thesis strives to advance its elimination, addressing important gaps in the literature that I hope will contribute to treatment guidelines and health policy, both in Vietnam and internationally.
Firstly, to define the hepatitis epidemic in Vietnam, I assimilate all published seroprevalence data since 1990 to estimate pooled prevalence of HBV, HCV and HDV in high and low risk populations. I show that although blood safety has improved, and HDV is largely confined to high-risk populations, a renewed focus on birth dose HBV vaccination and targeted HCV screening and treatment of people who inject drugs, is urgently required to meet elimination targets.
The next chapters address HCV therapy, namely predictive factors for selecting individuals who could be treated for shorter duration, treatment failure in relation to rare HCV subtypes, and the clinical importance of resistance mutations. I describe a prospective clinical trial evaluating the efficacy of shortened sofosbuvir and daclatasvir therapy, based on early virological response: firstly, in genotype 1 or 6-infected individuals with mild disease (chapter 3) and then in genotype 6-infected individuals with advanced liver fibrosis (chapter 4). I show that shortened therapy, with retreatment if needed, can reduce antiviral use while maintaining high cure rates, but that day 2 virologic response alone is not an adequate predictor of cure. I demonstrate that a high frequency of putative NS5A inhibitor resistance mutations in genotype 6 infection does not impact cure rates, negating the need for costly genotyping in Vietnam.
In my final data chapter, I explore an innovative means of decentralising HCV care. In two independent study populations from Vietnam and the UK, I show that an increase in routinely taken alanine transaminase after HCV therapy is a reliable screen for treatment failure that could substantially reduce reliance on nucleic acid testing in remote and resource-limited settings.Open Acces
Observable non-gaussianity from gauge field production in slow roll inflation, and a challenging connection with magnetogenesis
In any realistic particle physics model of inflation, the inflaton can be
expected to couple to other fields. We consider a model with a dilaton-like
coupling between a U(1) gauge field and a scalar inflaton. We show that this
coupling can result in observable non-gaussianity, even in the conventional
regime where inflation is supported by a single scalar slowly rolling on a
smooth potential: the time dependent inflaton condensate leads to amplification
of the large-scale gauge field fluctuations, which can feed-back into the
scalar/tensor cosmological perturbations. In the squeezed limit, the resulting
bispectrum is close to the local one, but it shows a sizable and characteristic
quadrupolar dependence on the angle between the shorter and the larger modes in
the correlation. Observable non-gaussianity is obtained in a regime where
perturbation theory is under control. If the gauge field is identified with the
electromagnetic field, the model that we study is a realization of the
magnetogenesis idea originally proposed by Ratra, and widely studied. This
identification (which is not necessary for the non-gaussianity production) is
however problematic in light of a strong coupling problem already noted in the
literature.Comment: 28 pages, no figures. Final versio
Phenomenology of a Pseudo-Scalar Inflaton: Naturally Large Nongaussianity
Many controlled realizations of chaotic inflation employ pseudo-scalar
axions. Pseudo-scalars \phi are naturally coupled to gauge fields through c
\phi F \tilde{F}. In the presence of this coupling, gauge field quanta are
copiously produced by the rolling inflaton. The produced gauge quanta, in turn,
source inflaton fluctuations via inverse decay. These new cosmological
perturbations add incoherently with the "vacuum" perturbations, and are highly
nongaussian. This provides a natural mechanism to generate large nongaussianity
in single or multi field slow-roll inflation. The resulting phenomenological
signatures are highly distinctive: large nongaussianity of (nearly) equilateral
shape, in addition to detectably large values of both the scalar spectral tilt
and tensor-to-scalar ratio (both being typical of large field inflation). The
WMAP bound on nongaussianity implies that the coupling, c, of the pseudo-scalar
inflaton to any gauge field must be smaller than about 10^{2} M_p^{-1}.Comment: 45 pages, 7 figure
Inhomogeneous tachyon condensation
We investigate the spacetime-dependent condensation of the tachyon in
effective field theories. Previous work identified singularities in the field
which appear in finite time: infinite gradients at the kinks, and (in the
eikonal approximation) caustics near local minima. By performing a perturbation
analysis, and with numerical simulations, we demonstrate and explain key
features of the condensation process: perturbations generically freeze, and
minima develop singular second derivatives in finite time (caustics). This last
has previously been understood in terms of the eikonal approximation to the
dynamics. We show explicitly from the field equations how this approximation
emerges, and how the caustics develop, both in the DBI and BSFT effective
actions. We also investigate the equation of state parameter of tachyon matter
showing that it is small, but generically non-zero. The energy density tends to
infinity near field minima with a charateristic profile. A proposal to regulate
infinities by modifying the effective action is also studied. We find that
although the infinities at the kinks are successfully regularised in the
time-dependent case, caustics still present.Comment: 4 figures,19p
Preheating After Modular Inflation
We study (p)reheating in modular (closed string) inflationary scenarios, with
a special emphasis on Kahler moduli/Roulette models. It is usually assumed that
reheating in such models occurs through perturbative decays. However, we find
that there are very strong non-perturbative preheating decay channels related
to the particular shape of the inflaton potential (which is highly nonlinear
and has a very steep minimum). Preheating after modular inflation, proceeding
through a combination of tachyonic instability and broad-band parametric
resonance, is perhaps the most violent example of preheating after inflation
known in the literature. Further, we consider the subsequent transfer of energy
to the standard model sector in scenarios where the standard model particles
are confined to a D7-brane wrapping the inflationary blow-up cycle of the
compactification manifold or, more interestingly, a non-inflationary blow up
cycle. We explicitly identify the decay channels of the inflaton in these two
scenarios. We also consider the case where the inflationary cycle shrinks to
the string scale at the end of inflation; here a field theoretical treatment of
reheating is insufficient and one must turn instead to a stringy description.
We estimate the decay rate of the inflaton and the reheat temperature for
various scenarios.Comment: 34 pages, 10 figures. Accepted for publication in JCA
Cosmological Fluctuations from Infra-Red Cascading During Inflation
We propose a qualitatively new mechanism for generating cosmological
fluctuations from inflation. The non-equilibrium excitation of interacting
scalar fields often evolves into infra-red (IR) and ultra-violet (UV)
cascading, resulting in an intermediate scaling regime. We observe elements of
this phenomenon in a simple model with inflaton \phi and iso-inflaton \chi
fields interacting during inflation via the coupling g^2 (\phi-\phi_0)^2
\chi^2. Iso-inflaton particles are created during inflation when they become
instantaneously massless at \phi=\phi_0, with occupation numbers not exceeding
unity. We point out that very quickly the produced \chi particles become heavy
and their multiple re-scatterings off the homogeneous condensate \phi(t)
generates bremschtrahlung radiation of light inflaton IR fluctuations with high
occupation numbers. The subsequent evolution of these IR fluctuations is
qualitatively similar to that of the usual inflationary fluctuations, but their
initial amplitude is different. The IR cascading generates a bump-shaped
contribution to the cosmological curvature fluctuations, which can even
dominate over the usual fluctuations for g^2>0.06. The IR cascading curvature
fluctuations are significantly non-gaussian and the strength and location of
the bump are model-dependent, through g^2 and \phi_0. The effect from IR
cascading fluctuations is significantly larger than that from the momentary
slowing-down of \phi(t). With a sequence of such bursts of particle production,
the superposition of the bumps can lead to a new broad band non-gaussian
component of cosmological fluctuations added to the usual fluctuations. Such a
sequence of particle creation events can, but need not, lead to trapped
inflation.Comment: 13 pages, 9 figures. Accepted for publication in Phys. Rev.
Towards a Resolution of the Cosmological Singularity in Non-local Higher Derivative Theories of Gravity
One of the greatest problems of standard cosmology is the Big Bang
singularity. Previously it has been shown that non-local ghostfree
higher-derivative modifications of Einstein gravity in the ultra-violet regime
can admit non-singular bouncing solutions. In this paper we study in more
details the dynamical properties of the equations of motion for these theories
of gravity in presence of positive and negative cosmological constants and
radiation. We find stable inflationary attractor solutions in the presence of a
positive cosmological constant which renders inflation {\it geodesically
complete}, while in the presence of a negative cosmological constant a cyclic
universe emerges. We also provide an algorithm for tracking the super-Hubble
perturbations during the bounce and show that the bouncing solutions are free
from any perturbative instability.Comment: 38 pages, 6 figures. V2: Added: a word to the title, clarifications,
an appendix, many references. To appear in JCA
FRW Cosmology with Non-positively Defined Higgs Potentials
We discuss the classical aspects of dynamics of scalar models with
non-positive Higgs potentials in the FRW cosmology. These models appear as
effective local models in non-local models related with string field theories.
After a suitable field redefinition these models have the form of local Higgs
models with a negative extra cosmological term and the total Higgs potential is
non-positively defined and has rather small coupling constant. The
non-positivity of the potential leads to the fact that on some stage of
evolution the expansion mode gives place to the mode of contraction, due to
that the stage of reheating is absent. In these models the hard regime of
inflation gives place to inflation near the hill top and the area of the slow
roll inflation is very small. Meanwhile one can obtain enough e-foldings before
the contraction to make the model under consideration admissible to describe
inflation.Comment: 40 pages, 20 figures, typos correcte
Localization of the SFT inspired Nonlocal Linear Models and Exact Solutions
A general class of gravitational models driven by a nonlocal scalar field
with a linear or quadratic potential is considered. We study the action with an
arbitrary analytic function , which has both simple and double roots.
The way of localization of nonlocal Einstein equations is generalized on models
with linear potentials. Exact solutions in the Friedmann-Robertson-Walker and
Bianchi I metrics are presented.Comment: 20 pages, 3 figures, published in the proceedings of the VIII
International Workshop "Supersymmetries and Quantum Symmetries" (SQS'09),
Dubna, Russia, July 29 - August 3, 2009, http://theor.jinr.ru/~sqs09
- …