arXiv subjects

Ignatios Antoniadis

Publications and source records attributed to Ignatios Antoniadis.

At least 91 records · Page 5Linked to original sources

arXiv2020

Inflation near a metastable de Sitter vacuum from moduli stabilisation

We study the cosmological properties of a metastable de Sitter vacuum obtained recently in the framework of type IIB flux compactifications in the presence of three D7-brane stacks, based on perturbative quantum corrections at both world-sheet and string loop level that are dominant at large volume and weak coupling. In the simplest case, the model has one effective parameter controlling the shape of the potential of the inflaton which is identified with the volume modulus. The model provides a phenomenological successful small-field inflation for a value of the parameter that makes the minimum very shallow and near the maximum. The horizon exit is close to the inflection point while most of the required e-folds of the Universe expansion are generated near the minimum, with a prediction for the ratio of tensor-to-scalar primordial fluctuations $r \sim 4 \times 10^{-4}$. Despite its shallowness, the minimum turns out to be practically stable. We show that it can decay only through the Hawking-Moss instanton leading to an extremely long decay rate. Obviously, in order to end inflation and obtain a realistic model, new low-energy physics is needed around the minimum, at intermediate energy scales of order $10^{12}$ GeV. An attractive possibility is by introducing a "waterfall"' field within the framework of hybrid inflation.

hep-th↗

arXiv2020

Scotogenic dark matter in an orbifold theory of flavor

We propose a flavour theory in which the family symmetry results naturally from a six-dimensional orbifold compactification. "Diracness" of neutrinos is a consequence of the spacetime dimensionality, and the fact that right-handed neutrinos live in the bulk. Dark matter is incorporated in a scotogenic way, as a result of an auxiliary $\mathbb{Z}_3$ symmetry, and its stability is associated to the conservation of a "dark parity" symmetry. The model leads naturally to a "golden" quark-lepton mass relation.

hep-ph↗

arXiv2020

Anomalous $U(1)$ Gauge Bosons as Light Dark Matter in String Theory

Present experiments are sensitive to very weakly coupled extra gauge symmetries which motivates further investigation of their appearance in string theory compactifications and subsequent properties. We consider extensions of the standard model based on open strings ending on D-branes, with gauge bosons due to strings attached to stacks of D-branes and chiral matter due to strings stretching between intersecting D-branes. Assuming that the fundamental string mass scale saturates the current LHC limit and that the theory is weakly coupled, we show that (anomalous) $U(1)$ gauge bosons which propagate into the bulk are compelling light dark matter candidates. We comment on the possible relevance of the $U(1)$ gauge bosons, which are universal in intersecting D-brane models, to the observed $3σ$ excess in XENON1T.

hep-th↗

arXiv2020

Weak Gravity Conjecture in de Sitter space-time

We propose a generalisation of the Weak Gravity Conjecture in de Sitter space by studying charged black-holes and comparing the gravitational and an abelian gauge forces. Using the same condition as in flat space, namely the absence of black-hole remnants, one finds that for a given mass $m$ there should be a state with a charge $q$ bigger than a minimal value $q_{\rm min}(m,l)$, depending on the mass and the de Sitter radius $l$, in Planck units. In the large radius flat space limit (large $l$), $q_{\rm min}\to m$ leading to the known result $q>m/\sqrt{2}$, while in the highly curved case (small $l$) $q_{\rm min}$ behaves as $\sqrt{ml}$. We also discuss the example of the gauged R-symmetry in $N=1$ supergravity.

hep-th↗

arXiv2020

$\mathcal N=2$ Supersymmetry Deformations, Electromagnetic Duality and Dirac-Born-Infeld Actions

We study the general deformation of $\mathcal N=2$ supersymmetry transformations of a vector multiplet that forms a (constant) triplet under the $SU(2)$ R-symmetry corresponding to the magnetic dual of the triplet of the Fayet-Iliopoulos (FI) parameters. We show that in the presence of both triplets, the induced scalar potential of a vector multiplet with generic prepotential has always a minimum that realises partial breaking of $\mathcal N=2\to \mathcal N=1$ supersymmetry. We then consider the impact of the deformation in the Dirac-Born-Infeld (DBI) action where one supersymmetry is non-linearly realised, described by a nilpotent constraint on the deformed $\mathcal N=2$ chiral-chiral superfield. We show that the generic magnetic deformation induces an ordinary FI D-term along the linear supersymmetry via the theta-angle. Moreover, we argue that the resulting action differs on-shell from the standard one (DBI+FI) by fermionic contributions.

hep-th↗

arXiv2020

Note on supersymmetric Dirac-Born-Infeld action with Fayet-Iliopoulos term

We study the Dirac-Born-Infeld (DBI) action with one linear and one non-linear supersymmetry in the presence of a constant Fayet-Iliopoulos (FI) D-term added explicitly or through a deformation of supersymmetry transformations. The linear supersymmetry appears to be spontaneously broken since the D auxiliary field gets a non-vanishing vacuum expectation value and an extra term proportional to the FI parameter involving fermions emerges in the non-linear formulation of the action written recently. However in this note, we show that on-shell this action is equivalent to a standard supersymmetric DBI action ${\it without}$ FI term but with redefined tension, at least up to order of mass-dimension 12 effective interactions.

hep-th↗

arXiv2020

Magnetic Deformation of Super-Maxwell Theory in Supergravity

A necessary condition for partial breaking of N=2 global supersymmetry is the presence of nonlinear deformations of the field transformations which cannot be generated by background values of auxiliary fields. This work studies the simplest of these deformations which already occurs in N=1 global supersymmetry, and its coupling to supergravity. It can be viewed as an imaginary constant shift of the D-auxiliary real field of an abelian gauge multiplet. We show how this deformation describes the magnetic dual of a Fayet-Iliopoulos term, a result that remains valid in supergravity, using its new-minimal formulation. Local supersymmetry and the deformation induce a positive cosmological constant. Moreover, the deformed U(1) Maxwell theory coupled to supergravity describes upon elimination of the auxiliary fields the gauging of R-symmetry, realised by the Freedman model of 1976. To this end, we construct the chiral spinor multiplet in superconformal tensor calculus by working out explicitly its transformation rules and use it for an alternative description of the new-minimal supergravity coupled to a U(1) multiplet. We also discuss the deformed Maxwell theory in curved superspace.

hep-th↗

arXiv2020

New Kähler invariant Fayet-Iliopoulos terms in supergravity and cosmological applications

Recently, a new type of constant Fayet-Iliopoulos (FI) terms was introduced in $\mathcal{N}=1$ supergravity, which do not require the gauging of the $R$-symmetry. We revisit and generalise these constructions, building a new class of Kähler invariant FI terms parametrised by a function of the gravitino mass as functional of the chiral superfields, which is then used to describe new models of inflation. They are based on a no-scale supergravity model of the inflaton chiral multiplet, supplemented by an abelian vector multiplet with the new FI-term. We show that the inflaton potential is compatible with the CMB observational data, with a vacuum energy at the minimum that can be tuned to a tiny positive value. Finally, the axionic shift symmetry can be gauged by the $U(1)$ which becomes massive. These models offer a mechanism for fixing the gravitino mass in no-scale supergravities, that corresponds to a flat direction of the scalar potential in the absence of the new FI-term; its origin in string theory is an interesting open problem.

hep-th↗

arXiv2020

Constant-roll in the Palatini-$R^2$ models

We consider models of a scalar field coupled to quadratic $R\!+\!R^2$ gravity in the framework of the Palatini formulation. The resulting Einstein-frame generalized $k$-inflation effective theory is analyzed assuming that the constant-roll condition holds. We focus on a quartic self-interaction potential, a case of particular appeal modelling Higgs inflation, considering the cases of minimal and non-minimal coupling of the inflaton to gravity. For an appropriate range of the model parameters in the large field domain the obtained values for the inflationary observables are found in agreement with current observations.

gr-qc↗

arXiv2020

$H_0$ tension and the String Swampland

[Abridged] We realize the Agrawal-Obied-Vafa (AOV) swampland proposal of fading dark matter by the model of Salam-Sezgin and its string realization of Cvetic-Gibbons-Pope. The model describes a compactification of 6-dimensional supergravity with a monopole background on a 2-sphere. In 4 dimensions, there are 2 scalar fields, $X$ and $Y $, and the effective potential in the Einstein frame is an exponential with respect to $Y$ times a quadratic polynomial in the field $e^{-X}$. When making the volume of the 2-sphere large, namely for large values of $Y$, there appears a tower of states, which according to the infinite distance swampland conjecture becomes exponentially massless. If the standard model fields are confined on Neveu-Schwarz 5-branes the 6-dimensional gauge couplings are independent of the string dilaton in the string frame, and upon compactification to 4 dimensions the 4-dimensional gauge couplings depend on $X$ (rather than the dilaton $Y$) which is fixed at the minimum of the potential. This avoids direct couplings of the dilaton to matter suppressing extra forces competing with gravity. We show that this set up has the salient features of the AOV models, and ergo can potentially ameliorate the tension between local distance ladder and cosmic microwave background estimates of the Hubble constant $H_0$. Although the AOV proposal does not fully resolve the tension in $H_0$ measurements, it provides a dynamical dark energy model of cosmology that satisfies the de Sitter swampland conjecture. We comment on a viable solution to overcome the tension between low- and high-redshift observations within the AOV background and discuss the implications for the swampland program.

hep-th↗

arXiv2020

Logarithmic loop corrections, moduli stabilisation and de Sitter vacua in string theory

We study string loop corrections to the gravity kinetic terms in type IIB compactifications on Calabi-Yau threefolds or their orbifold limits, in the presence of $D7$-branes and orientifold planes. We show that they exhibit in general a logarithmic behaviour in the large volume limit transverse to the $D7$-branes, induced by a localised four-dimensional Einstein-Hilbert action that appears at a lower order in the closed string sector, found in the past. Here, we compute the coefficient of the logarithmic corrections and use them to provide an explicit realisation of a mechanism for Kähler moduli stabilisation that we have proposed recently, which does not rely on non-perturbative effects and lead to de Sitter vacua. Our result avoids no-go theorems of perturbative stabilisation due to runaway potentials, in a way similar to the Coleman-Weinberg mechanism, and provides a counter example to one of the swampland conjectures concerning de Sitter vacua in quantum gravity, once string loop effects are taken into account; it thus paves the way for embedding the Standard Model of particle physics and cosmology in string theory.

hep-th↗

arXiv2019

Single-field inflation in models with an $R^2$ term

We present two cases where the addition of the $R^2$ term to an inflationary model leads to single-field inflation instead of two-field inflation as is usually the case. In both cases we find that the effect of the $R^2$ term is to reduce the value of the tensor-to-scalar ratio $r$.

gr-qc↗

arXiv2019

The String Geometry Behind Topological Amplitudes

It is shown that the generating function of $\mathscr{N}=2$ topological strings, in the heterotic weak coupling limit, is identified with the partition function of a six-dimensional Melvin background. This background, which corresponds to an exact CFT, realises in string theory the six-dimensional $\varOmega$-background of Nekrasov, in the case of opposite deformation parameters $ε_1=-ε_2$, thus providing the known perturbative part of the Nekrasov partition function in the field theory limit. The analysis is performed on both heterotic and type I strings and for the cases of ordinary $\mathscr{N}=2$ and $\mathscr{N}=2^*$ theories.

hep-th↗

arXiv2019

A microscopic model for inflation from supersymmetry breaking

We have proposed recently a framework for inflation driven by supersymmetry breaking with the inflaton being a superpartner of the goldstino, that avoids the main problems of supergravity inflation, allowing for: naturally small slow-roll parameters, small field initial conditions, absence of a (pseudo)scalar companion of the inflation, and a nearby minimum with tuneable cosmological constant. It contains a chiral multiplet charged under a gauged R-symmetry which is restored at the maximum of the scalar potential with a plateau where inflation takes place. The effective field theory relies on two phenomenological parameters corresponding to corrections to the Kähler potential up to second order around the origin. The first guarantees the maximum at the origin and the second allows the tuning of the vacuum energy between the F- and D-term contributions. Here, we provide a microscopic model leading to the required effective theory. It is a Fayet-Iliopoulos model with two charged chiral multiplets under a second U(1) R-symmetry coupled to supergravity. In the Brout-Englert-Higgs phase of this U(1), the gauge field becomes massive and can be integrated out in the limit of small supersymmetry breaking scale. In this work, we perform this integration and we show that there is a region of parameter space where the effective supergravity realises our proposal of small field inflation from supersymmetry breaking consistently with observations and with a minimum of tuneable energy that can describe the present phase of our Universe.

hep-th↗

arXiv2019

New Fayet-Iliopoulos terms in ${\mathcal N}=2$ supergravity

We present a new type of Fayet-Iliopoulos (FI) terms in ${\mathcal N}=2$ supergravity that do not require the gauging of the $R$-symmetry. We elaborate on the impact of such terms on the vacuum structure of the ${\mathcal N}=2$ theory and compare their properties with the standard Fayet-Iliopoulos terms that arise from gaugings. In particular, we show that, with the use of the new FI terms, models with a single physical ${\mathcal N}=2$ vector multiplet can be constructed that give stable de Sitter vacua.

hep-th↗

arXiv2019

Nonlinear ${\cal N}=2$ Global Supersymmetry

We study the partial breaking of ${\cal N}=2$ global supersymmetry, using a novel formalism that allows for the off-shell nonlinear realization of the broken supersymmetry, extending previous results scattered in the literature. We focus on the Goldstone degrees of freedom of a massive ${\cal N}=1$ gravitino multiplet which are described by deformed ${\cal N}=2$ vector and single-tensor superfields satisfying nilpotent constraints. We derive the corresponding actions and study the interactions of the superfields involved, as well as constraints describing incomplete ${\cal N}=2$ matter multiplets of non-linear supersymmetry (vectors and single-tensors).

hep-th↗

arXiv2019

Inflation from the internal volume in type IIB/F-theory compactification

We study cosmological inflation within a recently proposed framework of perturbative moduli stabilisation in type IIB/F theory compactifications on Calabi-Yau threefolds. The stabilisation mechanism utilises three stacks of magnetised 7-branes and relies on perturbative corrections to the Kähler potential that grow logarithmically in the transverse sizes of co-dimension two due to local tadpoles of closed string states in the bulk. The inflaton is the Kähler modulus associated with the internal compactification volume that starts rolling down the scalar potential from an initial condition around its maximum. Although the parameter space allows moduli stabilisation in de Sitter space, the resulting number of e-foldings is too low. An extra uplifting source of the vacuum energy is then required to achieve phenomenologically viable inflation and a positive (although tiny) vacuum energy at the minimum. Here we use, as an example, a new Fayet-Iliopoulos term proposed recently in supergravity that can be written for a non R-symmetry U(1) and is gauge invariant at the Lagrangian level; its possible origin though in string theory remains an open interesting problem.

hep-th↗

arXiv2019

Supersymmetric sphaleron configurations as the origin of the perplexing ANITA events

The ANITA experiment has observed two air shower events with energy ~ 500 PeV emerging from the Earth with exit angles of ~ 30 degrees. We explain ANITA events as arising from neutrino-induced supersymmetric sphaleron transitions. These high-multiplicity configurations could contain a large number of long-lived supersymmetric fermions, which can traverse the Earth and decay in the atmosphere to initiate upward-pointing air showers at large angles above the horizon. We comment on the sensitivity of new generation LHC detectors, designed to searching for displaced decays of beyond standard model long-lived particles, to test our model.

hep-ph↗