Title: Probing the electroweak sector of the μνSSM at the LHC
Abstract: The discovery of the Higgs boson at the Large Hadron Collider (LHC) constitutes the confirmation of the existence of the last building block of the Standard Model (SM) of particle physics. However, there are still important questions unanswered by this theoretical framework, such as for example the origin of neutrino masses, the nature of dark matter, the origin of the baryon asymmetry of the Universe or the strong CP problem. Many proposals have appeared in the last decades to address some of these questions, with the general feature that the extensions Beyond the Standard Model (BSM) should somehow manifest at energy scales no too far from the Electroweak Scale (EW). This constitutes an invitation to explore experimentally the physics at the TeV scale. Nevertheless, the range of possibilities to extend the standard model is huge, each of them motivated by different theoretical paradigms or experimental hints.
The experimental road-map at energies above the EW scale for the next decades consists of the exploitation of the full capabilities of the LHC, including the High Luminosity (HL) phase, and the addition of new detectors focused on specific signals hard to measure by the actual experiments, such as very long-lived particles. Also there are plans to build the International Linear Collider (ILC) in Japan, an electron-positron collider with energies of 500 GeV. Therefore, in the present and in the nearest future, the possibilities to test any BSM theory in terms of its influence on particle phenomenology are constrained to collider experiments around few TeV scale. Nevertheless, this is enough to reach a plethora of models capable of solving problems of the SM and leave a signal in collider experiments, both direct and indirect. Thus each model deserves a detailed analysis of the expected phenomenology, in order to be able to design efficient searches covering the most of the models and to differentiate between them in the case that a signal is finally detected.
TeV scale supersymmetry (SUSY) has been perhaps the most popular choice in the past decades to extend the SM at the EW scale. Supersymmetric extensions of the SM have proven themselves very useful solving the hierarchy problem and providing also a Dark Matter (DM) candidate. SUSY theories are in addition quite appealing since they make the gauge couplings become unified at some high energy scale after the inclusion of the effects of SUSY on the renormalization group equations.
SUSY predicts the existence of a new particle for each one in the Standard Model, with the same quantum numbers excepting the spin. Thus it predicts a scalar partner for each fermion, and a fermionic partner for each boson, and the masses of these particles are highly motivated to be around the TeV scale. Consequently, it predicts a large number of new particles that could be within the experimental reach of the LHC or the next generation of colliders.
In this thesis we discuss the phenomenological aspects of a SUSY model, the so-called called μνSSM, at the LHC. Besides the good common properties of the SUSY models, the μν can provide a solution of the μ-problem of the Minimal Supersymmetric Standard Model (MSSM) and simultaneously explain the origin of the measured properties of neutrinos. In addition, the Gravitino in the μνSSM is a good DM candidate also.
In the SUSY extensions of the SM the baryon number is no longer an accidental symmetry and should be achieved imposing a discrete symmetry to prevent fast decay of the proton. This role is usually played by \emph{R-parity} conservation (RPC), which forces each superpartner to be pair produced. The direct consequence of this is that the \emph{lightest supersymmetric particle} (LSP) would be stable. There exist cosmological constraints which bound this particle to be neutral, and play the role of the dark matter. At colliders, this family of models would produce events with neutral stable particles which manifest themselves as events with large missing transverse momentum (MET).
Instead, the μνSSM includes terms in the Lagrangian which do not allow for a proper assignment of R-parity charges and thus this symmetry is explicitly violated (RPV). Since the superpartners can decay to SM particles, the expected phenomenology is richer and should be carefully studied. Altough there exist other RPV models, the structure of the μνSSM is complex and leads to unique signals.
The objective of this thesis is to make an exhaustive exploration of interesting signals within the μνSSM framework. In particular, we focus on the electroweak sector of the model, which in principle can be explored with a certain ease in a hadron collider, and identify the signatures that can appear in the production of electroweak superpartners at the LHC. The text is organized as follows. The first chapter consist on an introduction and summary of the contents of the thesis. Chapter 2 is a brief introduction to SUSY, discussing the most important characteristics of the theoretical framework and presenting the most popular models, with its limitations.
In chapter 3, we introduce the μνSSM, describing its most important features and summarizing the previous phenomenological studies. Chapter 4 is devoted to an analysis of the most important phenomenological aspects of the left-handed neutrino superpartner, the left sneutrino, when it is the LSP. There we show the possible signals that can be generated at colliders when producing this particle. In chapter 5, we choose a selection of benchmark points representative of the left sneutrino phenomenology, that can produce detectable prompt signals at the LHC. In chapter 6, we use some of the current ATLAS searches for long lived particles to constrain the parameter space of the model, when the left sneutrino is the LSP and decays at significant distance of the interaction point, producing displaced vertices. In chapter 7, we utilize the ATLAS searches for electroweak production of SUSY particles in compressed scenarios, to obtain limits on the sneutrino mass, when the LSP is a bino-like neutralino and the sneutrino is the NLSP. In addition, we explain that the process described is compatible with the local excess of three leptons recently reported by the ATLAS collaboration.
Chapter 8 concludes the thesis summarizing the main results and outlining the future prospects of research.
Publication Year: 2019
Publication Date: 2019-01-22
Language: en
Type: dissertation
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Cited By Count: 1
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