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Publication# A Strong Sector at the LHC: Top Partners in Same-Sign Dileptons

Résumé

Heavy partners of the top quark are a common prediction of many models in which a new strongly-coupled sector is responsible for the breaking of the electroweak symmetry. In this paper, we investigate their experimental signature at the LHC, focusing on the particularly clean channel of same-sign dileptons. We show that, thanks to a strong interaction with the top quark which allows them to be singly produced at a sizable rate, the top partners will be discovered at the LHC if their mass is below 1.5 TeV, higher masses being possible in particularly favorable (but plausible) situations. Since the partners are expected to be lighter in both the Higgsless and composite-Higgs scenarios, then one of same-sign dileptons is found to be a very promising channel in which these models could be tested. We also discuss several experimental signatures which would allow, after the discovery of the excess, to attribute it uniquely to the top partners production and to measure the relevant physical parameters, i.e. the top partners' masses and couplings. We believe that our results constitute a valid starting point for a more detailed experimental study.

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L’interaction électrofaible, aussi appelée force électrofaible, est la description unifiée de deux des quatre interactions fondamentales de l'univers, à savoir l'électromagnétisme (appelé électrodynam

Quark top

Le quark top (en abrégé t) est un quark, une particule élémentaire de la physique des particules.
Propriétés
vignette|300px|Diagramme de désintégration des quarks.
Comme tous les quarks,

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Kyrylo Bondarenko, Pavlo Kashko

We consider the effective interaction of quarks with a new GeV-scale vector particle that couples to electroweak gauge bosons by the so-called effective Chern-Simons (CS) interaction. We call this particle the CS boson. We construct effective Lagrangian of the CS boson interaction with quarks of two different flavors. This interaction is given by a divergent loop diagram, however, it turns out that the divergent part is equal to zero as a consequence of the CKM matrix unitarity in the SM. Therefore, we are able to predict effective interaction of the CS boson with quarks of different flavors without introducing new unknown parameters to the model, using only parameters of the initial effective Lagrangian. Our result shows that the effective interaction of the CS boson with down-type quarks is sufficiently stronger compared with up-type quarks. Based on our results, we give a prediction for the production of CS bosons in mesons decays. Branching fractions were obtained for the main reactions of the CS production in meson decays. The results obtained will be useful for searching for the long-lived GeV-scale CS boson in intensity frontier experiments.

When a classical conservation law is broken by quantum corrections, the associated symmetry is said to be anomalous. This type of symmetry breaking can lead to interesting physics. For instance in strong interactions, the anomaly in the chiral current is important in the pion decay to two photons. In weak interactions, there is an anomaly in the baryon number current. Although anomalous baryon number violating transitions are strongly suppressed at small energies, they could be at the origin of the baryon asymmetry of the universe. In this thesis, we consider several issues related to the theoretical and phenomenological aspects of anomalies. Although our main aim is the study of the electroweak theory, most of the theoretical questions do not rely on its precise setup. In order to solve these problems, we design a 1+1 dimensional chiral Abelian Higgs model displaying similar nonperturbative physics as the electroweak theory and leading to many simplifications. This model contains sphaleron and instanton transitions and, as the electroweak theory, leads to anomalous fermion number nonconservation. The one-loop fermionic contribution to the probability of an instanton transition with fermion number violation is calculated in the chiral Abelian Higgs model where the fermions have a Yukawa coupling to the scalar field. These contributions are given by the determinant of the fermionic fluctuations. The dependence of the determinant on fermionic, scalar and vector mass is determined. We also show in detail how to renormalize the fermionic determinant in partial wave analysis. The 1+1 dimensional model has the remarkable property to enable the creation of an odd number of fractionally charged fermions. We point out that for 1+1 dimensions this process does not violate any symmetries of the theory, nor does it lead to any mathematical inconsistencies. We construct the proper definition of the fermionic determinant in this model and underline its non-trivial features that are of importance for realistic 3+1 dimensional models with fermion number violation. In theories with anomalous fermion number nonconservation, the level crossing picture is considered a faithful representation of the fermionic quantum number variation. It represents each created fermion by an energy level that crosses the zero-energy line from below. If several fermions of various masses are created, the level crossing picture contains several levels that cross the zero-energy line and cross each other. However, we know from quantum mechanics that the corresponding levels cannot cross if the different fermions are mixed via some interaction potential. The simultaneous application of these two requirements on the level behavior leads to paradoxes. For instance, a naive interpretation of the resulting level crossing picture gives rise to charge nonconservation. We resolve this paradox by a precise calculation of the transition probability, and discuss what are the implications for the electroweak theory. In particular, the nonperturbative transition probability is higher if top quarks are present in the initial state. Coming back to the electroweak theory, we point out that the results of many baryogenesis scenarios operating at or below the TeV scale are rather sensitive to the rate of anomalous fermion number violation across the electroweak crossover. Assuming the validity of the Standard Model of electroweak interactions, we estimate this rate for experimentally allowed values of the Higgs mass (mH = 100…300 GeV). We also discuss where the rate enters in the particle density evolution and how to compute the leading baryonic asymmetry.

This thesis presents a general discussion of the Composite Higgs scenario of Electro-Weak Symmetry Breaking (EWSB). We start by reviewing the Standard Model of Electro-Weak interaction, discussing its experimental tests and conceptual pitfalls. Emphasis is given to the effective field theory point of view. In particular, the inherent tension related to the stability of the Electro-Weak scale motivates us to explore the possibility of having the Higgs field emerging as a Nambu-Goldstone boson from a new strongly coupled sector. Our construction is to a large extent inspired by the picture of the long range dynamics of QCD. The main ingredients are the symmetry of the UV theory, the pattern of its spontaneous breakdown and the sources of explicit breaking. In QCD, the latter are provided by the light quark masses and by the electromagnetic interaction. In Composite Higgs models, the most relevant symmetry breaking couplings are those related to the generation of the third family quark Yukawas through partial compositeness. They generate a potential for the Higgs and thus trigger EWSB. The constraints on the scenario are exposed, with a particular emphasis on the composite Two Higgs Doublet Model (THDM). While a residual SO(4) symmetry is sufficient to ensure a realistic phenomenology in presence of a single composite Higgs doublet, an extended Higgs sector needs more symmetries. For two doublets we show how either CP or a ℤ2 symmetry can play this role and construct a model for each realisation relying on the SO(6)/SO(4) × SO(2) coset. Finally, we discuss the phenomenology of this scenario. In particular, we present de differences between an elementary and a composite THDM. We also conclude that composite fermions associated to the third family quarks seem to be the most promising experimental handles for these models. We discuss their discovery range at the LHC, and the possibility of measuring the structure of their couplings. This knowledge would allow important insight into the strong dynamics.