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Concept# Higgs boson

Summary

The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. In the Standard Model, the Higgs particle is a massive scalar boson with zero spin, even (positive) parity, no electric charge, and no colour charge that couples to (interacts with) mass. It is also very unstable, decaying into other particles almost immediately upon generation.
The Higgs field is a scalar field with two neutral and two electrically charged components that form a complex doublet of the weak isospin SU(2) symmetry. Its "Mexican hat-shaped" potential leads it to take a nonzero value everywhere (including otherwise empty space), which breaks the weak isospin symmetry of the electroweak interaction and, via the Higgs mechanism, gives mass to many particles.
Both the field and the boson are named after

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PHYS-416: Particle physics II

Presentation of the electroweak and strong interaction theories that constitute the Standard Model of particle physics. The course also discusses the new theories proposed to solve the problems of the Standard Model.

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Presentation of particle properties, their symmetries and interactions.
Introduction to quantum electrodynamics and to the Feynman rules.

PHYS-432: Quantum field theory II

The goal of the course is to introduce relativistic quantum field theory as the conceptual and mathematical framework describing fundamental interactions.

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Effective Field Theories (EFTs) allow a description of low energy effects of heavy new physics Beyond the Standard Model (BSM) in terms of higher dimensional operators among the SM fields. EFTs are not only an elegant and consistent way to describe heavy new physics but they represent, at the same time, a valuable experimental tool for collider searches. The Standard Model Effective Field Theory naturally parametrizes the space of models BSM and measuring its interactions is, nowadays, substantial part of the theoretical and the experimental program at the (HL-)LHC and at future colliders. In this thesis we address the theoretical challenges of this Beyond the Standard Model precision program, following three different paths.Firstly, we present some results towards the so-called high-$p_T$ program at the (HL-)LHC, targeting to measure energy growing effects of higher dimensional operators in the tail of kinematic distributions. Concretely, we focus on dilepton production and we study the sensitivity to flavor universal dimension-six operators interfering with the SM and enhanced by the energy. We produce theoretical predictions for the SM and the dim-6 EFT operators at NLO-QCD, including 1-loop EW logs. Our predictions are based on event reweighting of SM Montecarlo simulations and allow an easy scan of the multi-dimensional new physics parameter space on data. Furthermore we asses the impact of the various sources of theoretical uncertainties and we study the projected sensitivity of (HL-)LHC to the EFT interactions under consideration and to concrete BSM scenario.We then turn to future colliders and in particular to very high energy lepton colliders. In this context we study the potential of such machines with about 10 TeV center of mass energy to probe Higgs, ElectroWeak and Top physics at 100 TeV via precise measurements of EFT interactions. A peculiar aspect of so energetic ElectroWeak processes is the prominent phenomenon of the EW radiation. On one hand we find that consistent and sufficiently accurate predictions require resummations, that we perform at double logarithmic order. On the other hand we show how the study of the radiation pattern can enhance the sensitivity to new physics. We assess our results in Composite Higgs and Top scenarios and minimal Z' models.Finally, we move to a top-down perspective and we perform a phenomenological study of composite Higgs models with partially composite Standard Models quarks. Starting from maximally symmetric scenarios that realize minimal flavor violation, we test various assumptions for the flavor structure of the strong sector. Among the different models we consider, we find that there is an optimal amount of symmetries that protects from (chromo-)electric dipoles and reduces, at the same time, constraints from other flavor observables.

Matthieu Philippe Luther Marinangeli

New massive long-lived particles (LLP) are predicted by multiple beyond the Standard Model theories. This thesis presents the search of such long-lived particles decaying leptonically in the $e \mu \nu$ final state. The search is performed using 2.31 fb$^{-1}$ of proton-proton collisions collected by the LHCb detector at a center-of-mass energy of $\sqrt{s} = 13$ TeV, seeking for displaced vertices of electrons and muons of opposite charges.
LLPs with masses between 7 to $50$ GeV/c$^2$, and lifetime between 2 and 50 ps, are explored in this search.
Three kinds of LLP production modes are considered: the direct pair production of LLPs from quark interactions, the pair production from a Standard Model like Higgs boson with a mass of $125$ GeV/c$^2$, and charged current production from an on-shell $W$ boson with an additional lepton.
No evidence of these long-lived states has been observed, upper limits at $95 \%$ CL on the production cross-section times branching ratio have been set on the different production mode.

In the context of warped extra-dimensional models which address both the Planck-weak- and flavor-hierarchies of the Standard Model (SM), it has been argued that certain observables can be calculated within the 5D effective field theory only with the Higgs field propagating in the bulk of the extra dimension, just like other SM fields. The related studies also suggested an interesting form of decoupling of the heavy Kaluza-Klein (KK) fermion states in the warped 5D SM in the limit where the profile of the SM Higgs approaches the IR brane. We demonstrate that a similar phenomenon occurs when we include the mandatory KK excitations of the SM Higgs in loop diagrams giving dipole operators for SM fermions, where the earlier work only considered the SM Higgs (zero mode). In particular, in the limit of a quasi IR-localized SM Higgs, the effect from summing over KK Higgs modes is unsuppressed (yet finite), in contrast to the naive expectation that KK Higgs modes decouple as their masses become large. In this case, a wide range of KK Higgs modes have quasi-degenerate masses and enhanced couplings to fermions relative to those of the SM Higgs, which contribute to the above remarkable result. In addition, we find that the total contribution from KK Higgs modes in general can be comparable to that from the SM Higgs alone. It is also interesting that KK Higgs couplings to KK fermions of the same chirality as the corresponding SM modes have an unsuppressed overall contribution, in contrast to the result from the earlier studies involving the SM Higgs. Our studies suggest that KK Higgs bosons are generally an indispensable part of the warped 5D SM, and their phenomenology such as signals at the LHC are worth further investigation.