Strongly Interacting Electroweak Symmetry Breaking Sector

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Any: 2014, Tutor: Domènec EspriuUsing the equivalence theorem we study the scattering of longitudinally polarized gauge bosons in extensions of the Standard Model where anomalous Higgs couplings to gauge sector and higher order O(p4) operators are considered. Forcing the amplitudes to be unitary we nd new resonances for di erent values of the coupling constants. Hereby we would be able to narrow the range of values of the chiral parameters with new experiments at LHC

Unitarisation of WW Scattering and Interpretation of a 2 TeV Resonance in a Strongly Interacting Electroweak Symmetry-Breaking Sector

Màster Oficial d'Astrofísica, Física de Partícules i Cosmologia, Facultat de Física, Universitat de Barcelona, Curs: 2014-2015, Tutor: Domènec EspriuIn this work we introduce and combine an effective Lagrangian formalism with the inverse amplitude method to unitarize WLWL ! WLWL and ZLZL ! ZLZL amplitudes in an extended electroweak symmetry breaking sector. We derive the unitarisation method via dispersion relations, and discuss different ways of fnding the possible resonant states. We compare the two amplitudes calculated with the equivalence theorem approximation and with an exact computation. Moreover, since a diboson excess has been observed near 2 TeV |albeit with very limited statistical significance| in WW, WZ and ZZ final states at the LHC experiments, we constrain the Lagrangian low-energy parameters by limiting the region of the parameter space where a resonance around 2 TeV may appear, and also derive the width of these resonances. Computing the cross-section for two processes we get an idea of how the signal varies along the parameter space, and from the amplitudes we introduce unitarized form factors and vertex functions to allow for a proper treatment of the resonances in Monte Carlo generators and a more precise comparison with experimen

Building a Scenario of Physics Beyond the Standard Model in the Flavor Sector

[eng] Recently lepton flavor universality tests in colliders have reported tensions with the Standard Model, providing an experimental hint at low energies of the effect of a new high energy theory. These tensions are known as flavor anomalies and that is why in this thesis we focus on the study of the impact of the flavor anomalies on the flavor sector of the Standard Model of particle physics. We introduce the Standard Model of particle physics and list the relevant tensions of the theory with the experiments in B decays, commenting in a brief manner the possible solutions involving new physics, as well as the main advantages and inconveniences of each new physics scenario. We first focus in a model with new heavy scalars and fermions were we only account for left-handed couplings to the Standard Model particles. We consider two possible models: one with an additional scalar and two vector-like fermions and another with two additional scalars and one vector-like fermion. The purpose of this model is to solve the tensions in FCNC decays, together with the anomalous magnetic moment of the muon, which are induced at loop-level in both models. We list the relevant constraints and show that the models are able to solve the anomalies, albeit a relatively large coupling of the muons is required. In the case of semi-leptonic decays, stringent constraints arising from B oscillations can be relaxed if the new fermions are considered Majorana particles. Then, since we cannot explain the muon magnetic moment with only left-handed couplings, we construct a model with new scalars and fermions allowing also for right- handed couplings, were we list the relevant Wilson coefficients for b quark decays to a strange quark and two muons, as well as the relevant observables acting as constraints for any number of new scalars and fermions. In order to illustrate this generic approach we supplement the Standard Model with a fourth generation of quarks and leptons. With this model we can explain FCNC anomalies and the magnetic moment of the muon avoiding all the constraints. The second kind of model that we explore is an extension of the Standard Model with scalar leptoquarks. In this case, we compute the Z- and W-decays to leptons for each one of the scalar leptoquarks at next-to-leading-logarithm approximation, and show that the finite terms can account for 20% of the total contribution for leptoquark masses of below 1.5 TeV. We also show their phenomenological relevance in a model with a singlet and a triplet, where our computation pushes the fit towards a better explanation of data. Besides, we comment on the fact that the B mixing has to be implemented carefully as it is one of the main constraints that was missing in earlier studies of these kind of models, and we also illustrate its key role in a particular model model, since it spoils the pure left- handed scenario with 2018 data. Finally, we scrutinize the FCNC process of b quark dcaying into an s quark and two leptons, in the framework of a two Higgs doublet model. We compute all the relevant Wilson coefficients performing the matching of the full theory with the low energy theory showing that it is necessary to keep the external momenta for the scalar and pseudo-scalar operators. This is the first time computation of a proper matching including all the relevant operators. We perform a phenomenological analysis of the model with the strange B meson decaying to two muons and the B meson decaying into two muons and a K meson at high momenta, where we have control of the hadronic uncertainties.[cat] Des del 2012, una serie de mesures en experiments com BaBar, Belle i LHCb, han presentat tensions respecte el model estàndard en decaïments de mesons B. Aquestes desviacions són conegudes com les anomalies de sabor. En aquesta tesi interpretem les anomalies de sabor com a possibles efectes de nova física i proposem alguns models simples per poder acomodar les dades experimentals que difereixen del model estàndard. En primer lloc, proposem un model amb nous escalars i fermions pesats que només s’acoblen als fermions tipus esquerra del model estàndard. En aquest escenari intentem explicar una part de les anomalies ensems amb el valor anòmal del moment magnètic del muó. El resultat són uns acoblaments relativament grans de valor 2 aproximadament. Pel que fa a resoldre part de les anomalies, tenim la possibilitat de considerar els nous fermions com fermions de Majorana, que permet reduir el valor dels acoblaments. Per tal de poder relaxar el valor dels acoblaments d’una forma més general, en el següent capítol proposem un model semblant però amb acoblaments tipus dreta amb el model estàndard. En aquest model calculem totes les fórmules d’una manera genèrica i posteriorment ho particularitzem en un model de quarta generació. Amb la introducció d’aquests acoblaments tipus dreta combinats amb els esquerra podem explicar part de les anomalies de sabor i també el moment magnètic del muó. Seguidament, construïm un model amb leptoquarks escalars on estudiem alguns dels lligams més importants, com els decaïments de Z o les oscil·lacions del mesó B. Posteriorment, proposem models fenomenològics de 2 leptoquarks escalars on veiem l’impacte dels lligams estudiats anteriorment. També discutim com els models quedarien obsolets si no fos perquè les dades experimentals van canviar subtilment després de Moriond 2019. Per últim, fem un estudi de l’impacte de dos decaïments: del mesó B estrany a dos muons i del mesó B al mesó K i dos muons dins d’un model de dos doblets de Higgs. Aquí no ens centrem en les anomalies, sinó que mirem l’impacte d’aquests observables en el model i realitzem una comparació adequada entre la teoria efectiva i la teoria a altes enegies

Generic Loop Effects of New Scalars and Fermions in b→sℓ+ℓ−b\to s\ell^+\ell^-, (g−2)μ(g-2)_\mu and a Vector-like 4th4^{\rm th} Generation

In this article we investigate in detail the possibility of accounting for the $b\to s\ell^+\ell^-$ and $(g-2)_\mu$ anomalies via loop contributions involving with new scalars and fermions. For this purpose, we first write down the most general Lagrangian which can generate the desired effects and then calculate the generic expressions for all relevant $b\to s$ Wilson coefficients. Here we extend previous analysis by allowing that the new particles can also couple to right-handed Standard Model (SM) fermions as preferred by recent $b\to s\ell^+\ell^-$ data and the anomalous magnetic moment of the muon. In the second part of this article we illustrate this generic approach for a UV complete model in which we supplement the Standard Model by a $4^{\rm th}$ generation of vector-like fermions and a real scalar field. This model allows one to coherently address the observed anomalies in $b\to s\ell^+\ell^-$ transitions and in $a_\mu$ without violating the bounds from other observables (in particular $B_s -\bar B_s$ mixing) or LHC searches. In fact, we find that our global fit to this model, after the recent experimental updates, is very good and prefers couplings to right-handed SM fermions, showing the importance of our generic setup and calculation performed in the first part of the article.Comment: 44 pages, 10 figures, 7 table

Probing low energy scalar leptoquarks by the leptonic WW and ZZ couplings

We compute the generic one-loop contribution involving scalar leptoquarks (LQ) to the $W$ and $Z$ leptonic decay widths. In our computation we include for the first time the finite terms and the corrections due to the external momenta of the electroweak bosons, which is a step beyond the leading-logarithmic approximation considered in the literature so far. We show that the terms we include can be numerically quite significant. They amount to about $20\%$ for scalar LQ masses below $1.5$~TeV, as currently allowed by the direct searches at the LHC. To further illustrate the relevance of our results we revisit a model with two light scalar LQs, proposed to accommodate the $B$-physics anomalies. We show that the finite terms we computed can reduce the tension with the $Z$-pole data.Comment: Minor typos amende

Two Higgs doublet models and b→ s exclusive decays

We computed the leading order Wilson coefficients relevant to all the exclusive b→sℓ+ℓ− decays in the framework of the two Higgs doublet model (2HDM) with a softly broken Z2 symmetry by including the O(mb) corrections. We elucidate the issue of appropriate matching between the full and the effective theory when dealing with the (pseudo-)scalar operators for which keeping the external momenta different from zero is necessary. We then make a phenomenological analysis by using the measured B(Bs→μ+μ−) and B(B→Kμ+μ−)high−q2 , for which the hadronic uncertainties are well controlled, and we discuss their impact on various types of 2HDM. A brief discussion of the decays with τ -leptons in the final state is provided too

Loop effects of heavy new scalars and fermions in b → sμ + μ −

Recent measurements of b → sμ + μ − processes at LHCb and BELLE have revealed tensions at the 2 − 3σ level between the Standard Model (SM) prediction and the experimental results in the channels B → K * μ + μ − and B s → ϕμ + μ −, as well as in the lepton-flavor universality violating observable R K = Br(B → Kμ + μ −)/Br(B → Ke + e −). Combined global fits to the available b → sμ + μ − data suggest that these tensions might have their common origin in New Physics (NP) beyond the SM because some NP scenarios turn out to be preferred over the SM by 4 − 5σ. The fact that all these anomalies are related to muons further suggests a connection (and a common NP explanation) with the long-standing anomaly in the anomalous magnetic moment of the muon, a μ . In this article, we study the impact of a generic class of NP models featuring new heavy scalars and fermions that couple to the SM fermions via Yukawa-like interactions. We consider two different scenarios, introducing either one additional fermion and two scalars or two additional fermions and one scalar, and examine all possible representations of the new particles under the SM gauge group with dimension up to the adjoint one. The models induce one-loop contributions to b → sμ + μ − and a μ which are capable of solving the respective anomalies at the 2σ level, albeit a relatively large coupling of the new particles to muons is required. In the case of b → sμ + μ −, stringent constraints from Bs−B¯¯¯¯s mixing arise which can be relaxed if the new fermion is a Majorana particle

Interpreting a 2 TeV resonance in WW scattering

A diboson excess has been observed albeit with very limited statistical significance in WW, WZ, and ZZ final states at the LHC experiments using the accumulated 8 TeV data. Assuming that these signals are due to resonances resulting from an extended symmetry breaking sector in the standard model and exact custodial symmetry, we determine using unitarization methods the values of the relevant low-energy constants in the corresponding effective Lagrangian. Unitarity arguments also predict the widths of these resonances. We introduce unitarized form factors to allow for a proper treatment of the resonances in Monte Carlo generators and a more precise comparison with experiment

Strongly Interacting Electroweak Symmetry Breaking Sector

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Any: 2014, Tutor: Domènec EspriuUsing the equivalence theorem we study the scattering of longitudinally polarized gauge bosons in extensions of the Standard Model where anomalous Higgs couplings to gauge sector and higher order O(p4) operators are considered. Forcing the amplitudes to be unitary we nd new resonances for di erent values of the coupling constants. Hereby we would be able to narrow the range of values of the chiral parameters with new experiments at LHC