81 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
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
Quantum mechanics of 4-derivative theories
A renormalizable theory of gravity is obtained if the dimension-less
4-derivative kinetic term of the graviton, which classically suffers from
negative unbounded energy, admits a sensible quantisation. We find that a
4-derivative degree of freedom involves a canonical coordinate with unusual
time-inversion parity, and that a correspondingly unusual representation must
be employed for the relative quantum operator. The resulting theory has
positive energy eigenvalues, normalisable wave functions, unitary evolution in
a negative-norm configuration space. We present a formalism for quantum
mechanics with a generic norm.Comment: 25 pages. v2: clarifications added, final published versio
Agravity up to infinite energy
The self-interactions of the conformal mode of the graviton are controlled,
in dimensionless gravity theories (agravity), by a coupling that is not
asymptotically free. We show that, nevertheless, agravity can be a complete
theory valid up to infinite energy. When grows to large values, the
conformal mode of the graviton decouples from the rest of the theory and does
not hit any Landau pole provided that scalars are asymptotically conformally
coupled and all other couplings approach fixed points. Then, agravity can flow
to conformal gravity at infinite energy. We identify scenarios where the Higgs
mass does not receive unnaturally large physical corrections. We also show a
useful equivalence between agravity and conformal gravity plus two extra
conformally coupled scalars, and give a simpler form for the renormalization
group equations of dimensionless couplings as well as of massive parameters in
the presence of the most general matter sector.Comment: 24 pages, 1 figure; v2: revised version to appear in the European
Physical Journal
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