A composite light scalar, electro-weak symmetry breaking and the recent LHC searches

We construct a model in which electro-weak symmetry breaking is induced by a strongly coupled sector, which is described in terms of a five-dimensional model in the spirit of the bottom-up approach to holography. We compute the precision electro-weak parameters, and identify regions of parameter space allowed by indirect tests. We compute the spectrum of scalar and vector resonances, which contains a set of parametrically light states that can be identified with the electroweak gauge bosons and a light dilaton. There is then a little desert, up to 2-3 TeV, where towers of resonances of the vector, axial-vector and scalar particles appear.Comment: Version accepted for publication. Two footnotes and one reference adde

A light scalar from walking solutions in gauge-string duality

We consider the type-IIB background generated by the strong-coupling limit of Nc D5 branes wrapped on S2, and focus our attention on a special class of solutions that exhibit walking behavior. We compute numerically the spectrum of scalar fluctuations around vacua of this class. Besides two cuts, and sequences of single poles converging on one of the branch points, the spectrum contains one isolated scalar, the mass of which is suppressed by the length of the walking region. Approximate scale-invariance symmetry in the walking region suggests that this be interpreted as a light dilaton, the pseudo-Goldstone boson of dilatations.Comment: 4 pages, 3 figures. Superpotential formalism replaced with effective potential. Main results unchange

Aspects of Gauge-Gravity Duality

In this Ph.D. thesis, we study various backgrounds in Type IIB supergravity which admit interpretations in terms a dual field theory, and compute properties such as effective potentials and spectra, using both holographic and field theoretic methods. First, we study the phase structure of beta-deformed N=4 SYM on S3 at weak and strong 't Hooft coupling. We compute the one-loop effective potential, and find that at near critical chemical potential and small finite temperature, there is a metastable state at the origin of moduli space. We derive the gravitational background describing the theory at strong coupling, and by performing a probe-brane calculation, we find qualitative agreement between the weak and strong coupling results. Next, we study gravitational backgrounds obtained by wrapping Nc D5 color branes on an S2 inside a CY3-fold, and Nf D5 backreacting flavor branes on a non-compact two-cycle inside the same CY3-fold. These backgrounds are believed to be dual to certain SQCD-like theories. We compute how the spectrum depends on the number of flavors, and find that the mass of the lightest scalar glueball increases with the number of flavors until the point Nf=2Nc is reached after which the opposite behaviour is observed. Finally, we consider a class of backgrounds that exhibit walking behaviour, i.e. a suitably defined four-dimensional gauge coupling stays nearly constant in an intermediate energy regime. The breaking of approximate scale invariance has been conjectured to lead to the existence of a light scalar in the spectrum. This so-called dilaton would be the pseudo-Goldstone boson of dilatations. Using holographic techniques, we compute the spectrum and find a light state whose mass is suppressed by the length of the walking region, suggesting that this might be the dilaton.Comment: Ph.D. Thesis. 119 pages. 13 figure

Glueballs on the baryonic branch of Klebanov-Strassler: dimensional deconstruction and a light scalar particle

Within gauge/gravity duality, we compute the scalar and tensor mass spectrumin the boundary theory defined by the five-dimensional sigma-model coupled to gravityobtained by constraining to eight scalars the truncation on T(1,1) that corresponds to thePapadopoulos-Tseytlin (PT) ansatz. We study fluctuations around the 1-parameter familyof backgrounds that lift to the baryonic branch of the Klebanov-Strassler (KS) system,and interpolates between the KS background and the Maldacena-Nunez one (CVMN). Weadopt a gauge invariant formalism in the treatment of the fluctuations that we interpret asstates of the dual theory. The tensor spectrum interpolates between the discrete spectrumof the KS background and the continuum spectrum of the CVMN background, in particularshowing the emergence of a finite energy range containing a dense set of states, as expectedfrom dimensional deconstruction. The scalar spectrum shows analogous features, and inaddition it contains one state that becomes parametrically light far from the origin alongthe baryonic branch

Towards top-down holographic composite Higgs: minimal coset from maximal supergravity

Within the context of top-down holography, we study a one-parameter family of regular background solutions of maximal gauged supergravity in seven dimensions, dimensionally reduced on a 2-torus. The dual, four-dimensional confining field theory realises the global (spontaneous as well as explicit) symmetry breaking pattern SO(5) → SO(4). We compute the complete mass spectrum for the fluctuations of the 128 bosonic degrees of freedom of the five-dimensional gravity theory, which correspond to scalar, pseudoscalar, vector, axial-vector, and tensor bound states of the dual field theory, and includes particles with exotic SO(4) quantum numbers. We confirm the existence of tachyonic instabilities near the boundaries of the parameter space.We discuss the interplay between explicit and spontaneous symmetry breaking. The SO(5)/SO(4) coset might provide a first step towards the realisation of a calculable frame- work and ultraviolet completion of minimal composite Higgs models, if the four pseudo- Nambu-Goldstone bosons are identified with the real components of the Higgs doublet in the standard model (SM), and a subgroup of SO(4) with the SU(2)×U(1) SM gauge group. We exhibit an example with an additional localised boundary term that mimics the effect of a weakly-coupled external sector

Hyperscaling violation and electroweak symmetry breaking

We consider a class of simplified models of dynamical electroweak symmetry breaking built in terms of their five-dimensional weakly-coupled gravity duals, in the spirit of bottom-up holography. The sigma-model consists of two abelian gauge bosons and one real, non-charged scalar field coupled to gravity in five dimensions. The scalar potential is a simple exponential function of the scalar field. The background metric resulting from solving the classical equations of motion exhibits hyperscaling violation, at least at asymptotically large values of the radial direction. We study the spectrum of scalar composite states of the putative dual field theory by fluctuating the sigma-model scalars and gravity, and discuss in which cases we find a parametrically light scalar state in the spectrum. We model the spontaneous breaking of the (weakly coupled) gauge symmetry to the diagonal subgroup by the choice of IR boundary conditions. We compute the mass spectrum of spin-1 states, and the precision electroweak parameter S as a function of the hyperscaling coefficient. We find a general bound on the mass of the lightest spin-1 resonance, by requiring that the indirect bounds on the precision parameters be satisfied, that implies that precision electroweak physics excludes the possibility of a techni-rho meson with mass lighter than several TeV