Effective Field Theories for New and Old Physics

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.

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-

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.

EPFL2022

B°s --> J/[psi][eta]' decay and sensitivity to the Bs mixing phase at LHCb

Sergio Jiménez-Otero

The LHCb experiment is one of four main experiments to be setup at IP8 point of the proton-proton Large Hadron Collider (LHC) at CERN, Geneva. The data taking is scheduled to start end 2007. The LHCb detector is a forward single-arm spectrometer conceived to study CP violation and rare phenomena in the b-quark sector with very high precision. LHCb should provide a profound understanding of flavour physics in the framework of Standard Model (SM), so that the SM will be over-constrained, hopefully, it will exhibit some inconsistencies which may reveal a sign of New Physics beyond the SM. The physics studies being considered by the LHCb collaboration offer the possibility of furthering in the SM description of CP violation. The SM provides the theoretical framework for the violation of the CP symmetry in neutral B mesons as well as in neutral K mesons. The neutral Bs0 - Bs0 system stays at a privileged position in the quest for CP violation evidences. The Bs0 - Bs0 mixing phase, φs, has never been measured. This mixing phase has its origins in the interference between the decay and the mixing of those neutral mesons. b → ccs quark-level transitions decaying to CP eigenstates, may directly probe this electroweak phase by performing a time-dependent measurement of mixing-induced CP violation. Among those quark level transitions, we have the golden-plated channel to perform such a measurement that is Bs0 → J/ψφ and that requires an angular analysis to disentangle its CP eigenstates components. On the other hand, b → ccs transitions to pure CP eigenstates provide another probe to the φs not requiring such angular studies. In this dissertation we will be consider the Bs0 → J/ψ(μ+μ-) η' (π+π-η(γγ)) decays which are b → ccs transitions to pure CP even eigenstates. The reconstruction and selection of those decays will be presented here using full Monte Carlo simulations. An annual event yield of ∼ 2000 is assured with a background-over-signal level close to unity. The mass and proper time resolution will be also presented and some methods searching to improve those performances. In the last chapter of the dissertation, we will also present a study of the statistical sensitivity of Bs0 → J/ψ(μ+μ-) η' (π+π-η(γγ)) to the Bs0 - Bs0 mixing parameters, using a fast parameterised Monte Carlo simulation. Those simulations take into account in a realistic manner the outputs from the full Monte Carlo simulations. Specifically, we will be simulate the background-over-signal levels, the time dependent acceptance efficiency and tagging and reconstruction performances for that decay mode. It will also take as inputs the event-by-event proper time errors obtained from the full MC. The likelihood fit performed to extract the physics parameters requires a flavour-specific control sample, Bs0 → Ds π, which allows for the extraction of the Bs0 - Bs0 mixing frequency, ΔMs. The sensitivity achieved by our channel decay will be then combined to other b → ccs quark level transitions to pure CP eigenstates and finally also to the Bs0 → J/ψφ decay to admixture of CP eigenstates. The sensitivity obtained from the pure CP modes is σ(φs) = 0.056 rad whereas if we combine all decay channels into a single sensitivity measurement we achieve σ(φs) = 0.021 rad. The contribution from the pure CP modes is non-negligible, ∼ 14%, so that it is expected that these channels will certainly help in the measurement of φs.

EPFL2007

Magnetic fields at first order phase transition: a threat to electroweak baryogenesis

Andrea De Simone

The generation of the observed baryon asymmetry may have taken place during the electroweak phase transition, thus involving physics testable at LHC, a scenario dubbed electroweak baryogenesis. In this paper we point out that the magnetic field which is produced in the bubbles of a first order phase transition endangers the baryon asymmetry produced in the bubble walls. The reason being that the produced magnetic field couples to the sphaleron magnetic moment and lowers the sphaleron energy; this strengthens the sphaleron transitions inside the bubbles and triggers a more effective wash out of the baryon asymmetry. We apply this scenario to the Minimal Supersymmetric extension of the Standard Model (MSSM) where, in the absence of a magnetic field, successful electroweak baryogenesis requires the lightest CP-even Higgs and the right-handed stop masses to be lighter than about 127 GeV and 120 GeV, respectively. We show that even for moderate values of the magnetic field, the Higgs mass required to preserve the baryon asymmetry is below the present experimental bound. As a consequence electroweak baryogenesis within the MSSM should be confronted on the one hand to future measurements at the LHC on the Higgs and the right-handed stop masses, and on the other hand to more precise calculations of the magnetic field produced at the electroweak phase transition.

2011