Electroweak baryogenesis above the electroweak scale

Conventional scenarios of electroweak (EW) baryogenesis are strongly constrained by experimental searches for CP violation beyond the SM. We propose an alternative scenario where the EW phase transition and baryogenesis occur at temperatures of the order of a new physics threshold far above the Fermi scale, say, in the 100-1000 TeV range. This way the needed new sources of CP-violation, together with possible associated flavor-violating effects, decouple from low energy observables. The key ingredient is a new CP- and flavor-conserving sector at the Fermi scale that ensures the EW symmetry remains broken and sphalerons suppressed at all temperatures below .We analyze a minimal incarnation based on a linear O(N) model. We identify a specific large-N limit where the effects of the new sector are vanishingly small at zero temperature while being significant at finite temperature. This crucially helps the construction of realistic models. A number of accidental factors, ultimately related to the size of the relevant SM couplings, force N to be above approximate to 100. Such a large N may seem bizarre, but it does not affect the simplicity of the model and in fact it allows us to carry out a consistent re-summation of the leading contributions to the thermal effective potential. Extensions of the SM Higgs sector can be compatible with smaller values N approximate to 20-30.Collider signatures are all parametrically suppressed by inverse powers of N and may be challenging to probe, but present constraints from direct dark matter searches cannot be accommodated in the minimal model. We discuss various extensions that satisfy all current bounds. One of these involves a new gauge force confining at scales between approximate to 1 GeV and the weak scale.

Electroweak and Higgs Physics at Very High Energy

Lorenzo Ricci

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-

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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.

EPFL2022

Effective Field Theories for New and Old Physics

Alfredo Glioti

Effective Field Theories have changed our understanding of Quantum Field Theories. This thesis shows several applications of this powerful tool in the context of the Standard Model and for searches of New Physics.The thesis starts with a review of the Standard Model and its open questions and is followed by an updated and systematic study of models of flavor in the context of Partial Compositeness in Composite Higgs theories. Following that, the question on how to measure the Wilson coefficients of the Standard Model effective operators at present and future experiments is addressed: first by using modern Machine Learning techniques by studying angular distributions for diboson production, followed then by a study on ElectroWeak radiation at a future Muon Collider and how to use it to better probe the new physics parameter space.The fourth chapter deals instead with applying Non-Relativistic Effective Theories to the study of exotic mesons in the Standard Model. The two competing interpretations, a molecule formed of two mesons or a compact tetraquark state, and their consequences are studied. In particular this study is done on the X(3872) exotic charmonium and the consequences of the two accidental tunings of this system are discussed.The last chapter addresses the problem of baryogenesis from the ElectroWeak phase transition. A new scalar sector is introduced that decouples the physics responsible for the generation of the baryon asymmetry from the weak scale. This helps solving the main problems that ElectroWeak baryogenesis models face, namely the large modifications to the Higgs physics and the need of large CP violating new effects.

EPFL2022

Non-perturbative processes with Fermion number non-conservation in different models of particle physics

Yannis Burnier

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.

EPFL2006

Electroweak and Higgs Physics at Very High Energy

Lorenzo Ricci

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EPFL2022

Non-perturbative processes with Fermion number non-conservation in different models of particle physics

Yannis Burnier

EPFL2006