Search

We study holographic entanglement entropy and revisit thermodynamics and confinement in the dilaton-gravity system. Our analysis focuses on a solvable class of backgrounds that includes AdS and linear dilaton spacetimes as particular cases, with some results extended to general warped metrics. A general lesson is that the behavior of the holographic theory is tied to the bulk curvature singularities. We find that a singular background is confining if and only if i) the singularity coincides with a boundary or ii) it is the linear dilaton. In the former case, for which the singularity cuts off spacetime, we demonstrate that both entanglement entropy and thermodynamics exhibit a first order phase transition. In the linear dilaton case we find instead that both entanglement entropy and thermal phase transitions are of second order. Additionally, along the process we thoroughly derive the radion effective action at quadratic order., Comment: 43 pages, 8 figures, 1 table

We propose a setup for the origin of dark matter based on spacetime with a warped extra dimension and three branes: the Planck brane, the TeV brane, at a (few) TeV scale $\rho_T$, and a dark brane, at a (sub)-GeV scale $\rho_1\lesssim 100$ GeV $\ll\rho_T$. The Standard Model is localized in the TeV brane, thus solving the Higgs hierarchy problem, while the dark matter $\chi$, a Dirac fermion with mass $m_\chi<\rho_1$, is localized in the dark brane. The radion, with mass $m_r

Extrapolating the Standard Model Higgs potential at high energies, we study the barrier between the electroweak and Planck scale minima. The barrier arises by taking the central values of the relevant experimental inputs, that is the strong coupling constant and the top quark and Higgs masses. We then extend the Standard Model by including a non-minimal coupling to gravity, and explore the phenomenology of the Higgs inflation model. We point out that even configurations that would be metastable in the Standard Model, become viable for inflation if the non-minimal coupling is large enough to flatten the Higgs potential at field values below the barrier; we find that the required value of the non-minimal coupling is smaller than the one needed for the conventional Higgs inflation scenario (which relies on a stable Standard Model Higgs potential, without any barrier); in addition, values of the top mass which are slightly larger than those required in the conventional scenario are allowed., Comment: v1: 25 pages, 7 figures; v2: 30 pages, 9 figures, text and discussion improved

$R^2$-Higgs inflation stands out as one of the best-fit models of Planck data. Using a covariant formalism for the inflationary dynamics and the production of helical gauge fields, we show that the observed baryon asymmetry of the Universe (BAU) can be obtained when this model is supplemented by a dimension-six CP-violating term $\sim (R/\Lambda^2)\, B_{\mu\nu} \widetilde{B}^{\mu\nu}$ in the hypercharge sector. At linear order, values of $\Lambda\simeq 2.5\times10^{-5}\ M_{\rm P}$ produce, in the $R^2$-like regime, sufficient helical hypermagnetic fields to create the observed matter-antimatter asymmetry during the electroweak crossover. However, the Schwinger effect of fermion pair production can play a critical role in this context, and that scale is significantly lowered when the backreaction of the fermion fields on the gauge field production is included. In all cases, the helical field configurations can remain robust against washout after the end of inflation., Comment: matches published version

We study a solvable class of five-dimensional dilaton gravity models that continuously interpolate between anti-de Sitter (AdS$_5$), linear dilaton (LD$_5$) and positively curved spacetimes as a function of a continuous parameter $\nu$. The dilaton vacuum expectation value is set by a potential localized on a flat brane. We chart the elementary properties of these backgrounds for any admissible $\nu$, and determine stability conditions of the brane-dilaton system. We find that the spectrum of metric fluctuations can be either continuous or discrete. It features a massless graviton mode confined between the brane and the curvature singularity, and a massive radion mode tied to brane-dilaton stability. We show that, in the presence of a bulk black hole, the holographic theory living on the brane features a perfect fluid. The equation of state of the holographic fluid interpolates between radiation, pressureless matter and vacuum energy as a function of $\nu$. This extends earlier findings on holographic fluids. Our results suggest that the thermodynamics of the fluid mirrors precisely the thermodynamics of the bulk black hole., Comment: 27 pages. v2: Minor edits, references added. Added appendices revisiting gauge fixing and the graviton quadratic action in warped extra dimensions

Unstable particles decay sooner or later, so they are not described by asymptotic one-particle states and they should not be included as independent states in unitarity relations such as the optical theorem. The same applies to any countable collection of unstable particles. We show that the behaviour of unparticle stuff, that is, a continuous collection of particles with different masses and common decay channels, is pretty different: it has a non-vanishing probability of surviving for ever and the corresponding asymptotic states must be taken into account to comply with unitarity. We also discuss compressed spectra and the transition from the discrete to the continuous case., Comment: 55 pages, 16 figures, new summary section, improved discussion, references added

The linear dilaton background is the keystone of a string-derived holographic correspondence beyond AdS$_{d+1}$/CFT$_d$. This motivates an exploration of the $(d+1)$-dimensional linear dilaton spacetime (LD$_{d+1}$) and its holographic properties from the low-energy viewpoint. We first notice that the LD$_{d+1}$ space has simple conformal symmetries, that we use to shape an effective field theory (EFT) on the LD background. We then place a brane in the background to study holography at the level of quantum fields and gravity. We find that the holographic correlators from the EFT feature a pattern of singularities at certain kinematic thresholds. We argue that such singularities can be used to bootstrap the putative $d$-dimensional dual theory using techniques analogous to those of the Cosmological Bootstrap program. Turning on finite temperature, we study the holographic fluid emerging on the brane in the presence of a bulk black hole. We find that the holographic fluid is pressureless for any $d$ due to a cancellation between Weyl curvature and dilaton stress tensor, and verify consistency with the time evolution of the theory. From the fluid thermodynamics, we find a universal temperature and Hagedorn behavior for any $d$. This matches the properties of a CFT$_2$ with large $T\overline T$ deformation, and of little string theory for $d=6$. Both the fluid equation of state and the spectrum of quantum fluctuations suggest that the $d$-dimensional dual theory arising from LD$_{d+1}$ is generically gapped., Comment: 40+11 pages, 3 figures

We investigate the potential of the warped-extradimension framework as an explanation for the recently observed stochastic gravitational background at nHz frequencies in pulsar timing arrays (PTA). Our analysis reveals that the PTA data can be effectively accommodated by a first-order phase transition triggered by a radion at the MeV-GeV scale feebly coupled to the Standard Model. Remarkably, this outcome remains robust irrespective of the specific details of the warped extradimension embedding, providing a foundation for future investigations aiming to develop concrete extradimension descriptions of Nature. We also demonstrate that many existing embeddings are not viable, as their radion and graviton phenomenology clash with a MeV-GeV scale radion. As a possible way-out, we sketch a promising solution involving multiple branes, wherein the light radion, graviton, and ensuing light resonances remain consistent with collider bounds and gravity tests., Comment: 15 pages, 3 figures; v2 typos corrected, added references, figures improved, Secs. III and IV improved

If the inflaton field is coupled to the hypercharge Chern-Simons density $F\tilde F$, an explosive production of helical gauge fields when inflation ends can trigger baryogenesis at the electroweak phase transition. Besides, Higgs inflation identifies the inflaton with the Higgs field $\mathcal H$, thus relating cosmological observables to properties of electroweak physics. In this paper we merge both approaches: the helical gauge fields are produced at the end of Higgs inflation from the coupling $|\mathcal H|^2 F\tilde F$. In the metric formulation of gravity we found a window in the parameter space for electroweak baryogenesis consistent with all experimental observations. Conversely, for the Palatini formalism the non-gaussianity bounds strongly constrain the helicity produced at the end of inflation, forbidding an efficient baryogenesis., Comment: 22 pages, 4 figures

We study the cosmology of a three-brane in a specific five-dimensional scalar-gravity (i.e. soft-wall) background, known as the linear dilaton background. We discover that the Friedmann equation of the brane-world automatically contains a term mimicking pressureless matter. We propose to identify this term as dark matter. This dark matter arises as a projection of the bulk black hole on the brane, which contributes to the brane Friedmann equation via both the Weyl tensor and the scalar stress tensor. The nontrivial matter-like behavior is due to an exact cancellation between the Weyl and scalar pressures. We show that the Newtonian potential only receives a mild short-distance correction going as inverse distance squared, ensuring compatibility of the linear dilaton brane-world with observed 4D gravity. Our setup can be viewed as a consistent cosmological description of the holographic theories arising in the linear dilaton background. We also present more general scalar-gravity models where the brane cosmology features an effective energy density whose behavior smoothly interpolates between dark radiation, dark matter and dark energy depending on a model parameter., Comment: v2: 7 pages, 2 figures. Text improved, model in Sec. V.A. added, references added, matches journal version

Using two different warped five-dimensional (5D) models with two branes along the extra dimension, we study the Green's functions and the spectral properties of some of the fields propagating in the bulk. While the first model has a discrete spectrum of Kaluza-Klein (KK) modes, the second one has a continuous spectrum above a mass gap. We also study the positivity of the spectral functions, as well as the coupling of the graviton and the radion with SM matter fields., Comment: 8 pages, 3 figures. Talk given by E.Megias at the XVth Quark Confinement and the Hadron Spectrum Conference, Stavanger, Norway, 1 - 6 August 2022

We examine effective field theories (EFTs) with a continuum sector in the presence of gravity. We first explain, via arguments based on central charge and species scale, that an EFT with a free continuum cannot consistently couple to standard (i.e. 4D Einstein) gravity. It follows that EFTs with a free or nearly-free continuum must either have a finite number of degrees of freedom or nonstandard gravity. The latter claim is realized for holographically-defined continuum models. We demonstrate this by computing the deviations from standard gravity in a specific 5D scalar-gravity system that gives rise to a gapped continuum (i.e the linear dilaton background). We find an $R^{-2}$ deviation from the Newtonian potential. At finite temperature we find an energy density with matter-like behavior in the brane Friedmann equation, holographically induced from the bulk geometry. Thus, remarkably, a brane-world living in the linear dilaton background automatically contains dark matter. We also present a slightly more evolved asymptotically-AdS linear dilaton model, for which the deviations exhibit a transition between AdS and linear dilaton behaviors., Comment: 47 pages, 4 figures, v2: Minor text edits for clarity, references added, models in Sec. 5 updated, appendices added

Previous studies demonstrate that the inflaton, when coupled to the hypercharge Chern-Simons density, can source an explosive production of helical hypermagnetic fields. Then, in the absence of fermion production, those fields have the capability of preheating the Universe after inflation and triggering a successful baryogenesis mechanism at the electroweak phase transition. In the presence of fermion production however, we expect a strong damping of the gauge fields production from the fermion backreaction, a phenomenon called Schwinger effect, thus jeopardizing their original capabilities. Using numerical methods we study the backreaction on the generated gauge fields and revisit the processes of gauge preheating and baryogenesis in the presence of the Schwinger effect. We have found that gauge preheating is very unlikely, while still having a sizable window in the parameter space to achieve the baryon asymmetry of the Universe at the electroweak phase transition., Comment: fit the Physical Review D version, 30 pages, 8 figures

The higgsino Lightest Supersymmetric Particle (LSP) scenario opens up the possibility of decays of strongly produced particles to an intermediate neutralino, due to the Yukawa-suppressed direct decays to the higgsino. Those decays produce multijet signals with a Higgs or a $Z$ boson being produced in the decay of the intermediate neutralino to the LSP. In this paper we study the discovery prospects of squarks that produce $b$-jets and leptons in the final state. Our collider analysis provides signal significances at the 3$\sigma$ level for luminosities of 1 ab$^{-1}$, and at the 5$\sigma$ level if we project these results for 3 ab$^{-1}$., Comment: 10 pages, 6 figures, 2 tables

We study a modification of the Higgs inflation scenario where we introduce an extra scalar $\phi$, with mass $m$, coupled to the Ricci scalar as $g\phi^2 R$, and mixed with the Higgs field $h$ via the Lagrangian term $\mu \phi h^2$. Both fields participate in the inflation process in a unitary theory that predicts values of the cosmological observables in agreement with the results from the Planck/BICEP/Keck collaborations. In addition, by means of a $\mathcal{CP}$-odd effective operator that couples $\phi$ to the Chern-Simons term of the hypercharge gauge group as $f_\phi^{-1}\phi \,Y_{\mu\nu}\tilde Y^{\mu\nu}$, maximally helical magnetic fields are produced during the last $e$-folds of inflation. We found a window in the coupling $ f_\phi$ where these fields survive all constraints until the electroweak phase transition, and source the baryon asymmetry of the Universe through the Standard Model chiral anomaly. From a phenomenological perspective, the model can solve the Standard Model instability problem at the scale $\mathcal Q_I\simeq 10^{11}$ GeV, provided that $\mu\lesssim m \lesssim \mathcal Q_I$, and for $m\lesssim \mathcal{O}$(few)~TeV, the $\phi$-$h$ mixing becomes sizable while the theory turns natural. The latter thus predicts modifications of the trilinear and quartic couplings that could be explored at the HE-LHC, as well as at future colliders, and allows for direct $\phi$ production at the LHC followed by decay into $hh$. Present results from ATLAS and CMS already put (mild) bounds on the mass of the heavy scalar as $m\gtrsim 0.55$~TeV at 95\% C.L., Comment: Version to appear in Physical Review D, 56 pages, 11 figures

The assumption that strongly interacting sparticles will decay directly to the LSP plus jets breaks down in situations where those decays are Yukawa suppressed. That occurs when producing the first two generations of squarks and when, at the same time, there are several electroweakinos lighter than those squarks. In this paper we analyze the signal of pair production of squarks that subsequently decay to an intermediate neutralino ($\tilde\chi_3^0$) plus jets. The neutralino will then decay to the LSP (mainly higgsino) and a Higgs. We have simulated the events and designed a discovery strategy based on a signal of two jets, four $b$-quarks and missing transverse energy. We obtain very promising values for the LHC sensitivity at 14 TeV and 300 fb$^{-1}$., Comment: 11 pages, 5 figures, 4 tables. Corrections of typos and references added

Using a five dimensional (5D) warped model with two branes along the extra dimension, we study the Green's functions for gauge bosons with a mass gap $m_g = \rho/2$ and a continuum for $s > m_g^2$. We find that the Green's functions exhibit poles in the second Riemann sheet of the complex $s$ plane, denoting the existence of broad resonances. The positivity of the spectral functions is also analyzed. Finally, it is shown how the Green's functions can modify some Standard Model processes in $pp$ collisions., Comment: 5 pages, 3 figures. Talk given by E.Megias at the 24th High-Energy Physics International Conference in Quantum Chromodynamics (QCD 21), Montpellier, France, 5 - 9 July 2021

The possibility in supersymmetric scenarios that the dark matter candidate is a Higgsino-like neutralino means that its production can be associated with Higgs bosons. Taking advantage of this fact, we propose a LHC search strategy for gluinos with $\tau$ leptons in the final state, coming from the decay of a Higgs boson. We consider the strong production of a pair of gluinos, one of which decays into the Higgsino plus jets while the other decays into the bino plus jets. In turn, this bino decays into the Higgsino plus a Higgs boson which finally decays into a $\tau$-lepton pair. Therefore, the experimental signature under study consists of 4 jets, 2 $\tau$ leptons, and a large amount of missing transverse energy. Our cut-based search strategy allows us to reach, for a LHC center-of-mass energy of 14 TeV and a total integrated luminosity of 1 ab$^{-1}$, significances of up to 2 standard deviations, considering systematic uncertainties in the SM background of 30\%. The projections for 3 ab$^{-1}$ are encouraging, with significances at the evidence level, which in more optimistic experimental scenarios could exceed 4 standard deviations., Comment: 11 pages, 1 table, 3 figures

Green's functions with continuum spectra are a way of avoiding the strong bounds on new physics from the absence of new narrow resonances in experimental data. We model such a situation with a five-dimensional model with two branes along the extra dimension $z$, the ultraviolet (UV) and the infrared (IR) one, such that the metric between the UV and the IR brane is AdS$_5$, thus solving the hierarchy problem, and beyond the IR brane the metric is that of a linear dilaton model, which extends to $z\to\infty$. This simplified metric, which can be considered as an approximation of a more complicated (and smooth) one, leads to analytical Green's functions (with a mass gap $m_g \sim \textrm{TeV}$ and a continuum for $s > m_g^2$) which could then be easily incorporated in the experimental codes. The theory contains Standard Model gauge bosons in the bulk with Neumann boundary conditions in the UV brane. To cope with electroweak observables the theory is also endowed with an extra custodial gauge symmetry in the bulk, with gauge bosons with Dirichlet boundary conditions in the UV brane, and without zero (massless) modes. All Green's functions have analytical expressions and exhibit poles in the second Riemann sheet of the complex plane at $s=M_n^2-i M_n\Gamma_n$, denoting a discrete (infinite) set of broad resonances with masses ($M_n$) and widths ($\Gamma_n$). For gauge bosons with Neumann or Dirichlet boundary conditions, the masses and widths of resonances satisfy the (approximate) equation $s= - 4 m_g^2 \mathcal W_n^2[\pm (1+i)/4]$, where $\mathcal W_n$ is the $n$-th branch of the Lambert function., Comment: 46 pages, 16 figures; v2 extended version: added reference [52] and Fig. 15, extended discussion in Secs. 2, 3.1, 3.3, 3.4 and Appendix A. Typos corrected. It matches the version published in Journal of High Energy Physics

In the LHC searches for gluinos it is usually assumed that they decay predominantly into the lightest neutralino plus jets. In this work we perform a proof-of-concept collider analysis of a novel supersymmetric signal in which gluinos decay mostly into jets and the bino-like neutralino ($\tilde\chi_3^0$), which in turn decays into the lightest Higgsino-like neutralino ($\tilde\chi_1^0$), considered the dark matter candidate, together with the SM-like Higgs boson ($h$). This new physics signal then consists of an LHC final state made up by four light jets, four $b$-jets, and a large amount of missing transverse energy. We identify $t \bar t$, $V$+jets ($V$= $W$, $Z$), and $t \bar t + X$ ($X$ = $W$, $Z$, $\gamma^*$, $h$) productions as the most problematic backgrounds, and develop a search strategy for the high luminosity phase of the LHC, reaching signal significances at the evidence level for a luminosity of 1000 fb$^{-1}$. The prospects for a luminosity of 3000 fb$^{-1}$ are even more promising, with discovery-level significances., Comment: 11 pages, 2 tables, 3 figures

Continuum spectra can be a way out to alleviate the tension generated by the elusiveness of narrow resonances of new physics in direct experimental searches. Motivated by the latter, we consider the linear dilaton model with a continuum spectrum of KK modes. It is provided by a critical exponential bulk potential for the scalar field stabilizing the distance, between the UV boundary at $y=0$ and a naked (good) singularity at $y=y_s$, in proper coordinates, which corresponds in conformal coordinates to $z_s\to\infty$. The cutoff $M_s$ in this theory is an intermediate scale $M_s\simeq 10^{-5}M_{\rm Pl}$ and the warped factor solves the hierarchy between $M_s$ and the TeV, while the hierarchy between $M_{\rm Pl}$ and $M_s$ has to be solved by a (Little) String Theory with coupling $g_s\simeq 10^{-5}$. The Standard Model is localized on a 4D IR brane. The graviton and radion Green's and spectral functions have a continuum of states with a TeV mass gap, and isolated poles consisting on the 4D graviton and the light radion/dilaton. We construct the effective field theory below the mass gap where the continua of KK modes are integrated out, generating a set of dimension eight operators which contribute to low energy electroweak precision observables, and high energy violation of unitarity in vector boson scattering processes. The radion mass depends on the stabilizing UV brane potential and its wave function is localized toward the IR which enhances its coupling with the SM fields., Comment: 33 pages, 7 figures

We study the confinement/deconfinement phase transition of the radion field in a warped model with a polynomial bulk potential. The backreaction of the radion on the metric is taken into account by using the superpotential formalism, while the radion effective potential is obtained from a novel formulation which can incorporate the backreaction. The phase transition leads to a stochastic gravitational wave background that depends on the energy scale of the first Kaluza-Klein resonance, $m_{\textrm{KK}}$. This work completes previous studies in the following aspects: i) we detail the evaluation of the radion spectrum; ii) we report on the mismatches between the thick wall approximation and the numerical bounce solution; iii) we include a suppression factor in the spectrum of sound waves accounting for their finite lifetime; and, iv) we update the bound on $m_{\textrm{KK}}$ in view of the O3 LIGO and Virgo data. We find that the forthcoming gravitational wave interferometers can probe scenarios where $m_{\textrm{KK}} \lesssim 10^9$ TeV, while the O3-run bounds rule out warped models with $10^4 \textrm{TeV} \lesssim m_{\textrm{KK}} \lesssim 10^7$ TeV exhibiting an extremely strong confinement/deconfinement phase transition., Comment: 16 pages, 7 figures; v2 extended version: added references and Figs. 2, 3, 5 and 7 (lower panels), Figs. 6 and 7 (upper panels) updated, extended discussion in Secs. 3.3, 4, 5 and 6. Talk given by E.Megias at the 9th International Conference on New Frontiers in Physics (ICNFP 2020), 4 Sep - 2 Oct 2020, Kolymbari, Crete, Greece

We show that the baryon asymmetry of the universe can be explained in models where the Higgs couples to the Chern-Simons term of the hypercharge group and is away from the late-time minimum of its potential during inflation. The Higgs then relaxes toward this minimum once inflation ends which leads to the production of (hyper)magnetic helicity. We discuss the conditions under which this helicity can be approximately conserved during its joint evolution with the thermal plasma. At the electroweak phase transition the helicity is then converted into a baryon asymmetry by virtue of the chiral anomaly in the standard model. We propose a simple model which realizes this mechanism and show that the observed baryon asymmetry of the universe can be reproduced., Comment: 35 pages, 10 figures, 2 tables; v2: small improvements, added comments and references, matches published version

In theories with a warped extra dimension, composite fermions, as e.g. the right-handed top quark, can be very strongly coupled to Kaluza-Klein (KK) fields. In particular, the KK gluons in the presence of such composite fields become very broad resonances, thus remarkably modifying their experimental signatures. We have computed the pole mass and the pole width of the KK gluon, triggered by its interaction with quarks, as well as the prediction for proton-proton cross-sections using the full propagator and compared it with that obtained from the usual Breit-Wigner approximation. We compare both approaches, along with the existing experimental data from ATLAS and CMS, for the $t\bar t$, $t\bar t W$, $t\bar t Z$, $t\bar t H$, and $t\bar t t\bar t$ channels. We have found differences between the two approaches of up to about 100%, highlighting that the effect of broad resonances can be dramatic on present, and mainly future, experimental searches. The channel $t\bar t t\bar t$ is particularly promising because the size of the cross-section signal is of the same order of magnitude as the Standard Model prediction, and future experimental analyses in this channel, especially for broad resonances, can shed light on the nature of possible physics beyond the Standard Model., Comment: 32 pages, 14 figures; v2 includes additional references and correct units for cross-section on vertical axis of Fig. 14

A comologically stable neutral component from a nearly pure $SU(2)$ doublet, with a mass $\sim$1.1 TeV, is one appealing candidate for dark matter (DM) consistent with all direct dark matter searches. We have explored this possibility in the context of the Minimal Supersymmetric extension of the Standard Model (MSSM), with the Higgsino playing the role of DM, in theories where supersymmetry breaking is transmitted by gravitational interactions at the unification scale $M\simeq 2\times 10^{16}$ GeV. We have focussed our work in the search of "light" supersymmetric spectra, which could be at reach of present and/or future colliders, in models with universal and non-universal Higgs and gaugino Majorana masses. The lightest supersymmetric particles of the spectrum are, by construction, two neutralinos and one chargino, almost degenerate, with a mass $\sim $1.1 TeV, and a mass splitting of a few GeV. Depending on the particular scenario the gluino can be at its experimental mass lower bound $\sim$ 2.2 TeV; in the squark sector, the lightest stop can be as light as $\sim$ 1.3 TeV, and the lightest slepton, the right-handed stau, can have a mass as light as $1.2$ TeV. The lightest neutralino can be found at the next generation of direct dark matter experimental searches. In the most favorable situation, the gluino, with some specific decay channels, could be found at the next run of the Large Hadron Collider (LHC), and the lightest stop at the High-Luminosity LHC run., Comment: 19 pages, 12 figures, minor changes, version to be published in PRD

We systematically study the holographic phase transition of the radion field in a five-dimensional warped model which includes a scalar potential with a power-like behavior. We consider Kaluza-Klein (KK) resonances with masses $m_{\rm KK}$ at the TeV scale or beyond. The backreaction of the radion field on the gravitational metric is taken into account by using the superpotential formalism. The confinement/deconfinement first order phase transition leads to a gravitational wave stochastic background which mainly depends on the scale $m_{\rm KK}$ and the number of colors, $N$, in the dual theory. Its power spectrum peaks at a frequency that depends on the amount of tuning required in the electroweak sector. It turns out that the present and forthcoming gravitational wave observatories can probe scenarios where the KK resonances are very heavy. Current aLIGO data already rule out vector boson KK resonances with masses in the interval $m_{\rm KK}\sim(1 - 10) \times 10^5$ TeV. Future gravitational experiments will be sensitive to resonances with masses $m_{\rm KK}\lesssim 10^5$ TeV (LISA), $10^8$ TeV (aLIGO Design) and $10^9$ TeV (ET). Finally, we also find that the Big Bang Nucleosynthesis bound in the frequency spectrum turns into a lower bound for the nucleation temperature as $T_n \gtrsim 10^{-4}\sqrt{N} \,m_{\rm KK}$., Comment: 11 pages, 6 figures; v2 extended version: added references and Figs. 1 (right), 2, 3, 4 (lower panels) and 5, Sec. IV, and extended discussion in Secs. V, VI and VII; v3 added references, extended discussion in Sec. VI. It matches the version published in Physical Review D

In this work we study a warped five-dimensional (5D) model with ultraviolet (UV) and infrared (IR) branes, that solves the hierarchy problem with a fundamental 5D Planck scale $M$, and curvature parameter $k$, of the order of the 4D Planck mass $M_{\textrm{Planck}}\equiv 2.4\times 10^{15}$ TeV. The model exhibits a continuum of Kaluza-Klein (KK) modes with different mass gaps, at the TeV scale, for all fields. We have computed the Green's functions and spectral densities, and shown how the presence of a continuum KK spectrum can produce an enhancement in the cross section of some Standard Model processes. The metric is linear near the IR, in conformal coordinates, as in the linear dilaton and 5D clockwork models, for which $M\sim$ TeV. We also analyze a pure (continuum) linear dilaton scenario, solving the hierarchy problem with more conventional fundamental $M$ and $k$ scales of the order of $M_{\textrm{Planck}}$, and a continuum spectrum., Comment: 18 pages, 7 figures, 1 table. Talk given by E.Megias at the 8th International Conference on New Frontiers in Physics (ICNFP 2019), 21-29 Aug 2019, Kolymbari, Crete, Greece

We explore the generation of the baryon asymmetry in an extension of the Standard Model where the lepton number is promoted to a $U(1)_\ell$ gauge symmetry with an associated $Z^\prime$ gauge boson. This is based on a novel electroweak baryogenesis mechanism first proposed by us in Ref. \cite{Carena:2018cjh}. Extra fermionic degrees of freedom - including a fermionic dark matter $\chi$ - are introduced in the dark sector for anomaly cancellation. Lepton number is spontaneously broken at high scale and the effective theory, containing the Standard Model, the $Z^\prime$, the fermionic dark matter, and an additional complex scalar field $S$, violates CP in the dark sector. The complex scalar field couples to the Higgs portal and is essential in enabling a strong first order phase transition. Dark CP violation is diffused in front of the bubble walls and creates a chiral asymmetry for $\chi$, which in turn creates a chemical potential for the Standard Model leptons. Weak sphalerons are then in charge of transforming the net lepton charge asymmetry into net baryon number. We explore the model phenomenology related to the leptophilic $Z^\prime$, the dark matter candidate, the Higgs boson and the additional scalar, as well as implications for electric dipole moments. We also discuss the case when baryon number $U(1)_B$ is promoted to a gauge symmetry, and discuss electroweak baryogenesis and its corresponding phenomenology., Comment: 44 pages, 12 figures, 1 table. v2 matches the published version

We find new limits on the $\lambda'_{3jk}$ R-parity violating (RPV) couplings of the minimal supersymmetric standard model, using Drell-Yan differential cross sections at the LHC and electroweak precision measurements from LEP and SLC. Specifically, limits on six out of the nine $\lambda'_{3jk}$-couplings, with $j=1\text{ or }2$, are obtained using Drell-Yan data, with the remaining three (for $j=3$) bounded by precision electroweak data. We also update the limits on all $\lambda'_{ijk}$-couplings using electroweak data and find new bounds on $\lambda'_{132}$ and $\lambda'_{232}$ that are stronger than obtained elsewhere. A table of all current bounds on $\lambda'_{ijk}$ is given in an appendix., Comment: 9 pages, 3 figures, 3 tables

We consider a warped five-dimensional model with an ultraviolet (UV) brane and, on top of the Standard Model isolated modes, continua of KK modes with different mass gaps for all particles: gauge bosons, fermions, graviton, radion and Higgs boson. The model can be considered as a modelization in five dimensions of gapped unparticles. The five dimensional metric has a singularity, at a finite (infinite) value of the proper (conformal) coordinate, which is admissible as it supports finite temperature in the form of a black hole horizon. An infrared (IR) brane, with particular jumping conditions, is introduced to trigger correct electroweak breaking. The gravitational metric is AdS$_5$ near the UV brane, to solve the hierarchy problem with a fundamental Planck scale, and linear, in conformal coordinates, near the IR, as in the linear dilaton and five-dimensional clockwork models. The branes, and singularity, distances are fixed, \`a la Goldberger-Wise, by a bulk scalar field with brane potentials explicitly breaking the conformal symmetry. The bosonic continuum of KK modes with the smallest mass gap are those of gauge bosons, and so they are the most likely produced at the LHC. Mass gaps of the continuum of KK fermions do depend on their localization in the extra dimension. We have computed the spectral functions, and arbitrary Green's functions, and shown how they can modify some Standard Model processes., Comment: 39 pages, 12 figures, 1 table; v2 sec. 5.5 modified, added references; v3 typos corrected. It matches the version published in Journal of High Energy Physics

We consider a minimal natural supersymmetric model based on an extra dimension with supersymmetry breaking provided by the Scherk-Schwarz mechanism. The lightest supersymmetric particle is a neutral, quasi-Dirac Higgsino and, unlike in previous studies, we assume that all Standard Model fields are propagating in the bulk. The resulting setup is minimal, as neither extra matter, effective operators, nor extra $U(1)$ groups are needed in order to be viable. The model has three free parameters which are fixed by the Higgsino mass -- set to the range 1.1-1.2 TeV so it can play the role of Dark Matter, and by the requirements of correct electroweak breaking and the mass of the Higgs. After imposing the previous conditions we find a benchmark scenario that passes all experimental constrains with an allowed range for the supersymmetric parameters. In particular we have found gluinos in the range 2.0-2.1 TeV mass, electroweakinos and sleptons almost degenerate in the range 1.7-1.9 TeV and squarks degenerate in the range 1.9-2.0 TeV. The best discovery prospects are: i.) gluino detection at the high luminosity LHC ($\gtrsim 3\, \text{ab}^{-1}$), and ii.) Higgsino detection at next-generation dark matter direct detection experiments. The model is natural, as the fine-tuning for the fixed values of the parameters is moderate mainly because supersymmetry breaking parameters contribute linearly to the Higgs mass parameter, rather than quadratically as in most models., Comment: 20 pages, 4 figures, new range for Higgsino mass considered and LHC bounds recalculated

We find constraints on R-Parity Violating (RPV) couplings of the minimal supersymmetric standard model, using Drell-Yan differential cross sections at the LHC. Specifically, we look at the constraints on $\lambda'LQD^c$ couplings from monolepton and dilepton data published by ATLAS, with either electrons or muons in the final state. Out of the 18 RPV couplings to which the LHC is at least potentially sensitive by this technique, we find new limits on 12 (or 13) of them, for squarks masses above 1 (or 2) TeV. We also show that one can employ our techniques to achieve significantly stronger bounds at a high-luminosity upgrade of the LHC., Comment: 5 pages, 1 figure, 2 tables

Using a five-dimensional warped model including a scalar potential with an exponential behavior in the infrared, and strong back-reaction over the metric, we study the electroweak phase transition, and explore parameter regions that were previously inaccessible. The model exhibits gravitational waves and predicts a stochastic gravitational wave background observable, both at the Laser Interferometer Space Antenna and at the Einstein Telescope. Moreover, concerning the collider signatures predictions, the radion evades current constraints but may show up in future LHC runs., Comment: 8 pages, 2 figures, 2 tables. Talk given by E.Megias at the XIIIth Conference on Quark Confinement and the Hadron Spectrum (Confinement 2018), 31 Jul - 6 Aug 2018, Maynooth, Ireland

We present a novel mechanism of electroweak baryogenesis where \textit{CP} violation occurs in a dark sector, comprised of standard model gauge singlets, thereby evading the strong electric dipole moment constraints. In this framework, the background of time-like component of a new gauge boson $Z^\prime_\mu$, generated at electroweak temperatures, drives the electroweak sphaleron processes to create the required baryon asymmetry. We first discuss the crucial ingredients for this mechanism to work, and then show that all of them can be elegantly embedded in ultraviolet completions with spontaneously broken gauged lepton number. The models under consideration have a rich phenomenology and can be experimentally probed in leptophilic $Z^\prime$ searches, dark matter searches, heavy Majorana neutrino searches, as well as through hunting for new Higgs portal scalars in multi-lepton channels at colliders., Comment: 6 pages, 1 figure

Flavor physics experiments allow to probe the accuracy of the Standard Model (SM) description at low energies, and are sensitive to new heavy gauge bosons that couple to quarks and leptons in a relevant way. The apparent anomaly in the ratios of the decay of $B$-mesons into $D$-mesons and different lepton flavors, $R_{D^{(\ast)}} = \mathcal B(B \to D^{(\ast)} \tau \nu)/ \mathcal B(B \to D^{(\ast)} \ell \nu )$ is particularly intriguing, since these decay processes occur at tree-level in the SM. Recently, it has been suggested that this anomaly may be explained by new gauge bosons coupled to right-handed currents of quarks and leptons, involving light right-handed neutrinos. In this work we present a well-motivated ultraviolet complete realization of this idea, embedding the SM in a warped space with an $SU(2)_L \otimes SU(2)_R \otimes U(1)_{B-L}$ bulk gauge symmetry. Besides providing a solution to the hierarchy problem, we show that this model, which has an explicit custodial symmetry, can explain the $R_{D^{(\ast)}}$ anomaly and at the same time allow for a solution to the $R_{K^{(\ast)}}$ anomalies, related to the decay of $B$-mesons into $K$-mesons and leptons, $R_{K^{(*)}} = \mathcal B(B\to K^{(*)} \mu \mu)/ \mathcal B(B \to K^{(*)} e e)$. In addition, a model prediction is an anomalous value of the forward-backward asymmetry $A^b_{FB}$, driven by the $Z\bar b_R b_R$ coupling, in agreement with LEP data., Comment: 43 pages, 12 figures, 1 table; v2 added references; v3 typos removed and added references. It matches the version published in Journal of High Energy Physics

We forecast the prospective of detection for a stochastic gravitational wave background sourced by cosmological first-order phase transitions. We focus on first-order phase transitions with negligible plasma effects, and consider the experimental infrastructures built by the end of the LISA mission. We make manifest the synergy among LISA, pulsar time array experiments, and ground-based interferometers. For phase transitions above the TeV scale or below the electroweak scale, LISA can detect the corresponding gravitational wave signal together with Einstein Telescope, SKA or even aLIGO-aVIRGO-KAGRA. For phase transitions at the electroweak scale, instead, LISA can be the only experiment observing the gravitational wave signal. In case of detection, by using a parameter reconstruction method that we anticipate in this work, we show that LISA on its own has the potential to determine when the phase transition occurs and, consequently, the energy scale above which the standard model of particle physics needs to be modified. The result may likely guide the collider community in the post-LHC era., Comment: 7 pages, 6 figures; v3: some clarifications, extra references, matches published version

We study the electroweak phase transition within a 5D warped model including a scalar potential with an exponential behavior, and strong back-reaction over the metric, in the infrared. By means of a novel treatment of the superpotential formalism, we explore parameter regions that were previously inaccessible. We find that for large enough values of the t'Hooft parameter (e.g. $N\simeq 25$) the holographic phase transition occurs, and it can force the Higgs to undergo a first order electroweak phase transition, suitable for electroweak baryogenesis. The model exhibits gravitational waves and colliders signatures. It typically predicts a stochastic gravitational wave background observable both at the Laser Interferometer Space Antenna and at the Einstein Telescope. Moreover the radion tends to be heavy enough such that it evades current constraints, but may show up in future LHC runs., Comment: 58 pages, 8 figures, 6 tables; v2 added references, comments added in Secs. 3, 9 and 10

We study the recently found anomalies in $B$-meson decays within a scenario with a warped extra dimension where the Standard Model fermions are propagating in the bulk. The anomalies are then interpreted as the result of the exchange of heavy vector resonances with electroweak quantum numbers. The model naturally leads to lepton-flavor universality violation when different flavor fermions are differently localized along the extra dimension, signaling a different degree of compositeness in the dual holographic theory., Comment: 6 pages, 8 figures. Talk given by E.Megias at the 20th International Conference in Quantum Chromodynamics (QCD 17), Montpellier, France, 3 - 7 Jul 2017

We explore the limits on lepton-flavor universality (LFU) violation in theories where the hierarchy problem is solved by means of a warped extra dimension. In those theories LFU violation, in fermion interaction with Kaluza-Klein modes of gauge bosons, is provided ab initio when different flavor of fermions are differently localized along the extra dimension. As this fact arises from the mass pattern of quarks and leptons, LFU violation is natural in this class of theories. We analyze the experimental data pointing towards LFU violation, as well as the most relevant electroweak and flavor observables, and the LFU tests in the $\mu/e$ and $\tau/\mu$ sectors. We find agreement with $R_{K^{(\ast)}}$ and $R_{D^{(\ast)}}$ data at 95% CL, provided the third generation left-handed fermions are composite ($0.14 < c_{b_L} < 0.28$ and $0.27 < c_{\tau_L} < 0.33$), and find the absolute limits $R_{K^{(\ast)}}\gtrsim 0.79$ and $R_{D^{(\ast)}}/R_{D^{(\ast)}}^{\rm SM}\lesssim 1.13$. Moreover we predict $\mathcal B( B\to K\nu\bar\nu)\gtrsim 1.14\times 10^{-5}$ at 95% CL, smaller than the present experimental upper bound but a few times larger than the Standard Model prediction., Comment: 6 pages, 4 figures; v2 added references, improved discussion

We study the possibility of explaining the recently found anomalies in B-meson decays within scenarios with a composite Higgs boson. This class of models provides a natural way to fit the experimental results, interpreting the anomalies as the result of the exchange of heavy vector resonances with electroweak quantum numbers. The anomalies are tightly related to deviations in Delta F = 2 transitions and to deformations of the Z and W couplings, whose size is of the order of the present experimental bounds. This leads to a very predictive scenario which could be soon tested at collider experiments., Comment: Proceedings of the Rencontres de Moriond EW 2017

Some anomalies in the processes $b\to s\ell\ell$ ($\ell=\mu,e$) and $b\to c \ell\bar\nu_\ell$ ($\ell=\tau,\mu,e$), in particular in the observables $R_K$ and $R_{D^{(\ast)}}$, have been found by the BaBar, LHCb and Belle Collaborations, leading to a possible lepton flavor universality violation. If these anomalies were confirmed they would inevitably lead to physics beyond the Standard Model. In this paper we try to accommodate the present anomalies in an extra dimensional theory, solving the naturalness problem of the Standard Model by means of a warped metric with a strong conformality violation near the infra-red brane. The $R_K$ anomaly can be accommodated provided that the left-handed bottom quark and muon lepton have some degree of compositeness in the dual theory. The theory is consistent with all electroweak and flavor observables, and with all direct searches of Kaluza-Klein electroweak gauge bosons and gluons. The fermion spectrum, and fermion mixing angles, can be reproduced by mostly elementary right-handed bottom quarks, and tau and muon leptons. Moreover the $R_{D^{(\ast)}}$ anomaly requires a strong degree of compositeness for the left-handed tau leptons, which turns out to be in tension with experimental data on the $g_{\tau_L}^Z$ coupling, possibly unless some degree of fine-tuning is introduced in the fixing of the CKM matrix., Comment: 40 pages, 20 figures; v2: Extended version with analyses of non-observation of FUV in the mu/e sector, LFU tests in tau decays, B->K nu nu and B->K tau tau; v3: Revised version where radiative corrections to observables $\tau\to\ell \nu\nu$ and the $Z\bar\ell\ell$ couplings ($\ell=\tau,\mu$), as well as the LHC dilepton resonance from bottom-bottom fusion, are considered. Some conclusions changed

Stop searches in supersymmetric frameworks with $R$-parity conservation usually assume the lightest neutralino to be the lightest supersymmetric particle. In this paper we consider an alternative scenario in which the left-handed tau sneutrino is lighter than neutralinos and stable at collider scales, but possibly unstable at cosmological scales. Moreover the (mostly right-handed) stop $\widetilde t$ is lighter than all electroweakinos, and heavier than the scalars of the third generation doublet, whose charged component, $\widetilde\tau$, is heavier than the neutral one, $\widetilde\nu$. The remaining supersymmetric particles are decoupled from the stop phenomenology. In most of the parameter space, the relevant stop decays are only into $t \widetilde\tau \tau$, $t\widetilde\nu\nu$ and $b \widetilde\nu \tau$ via off-shell electroweakinos. We constrain the branching ratios of these decays by recasting the most sensitive stop searches. Due to the "double invisible" kinematics of the $\widetilde t\to t\widetilde\nu\nu$ process, and the low efficiency in tagging the $t\widetilde\tau\tau$ decay products, light stops are generically allowed. In the minimal supersymmetric standard model with $\sim$ 100 GeV sneutrinos, stops with masses as small as $\sim$ 350 GeV turn out to be allowed at 95% CL., Comment: 25 pages

The experimental value of the anomalous magnetic moment of the muon, as well as the LHCb anomalies, point towards new physics coupled non-universally to muons and electrons. Working in extra dimensional theories, which solve the electroweak hierarchy problem with a warped metric, strongly deformed with respect to the AdS$_5$ geometry at the infra-red brane, the LHCb anomalies can be solved by imposing that the bottom and the muon have a sizable amount of compositeness, while the electron is mainly elementary. Using this set-up as starting point we have proven that extra physics has to be introduced to describe the anomalous magnetic moment of the muon. We have proven that this job is done by a set of vector-like leptons, mixed with the physical muon through Yukawa interactions, and with a high degree of compositeness. The theory is consistent with all electroweak indirect, direct and theoretical constraints, the most sensitive ones being the modification of the $Z\bar\mu\mu$ coupling, oblique observables and constraints on the stability of the electroweak minimum. They impose lower bounds on the compositeness ($c\lesssim 0.37$) and on the mass of the lightest vector-like lepton ($\gtrsim 270$ GeV). Vector-like leptons could be easily produced in Drell-Yan processes at the LHC and detected at $\sqrt{s}=13$ TeV., Comment: 42 pages, 15 figures; v2 added references

We study the extension of the Standard Model (SM) with a light dilaton in a five dimensional warped model. In particular, we analyze the coupling of the dilaton with the SM matter fields, compare the model predictions with Electroweak Precisions Tests and find the corresponding bounds on the mass of the lightest Kaluza-Klein modes. We also investigate the possibility that the Higgs-like resonance found at the LHC can be a dilaton. Finally, we show that our set-up can also provide an explanation of the anomalies recently observed in $B$-meson decays., Comment: 5 pages, 7 figures. Talk given by E.Megias at the 19th International Conference in Quantum Chromodynamics (QCD 16), Montpellier, France, 4 - 8 Jul 2016

We show that natural supersymmetry can be embedded in a five-dimensional theory with supersymmetry breaking \`a la Scherk-Schwarz (SS). There is no 'gluino-sucks' problem for stops localized in the four-dimensional brane and gluinos propagating in the full five-dimensional bulk, and sub-TeV stops are easily accommodated. The $\mu / B_\mu$ problem is absent as well; the SS breaking generates a Higgsino Dirac mass and no bilinear Higgs mass parameter in the superpotential is required. Moreover, for non-maximal SS twists leading to $\tan \beta \simeq 1$, the Higgs spectrum is naturally split, in agreement with LHC data. The 125-GeV Higgs mass and radiative electroweak symmetry breaking can be accommodated by minimally extending the Higgs sector with $Y=0$ $SU(2)_L$ triplets., Comment: 34 pages, 12 figures; v2, matches PRD published version

The anomalies recently found by the LHCb collaboration in $B$-meson decays seem to point towards the existence of new physics coupled non-universally to muons and electrons. We show that a beyond-the-Standard-Model dynamics with these features naturally arises in models with a warped extra-dimension that aim to solve the electroweak Hierarchy Problem. The attractiveness of our set-up is the fact that the dynamics responsible for generating the flavor anomalies is automatically present, being provided by the massive Kaluza--Klein excitations of the electroweak gauge bosons. The flavor anomalies can be easily reproduced by assuming that the bottom and muon fields have a sizable amount of compositeness, while the electron is almost elementary. Interestingly enough, this framework correlates the flavor anomalies to a pattern of corrections in the electroweak observables and in flavor-changing processes. In particular the deviations in the bottom and muon couplings to the $Z$-boson and in $\Delta F = 2$ flavor-changing observables are predicted to be close to the present experimental bounds, and thus potentially testable in near-future experiments., Comment: 33 pages, 14 figures, 1 table. References added

Extensions of the Standard Model Higgs sector with electroweak charged scalars can possess exotic `Higgs' bosons with vanishing or suppressed couplings to Standard Model fermions. These `fermiophobic' scalars, which cannot be produced via gluon fusion, are constrained by LHC measurements of the 125 GeV Higgs boson to have a small vacuum expectation value. This implies that vector boson fusion and associated vector boson production are in general suppressed rendering conventional Higgs searches insensitive. However, Drell-Yan Higgs pair production, which is not present in the SM, can be sizeable even in the limit of vanishing exotic Higgs vacuum expectation value. We utilize this to show that diphoton searches at 8 TeV LHC already rule out a large class of neutral fermiophobic Higgs bosons below around 110 GeV. This includes fermiophobic scalars found in two Higgs doublet as well as Higgs triplet and Georgi-Machacek type models. Our results extend the only relevant limit on fermiophobic Higgs bosons obtained by a recent CDF analysis of $4\gamma + X$ Tevatron data. Furthermore, diphoton limits are independent of the decay of the second Higgs boson and thus apply even for degenerate masses in contrast to the CDF search. We also find that if the fermiophobic Higgs has very enhanced couplings to photons, masses as large as $\sim 150$ GeV can be ruled out while if these couplings are somehow highly suppressed, masses below $\sim 90$ GeV can still be ruled out. Finally, we show that $WW$ and $ZZ$ diboson searches may serve as complementary probes for masses above the diphoton limit up to $\sim 250$ GeV and discuss prospects at 13 TeV LHC., Comment: references and small comments added

The Supersymmetric Custodial Triplet Model, a supersymmetric generalization of the Georgi-Machacek model, has proven to be an interesting modification of the MSSM. It extends the MSSM Higgs sector by three extra SU(2)L triplets in such a way that approximate custodial invariance is preserved and rho-parameter deviations are kept under control. By means of a sizeable triplet contribution to electroweak breaking the model is able to generate a barrier at tree level between the false vacuum and the electroweak one. This will result in a strong first order phase transition for an important region of the parameter space. We also look at the gravitational waves that could be generated as a result of the phase transition and show how future interferometers could be used as a probe of the model., Comment: 18 pages, 8 figures

We discuss the presence of a light dilaton in Conformal Field Theories deformed by a single scalar operator, in the holographic realization consisting of confining Renormalization Group flows. Then, we apply this formalism to study the extension of the Standard Model with a light dilaton in a 5D warped model. We study the spectrum of scalar and vector perturbations, compare the model predictions with Electroweak Precision Tests and find the corresponding bounds for the lightest modes. Finally, we analyze the possibility that the Higgs resonance found at the LHC be a dilaton., Comment: 6 pages, 3 figures. Presented by E.Megias at the 4th International Conference on New Frontiers in Physics (ICNFP 2015), 23-30 August 2015, Kolymbari, Crete, Greece