arXiv subjects

Alberto Salvio

Publications and source records attributed to Alberto Salvio.

68 records · Page 4Linked to original sources

arXiv2015

Dynamically Induced Planck Scale and Inflation

Theories where the Planck scale is dynamically generated from dimensionless interactions provide predictive inflationary potentials and super-Planckian field variations. We first study the minimal single-field realisation in the low-energy effective field theory limit, finding the predictions $n_s \approx 0.96$ for the spectral index and $r \approx 0.13$ for the tensor-to-scalar ratio, which can be reduced down to $\approx 0.04$ in presence of large couplings. Next we consider agravity as a dimensionless quantum gravity theory finding a multi-field inflation that converges towards an attractor trajectory that predicts $n_s\approx 0.96$ and $0.003<r<0.13$, interpolating between the quadratic and Starobinsky inflation. These theories relate the smallness of the weak scale to the smallness of inflationary perturbations: both arise naturally because of small couplings, implying a reheating temperature of $10^{7-9}$ GeV. A measurement of $r$ by Keck/Bicep3 would give us information on quantum gravity in the dimensionless scenario.

astro-ph.CO↗

arXiv2015

Softened Gravity and the Extension of the Standard Model up to Infinite Energy

Attempts to solve naturalness by having the weak scale as the only breaking of classical scale invariance have to deal with two severe difficulties: gravity and the absence of Landau poles. We show that solutions to the first problem require premature modifications of gravity at scales no larger than $10^{11}$ GeV, while the second problem calls for many new particles at the weak scale. To build models that fulfil these properties, we classify 4-dimensional Quantum Field Theories that satisfy Total Asymptotic Freedom (TAF): the theory holds up to infinite energy, where all coupling constants flow to zero. We develop a technique to identify such theories and determine their low-energy predictions. Since the Standard Model turns out to be asymptotically free only under the unphysical conditions $g_1 = 0$, $M_t = 186$ GeV, $M_τ= 0$, $M_h = 163$ GeV, we explore some of its weak-scale extensions that satisfy the requirements for TAF.

hep-ph↗

arXiv2015

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.

hep-ph↗

arXiv2014

On shape dependence of holographic mutual information in AdS4

We study the holographic mutual information in AdS4 of disjoint spatial domains in the boundary which are delimited by smooth closed curves. A numerical method which approximates a local minimum of the area functional through triangulated surfaces is employed. After some checks of the method against existing analytic results for the holographic entanglement entropy, we compute the holographic mutual information of equal domains delimited by ellipses, superellipses or the boundaries of two dimensional spherocylinders, finding also the corresponding transition curves along which the holographic mutual information vanishes.

hep-th↗

arXiv2014

Investigating the near-criticality of the Higgs boson

We extract from data the parameters of the Higgs potential, the top Yukawa coupling and the electroweak gauge couplings with full 2-loop NNLO precision, and we extrapolate the SM parameters up to large energies with full 3-loop NNLO RGE precision. Then we study the phase diagram of the Standard Model in terms of high-energy parameters, finding that the measured Higgs mass roughly corresponds to the minimum values of the Higgs quartic and top Yukawa and the maximum value of the gauge couplings allowed by vacuum metastability. We discuss various theoretical interpretations of the near-criticality of the Higgs mass.

hep-ph↗

arXiv2013

Towards leptogenesis at NLO: the right-handed neutrino interaction rate

We compute quantum and thermal corrections to the right-handed neutrino interaction rate in the early universe at next-to-leading order in all the relevant SM couplings (gauge, top Yukawa and higgs couplings). Previous computations considered 2 -> 2 scatterings, finding infra-red divergences. The KLN theorem demands that infra-red divergences cancel in the full result: after adding 1 -> 3 and one-loop virtual corrections that enter at the same order we find a simple result.

hep-ph↗

arXiv2013

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.

hep-ph↗

arXiv2013

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).

hep-th↗

arXiv2013

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.

hep-th↗

arXiv2013

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.

hep-th↗

arXiv2012

Holographic Superfluids and Superconductors in Dilaton-Gravity

We investigate holographic models of superfluids and superconductors in which the gravitational theory includes a dilatonic field. Dilaton extensions are interesting as they allow us to obtain a better description of low temperature condensed matter systems. We focus on asymptotically AdS black hole configurations, which are dual to field theories with conformal ultraviolet behavior. A nonvanishing value of the dilaton breaks scale invariance in the infrared and is therefore compatible with the normal phase being insulating (or a solid in the fluid mechanical interpretation); indeed we find that this is the case at low temperatures and if one appropriately chooses the parameters of the model. Not only the superfluid phase transitions, but also the response to external gauge fields is analyzed. This allows us to study, among other things, the vortex phase and to show that these holographic superconductors are also of Type II. However, at low temperatures they can behave in a qualitatively different way compared to their analogues without the dilaton: the critical magnetic fields and the penetration depth can remain finite in the small T/T_c limit.

hep-th↗

arXiv2012

Magnetic Response in the Holographic Insulator/Superconductor Transition

We study the magnetic response of holographic superconductors exhibiting an insulating "normal" phase. These materials can be realized as a CFT compactified on a circle, which is dual to the AdS Soliton geometry. We study the response under i) magnetic fields and ii) a Wilson line on the circle. Magnetic fields lead to formation of vortices and allows one to infer that the superconductor is of type II. The response to a Wilson line is in the form of Aharonov-Bohm-like effects. These are suppressed in the holographic conductor/superconductor transition but, instead, they are unsuppressed for the insulator case. Holography, thus, predicts that generically insulators display stronger Aharonov-Bohm effects than conductors. In the fluid-mechanical limit the AdS Soliton is interpreted as a supersolid. Our results imply that supersolids display unsuppressed Aharonov-Bohm (or "Sagnac") effects - stronger than in superfluids.

hep-th↗

arXiv2011

Flux Periodicities and Quantum Hair on Holographic Superconductors

Superconductors in a cylindrical geometry respond periodically to a cylinder-threading magnetic flux, with the period changing from hc/2e to hc/e depending on whether the Aharonov-Bohm effects are suppressed or not. We show that Holographic Superconductors present a similar phenomenon, and that the different periodicities follow from classical no-hair theorems. We also give the Ginzburg-Landau description of the period-doubling phenomenon.

hep-th↗

arXiv2010

Emergent Gauge Fields in Holographic Superconductors

Holographic superconductors have been studied so far in the absence of dynamical electromagnetic fields, namely in the limit in which they coincide with holographic superfluids. It is possible, however, to introduce dynamical gauge fields if a Neumann-type boundary condition is imposed on the AdS-boundary. In 3+1 dimensions, the dual theory is a 2+1 dimensional CFT whose spectrum contains a massless gauge field, signaling the emergence of a gauge symmetry. We study the impact of a dynamical gauge field in vortex configurations where it is known to significantly affect the energetics and phase transitions. We calculate the critical magnetic fields H_c1 and H_c2, obtaining that holographic superconductors are of Type II (H_c1 < H_c2). We extend the study to 4+1 dimensions where the gauge field does not appear as an emergent phenomena, but can be introduced, by a proper renormalization, as an external dynamical field. We also compare our predictions with those arising from a Ginzburg-Landau theory and identify the generic properties of Abrikosov vortices in holographic models.

hep-th↗