933 research outputs found
A minimal scale invariant axion solution to the strong CP-problem
We present a scale invariant extension of the Standard model allowing for the
Kim-Shifman-Vainstein-Zakharov (KSVZ) axion solution of the strong CP problem
in QCD. We add the minimal number of new particles and show that the
Peccei-Quinn scalar might be identified with the complex dilaton field. Scale
invariance, together with the Peccei-Quinn symmetry, is broken spontaneously
near the Planck scale before inflation, which is driven by the Standard Model
Higgs field. We present a set of general conditions which makes this scenario
viable and an explicit example of an effective theory possessing spontaneous
breaking of scale invariance. We show that this description works both for
inflation and low-energy physics in the electroweak vacuum. This scenario can
provide a self-consistent inflationary stage and, at the same time,
successfully avoid the cosmological bounds on the axion. Our general
predictions are the existence of coloured TeV mass fermion and the QCD axion.
The latter has all the properties of the KSVZ axion but does not contribute to
dark matter. This axion can be searched via it's mixing to a photon in an
external magnetic field
Scale-invariance as the origin of dark radiation?
Recent cosmological data favour R^2-inflation and some amount of non-standard
dark radiation in the Universe. We show that a framework of high energy scale
invariance can explain these data. The spontaneous breaking of this symmetry
provides gravity with the Planck mass and particle physics with the electroweak
scale. We found that the corresponding massless Nambu--Goldstone bosons --
dilatons -- are produced at reheating by the inflaton decay right at the amount
needed to explain primordial abundances of light chemical elements and
anisotropy of the cosmic microwave background. Then we extended the discussion
on the interplay with Higgs-inflation and on general class of inflationary
models where dilatons are allowed and may form the dark radiation. As a result
we put a lower limit on the reheating temperature in a general scale invariant
model of inflation.Comment: 15 pages, 4 figures; v2: replaced with revised version recently
publishe
On the dark radiation problem in the axiverse
String scenarios generically predict that we live in a so called axiverse:
the Universe with about a hundred of light axion species which are decoupled
from the Standard Model particles. However, the axions can couple to the
inflaton which leads to their production after inflation. Then, these axions
remain in the expanding Universe contributing to the dark radiation component,
which is severely bounded from present cosmological data. We place a general
constraint on the axion production rate and apply it to several variants of
reasonable inflaton-to-axion couplings. The limit merely constrains the number
of ultralight axions and the relative strength of inflaton-to-axion coupling.
It is valid in both large and small field inflationary models irrespectively of
the axion energy scales and masses. Thus, the limit is complementary to those
associated with the Universe overclosure and axion isocurvature fluctuations.
In particular, a hundred of axions is forbidden if inflaton universally couples
to all the fields at reheating. In the case of gravitational sector being
responsible for the reheating of the Universe (which is a natural option in all
inflationary models with modified gravity), the axion production can be
efficient. We find that in the Starobinsky -inflation even a single axion
(e.g. the standard QCD-axion) is in tension with the Planck data, making the
model inconsistent with the axiverse. The general conclusion is that an
inflation with inefficient reheating mechanism and low reheating temperature
may be in tension with the presence of light scalars
A high-order nonconservative approach for hyperbolic equations in fluid dynamics
It is well known, thanks to Lax-Wendroff theorem, that the local conservation
of a numerical scheme for a conservative hyperbolic system is a simple and
systematic way to guarantee that, if stable, a scheme will provide a sequence
of solutions that will converge to a weak solution of the continuous problem.
In [1], it is shown that a nonconservative scheme will not provide a good
solution. The question of using, nevertheless, a nonconservative formulation of
the system and getting the correct solution has been a long-standing debate. In
this paper, we show how get a relevant weak solution from a pressure-based
formulation of the Euler equations of fluid mechanics. This is useful when
dealing with nonlinear equations of state because it is easier to compute the
internal energy from the pressure than the opposite. This makes it possible to
get oscillation free solutions, contrarily to classical conservative methods.
An extension to multiphase flows is also discussed, as well as a
multidimensional extension
Random Sequential Adsorption of Objects of Decreasing Size
We consider the model of random sequential adsorption, with depositing
objects, as well as those already at the surface, decreasing in size according
to a specified time dependence, from a larger initial value to a finite value
in the large time limit. Numerical Monte Carlo simulations of two-dimensional
deposition of disks and one-dimensional deposition of segments are reported for
the density-density correlation function and gap-size distribution function,
respectively. Analytical considerations supplement numerical results in the
one-dimensional case. We investigate the correlation hole - the depletion of
correlation functions near contact and, for the present model, their vanishing
at contact - that opens up at finite times, as well as its closing and
reemergence of the logarithmic divergence of correlation properties at contact
in the large time limit.Comment: Submitted for publicatio
To Positivity and Beyond, where Higgs-Dilaton Inflation has never gone before
We study the consequences of (beyond) positivity of scattering amplitudes in
the effective field theory description of the Higgs-Dilaton inflationary model.
By requiring the EFT to be compatible with a unitary, causal, local and Lorentz
invariant UV completion, we derive constraints on the Wilson coefficients of
the first higher order derivative operators. We show that the values allowed by
the constraints are consistent with the phenomenological applications of the
Higgs-Dilaton model.Comment: 27 pages, 6 figures; matches the published versio
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