1,215 research outputs found
Initial Conditions for Critical Higgs Inflation
It has been pointed out that a large non-minimal coupling between the
Higgs and the Ricci scalar can source higher derivative operators, which may
change the predictions of Higgs inflation. A variant, called critical Higgs
inflation, employs the near-criticality of the top mass to introduce an
inflection point in the potential and lower drastically the value of . We
here study whether critical Higgs inflation can occur even if the
pre-inflationary initial conditions do not satisfy the slow-roll behaviour
(retaining translation and rotation symmetries). A positive answer is found:
inflation turns out to be an attractor and therefore no fine-tuning of the
initial conditions is necessary. A very large initial Higgs time-derivative (as
compared to the potential energy density) is compensated by a moderate increase
in the initial field value. These conclusions are reached by solving the exact
Higgs equation without using the slow-roll approximation. This also allows us
to treat consistently the inflection point, where the standard slow-roll
approximation breaks down. Here we make use of an approach that is independent
of the UV completion of gravity, by taking initial conditions that always
involve sub-planckian energies.Comment: 8 pages, 5 figures; v2: comments and references added, version
accepted for publication in Physics Letters
A Simple Motivated Completion of the Standard Model below the Planck Scale: Axions and Right-Handed Neutrinos
We study a simple Standard Model (SM) extension, which includes three
families of right-handed neutrinos with generic non-trivial flavor structure
and an economic implementation of the invisible axion idea. We find that in
some regions of the parameter space this model accounts for all experimentally
confirmed pieces of evidence for physics beyond the SM: it explains neutrino
masses (via the type-I see-saw mechanism), dark matter, baryon asymmetry
(through leptogenesis), solve the strong CP problem and has a stable
electroweak vacuum. The last property may allow us to identify the Higgs field
with the inflaton.Comment: 8 pages, 4 figures. Phys. Lett. B version: references and discussion
on light right-handed neutrinos adde
Critical Higgs inflation in a Viable Motivated Model
An extension of the Standard Model with three right-handed neutrinos and a
simple invisible axion model can account for all experimentally confirmed
signals of new physics (neutrino oscillations, dark matter and baryon
asymmetry) in addition to solving the strong CP problem, stabilizing the
electroweak vacuum and satisfying all current observational bounds. We show
that this model can also implement critical Higgs inflation, which corresponds
to the frontier between stability and metastability of the electroweak vacuum.
This leads to a value of the non-minimal coupling between the Higgs and the
Ricci scalar that is much lower than the one usually quoted in Higgs inflation
away from criticality. Then, an advantage is that the scale of perturbative
unitarity breaking on flat spacetime can be very close to the Planck mass,
where anyhow new physics is required. The higher dimensional operators are
under control in this inflationary setup. The dependence of the cutoff on the
Higgs background is also taken into account as appropriate when the Higgs is
identified with the inflaton. Furthermore, critical Higgs inflation enjoys a
robust inflationary attractor that makes it an appealing setup for the early
universe. In the proposed model, unlike in the Standard Model, critical Higgs
inflation can be realized without any tension with the observed quantities,
such as the top mass and the strong coupling.Comment: 13 pages, 6 figures; v2: explanations and references added; v3: title
changed and more explanations added (version accepted by PRD
Transitions in Dilaton Holography with Global or Local Symmetries
We study various transitions in dilaton holography, including those
associated with the spontaneous breaking of a global (superfluid case) or local
(superconductor case) U(1) symmetry in diverse dimensions d. By analyzing the
thermodynamics of the dilaton-gravity system we find that scale invariance is
broken at low temperatures, as shown by a nontrivial hyperscaling violation
exponent in the infrared; increasing the temperature we recover scale symmetry
in a d dependent way: while for d=2+1 a phase transition is found, for d=3+1
the transition is rather a crossover. This is the expected behavior of QCD
where the number of colors N_c equals three (although in our holographic
calculations N_c goes to infinity). When the U(1) is preserved and at low
temperatures, the system is insulating for arbitrary d if the dilaton is
appropriately coupled to the gauge field; for other couplings we also find a
linear in temperature resistivity. We then determine the prediction of these
models for several quantities in the superconducting phase: the DC and AC
conductivity, the gap for charged excitations, the superfluid density, the
vortex profiles, the coherence length, the penetration depth and the critical
magnetic fields. We show that at low temperatures some of these quantities
differ qualitatively compared with the corresponding models without the
dilaton, although the superconductor is robustly of Type II. The ratio of the
gap over the critical temperature of the superconductor is studied in detail
varying d and the couplings of the dilaton and then compared with the BCS
value. A holographic renormalization is required in d>2+1 to compute some
quantities (such as the AC conductivity and the penetration depth) and we
explain in detail how to perform it.Comment: 21 pages + references, 27 figures; v2: few comments and references
added, matches the article published in JHE
Bulk Renormalization and Particle Spectrum in Codimension-Two Brane Worlds
We study the Casimir energy due to bulk loops of matter fields in
codimension-two brane worlds and discuss how effective field theory methods
allow us to use this result to renormalize the bulk and brane operators. In the
calculation we explicitly sum over the Kaluza-Klein (KK) states with a new
convenient method, which is based on a combined use of zeta function and
dimensional regularization. Among the general class of models we consider we
include a supersymmetric example, 6D gauged chiral supergravity. Although much
of our discussion is more general, we treat in some detail a class of
compactifications, where the extra dimensions parametrize a rugby ball shaped
space with size stabilized by a bulk magnetic flux. The rugby ball geometry
requires two branes, which can host the Standard Model fields and carry both
tension and magnetic flux (of the bulk gauge field), the leading terms in a
derivative expansion. The brane properties have an impact on the KK spectrum
and therefore on the Casimir energy as well as on the renormalization of the
brane operators. A very interesting feature is that when the two branes carry
exactly the same amount of flux, one half of the bulk supersymmetries survives
after the compactification, even if the brane tensions are large. We also
discuss the implications of these calculations for the natural value of the
cosmological constant when the bulk has two large extra dimensions and the bulk
supersymmetry is partially preserved (or completely broken).Comment: 18 pages, partly based on a talk given at IARD 2012: the eighth
biennial conference on classical and quantum relativistic dynamics of
particles and fields; v2: version published in Journal of Physics: Conference
Serie
Superconductivity, Superfluidity and Holography
This is a concise review of holographic superconductors and superfluids. We
highlight some predictions of the holographic models and the emphasis is given
to physical aspects rather than to the technical details, although some
references to understand the latter are systematically provided. We include
gapped systems in the discussion, motivated by the physics of high-temperature
superconductivity. In order to do so we consider a compactified extra dimension
(with radius R), or, alternatively, a dilatonic field. The first setup can also
be used to model cylindrical superconductors; when these are probed by an axial
magnetic field a universal property of holography emerges: while for large R
(compared to the other scales in the problem) non-local operators are
suppressed, leading to the so called Little-Parks periodicity, the opposite
limit shows non-local effects, e.g. the uplifting of the Little-Parks
periodicity. This difference corresponds in the gravity side to a Hawking-Page
phase transition.Comment: 10 pages, partly based on a talk given at DICE2012 (Spacetime -
Matter - Quantum Mechanics from the Planck scale to emergent phenomena) and
on a seminar given at the Gran Sasso National Laboratory; v2 references adde
Higgs Inflation at NNLO after the Boson Discovery
We obtain the bound on the Higgs and top masses to have Higgs inflation
(where the Higgs field is non-minimally coupled to gravity) at full
next-to-next-to-leading order (NNLO). Comparing the result obtained with the
experimental values of the relevant parameters we find some tension, which we
quantify. Higgs inflation, however, is not excluded at the moment as the
measured values of the Higgs and top masses are close enough to the bound once
experimental and theoretical uncertainties are taken into account.Comment: 7 pages, 3 figures; v2: few comments added to emphasize the
importance of the results, published in Phys. Lett.
Quadratic Gravity
Adding terms quadratic in the curvature to the Einstein-Hilbert action
renders gravity renormalizable. This property is preserved in the presence of
the most general renormalizable couplings with (and of) a generic quantum field
theory (QFT). The price to pay is a massive ghost, which is due to the higher
derivatives that the terms quadratic in the curvature imply. In this paper the
quadratic gravity scenario is reviewed including recent progress on the related
stability problem of higher derivative theories. The renormalization of the
theory is also reviewed and the final form of the full renormalization group
equations in the presence of a generic renormalizable QFT is presented. The
theory can be extrapolated up to infinite energy through the renormalization
group if all matter couplings flow to a fixed point (either trivial or
interacting). Moreover, besides reviewing the above-mentioned topics some
further insight on the ghost issue and the infinite energy extrapolation is
provided. There is the hope that in the future this scenario might provide a
phenomenologically viable and UV complete relativistic field theory of all
interactions.Comment: 46 pages, 4 figures, 2 tables. Review article prepared for Frontiers;
v2 matches version published in Frontier
4D Effective Theory and Geometrical Approach
We consider the 4D effective theory for the light Kaluza-Klein (KK) modes.
The heavy KK mode contribution is generally needed to reproduce the correct
physical predictions: an equivalence, between the effective theory and the
D-dimensional (or geometrical) approach to spontaneous symmetry breaking (SSB),
emerges only if the heavy mode contribution is taken into account. This happens
even if the heavy mode masses are at the Planck scale. In particular, we
analyze a 6D Einstein-Maxwell model coupled to a charged scalar and fermions.
Moreover, we briefly review non-Abelian and supersymmetric extensions of this
theory.Comment: 7 pages, 2 figures. Proceeding for the Cairo International Conference
on High Energy Physics (CICHEP II), 14 - 17 January 200
Chiral Asymmetry from a 5D Higgs Mechanism
An intriguing feature of the Standard Model is that the representations of
the unbroken gauge symmetries are vector-like whereas those of the
spontaneously broken gauge symmetries are chiral. Here we provide a toy model
which shows that a natural explanation of this property could emerge in higher
dimensional field theories and discuss the difficulties that arise in the
attempt to construct a realistic theory. An interesting aspect of this type of
models is that the 4D low energy effective theory is not generically gauge
invariant. However, the non-invariant contributions to the observable
quantities are very small, of the order of the square of the ratio between the
light particle mass scale and the Kaluza-Klein mass scale. Remarkably, when we
take the unbroken limit both the chiral asymmetry and the non-invariant terms
disappear.Comment: 30 pages, 5 figures, uses axodraw.sty. Extended version, matches the
article published on JHE
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