Stefania De Curtis

The 4D Composite Higgs

A bstractWe propose a four dimensional description of Composite Higgs Models which represents a complete framework for the physics of the Higgs as a pseudo-Nambu-Goldstone boson. Our setup captures all the relevant features of 5D models and more in general of composite Higgs models with partial compositeness. We focus on the minimal scenario where we include a single multiplet of resonances of the composite sector, as these will be the only degrees of freedom which might be accessible at the LHC. This turns out to be sufficient to compute the effective potential and derive phenomenological consequences of the theory. Moreover our simplified approach is well adapted to simulate these models at the LHC. We also consider the impact of non-minimal terms in the effective lagrangian which do not descend from a 5D theory and could be of phenomenological relevance, for example contributing to the S − parameter.

Framework for model independent analyses of multiple extra quark scenarios

A bstractIn this paper we present an analysis strategy and a dedicated tool to determine the exclusion confidence level for any scenario involving multiple heavy extra quarks with generic decay channels, as predicted in several extensions of the Standard Model. We have created, validated and used a software package, called XQCAT (eXtra Quark Combined Analysis Tool), which is based on publicly available experimental data from direct searches for top partners and from Supersymmetry inspired searches. By means of this code, we recast the limits from CMS on new heavy extra quarks considering a complete set of decay channels. The resulting exclusion confidence levels are presented for some simple scenarios with multiple states and general coupling assumptions. Highlighting the importance of combining multiple topology searches to obtain accurate re-interpretations of the existing searches, we discuss the reach of the SUSY analyses so as to set bounds on new quark resonances. In particular, we report on the re-interpretation of the existing limits on benchmark scenarios with one and multiple pair-produced top partners having non-exclusive couplings to the third Standard Model generation of quarks.

Nearby resonances beyond the Breit-Wigner approximation

We consider a description of propagators for particle resonances which takes into account the quantum-mechanical interference due to the width of two or more nearby states that have common decay channels, by incorporating the effects arising from the imaginary parts of the one-loop self-energies. Depending on the couplings to the common decay channels, the interference effect, not taken into account in the usual Breit–Wigner approximation, can significantly modify the cross section or make the more long-lived resonance narrower. We give few examples of New Physics models for which the effect is sizable, namely a generic two and multiple Higgs model and neutral vector resonances in Higgsless models. Based on these results we suggest the implementation of a proper treatment of nearby resonances into Monte Carlo generators.

Interference effects in heavy W'-boson searches at the LHC

Interference effects are widely neglected in searches for new physics. This is the case in recent publications on searches for W?-bosons using leptonic final states. We examine the effects of interference on distributions frequently used to determine mass limits for possible W?-bosons and show that there are important qualitative effects on the behaviour of the new physics signal. There are two main consequences. The first is that current exclusion limits, where interferences effects have not been considered, are likely to have been overestimated. The second is that the way observed limits on the cross-section are presented, in terms of the contribution of new physics to the total cross-section, is not meaningful.

Drell-Yan production at the LHC in a four site Higgsless model

We consider a four site Higgsless model based on the SU(2){sub L}xSU(2){sub 1}xSU(2){sub 2}xU(1){sub Y} gauge symmetry, which predicts two neutral and four charged extra gauge bosons, Z{sub 1,2} and W{sub 1,2}{sup {+-}}. We compute the properties of the new particles, and derive indirect and direct limits on their masses and couplings from the CERN LEP and Tevatron data. In contrast to other Higgsless models, characterized by fermiophobic extra gauge bosons, here sizeable fermion-boson couplings are allowed by the electroweak precision data. The prospects of detecting the new predicted particles in the favored Drell-Yan channel at the CERN LHC are thus investigated. The outcome is that all six extra gauge bosons could be discovered in the early stage of the LHC low-luminosity run.

Strong tree level unitarity violations in the extra dimensional standard model with scalars in the bulk

We show how the tree-level unitarity violations of compactified extra dimensional extensions of the standard model become much stronger when the scalar sector is included in the bulk. This effect occurs when the couplings are not suppressed for larger Kaluza-Klein levels, and could have relevant consequences for the phenomenology of the next generation of colliders. We also introduce a simple and generic formalism to obtain unitarity bounds for finite energies, taking into account coupled channels including the towers of Kaluza-Klein excitations.

UV-complete composite Higgs models

We study confining gauge theories with fermions vectorial under the Standard Model (SM) that produce a Higgs doublet as a Nambu-Goldstone boson. The vacuum misalignment required to break the electroweak symmetry is induced by an elementary Higgs doublet with Yukawa couplings to the new fermions. The physical Higgs is a linear combination of elementary and composite Higgses while the SM fermions remain elementary. The full theory is renormalizable and the SM Yukawa couplings are generated from the ones of the elementary Higgs allowing to eliminate all flavor problems but with interesting effects for electric dipole moments of SM particles. We also discuss how ideas on the relaxation of the electroweak scale could be realized within this framework.

On the complementarity of Higgs and radion searches at LHC

Models with 3-branes in extra dimensions typically imply the existence of a radion, φ, that can mix with the Higgs, h, thereby modifying the Higgs properties and the prospects for its detectability at the LHC. The presence of the φ will extend the scope of the LHC searches. Detection of both the φ and the h might be possible. In this paper, we study the complementarity of the observation of gg → h, with h → γγ or h → Z 0 Z 0� → 4 l, and gg → φ → Z 0 Z 0(�) → 4 l at the LHC in the context of the Randall-Sundrum model. The potential for determining the nature of the detected scalar(s) at the LHC and at an e + e linear collider is discussed, both separately and in combination.

The equivalence theorem for gauge boson scattering in a five-dimensional Standard Model

We present an equivalence theorem for the longitudinal components of the gauge bosons in a compactified five-dimensional extension of the Standard Model, whose spontaneous symmetry breaking is driven either by one Higgs in the bulk or by one on a brane or by both together. We also show some implications for the unitarity bounds on Higgs masses.

Effects of custodial symmetry breaking in the Georgi-Machacek model at high energies

The model proposed by Georgi and Machacek enables the Higgs sector to involve isospin triplet scalar fields while retaining a custodial