Sterile Neutrinos and B-L Symmetry

We revisit the relation between the neutrino masses and the spontaneous breaking of the B-L gauge symmetry. We discuss the main scenarios for Dirac and Majorana neutrinos and point out two simple mechanisms for neutrino masses. In this context the neutrino masses can be generated either at tree level or at quantum level and one predicts the existence of very light sterile neutrinos with masses below the eV scale. The predictions for lepton number violating processes such as mu to e and mu to e gamma are discussed in detail. The impact from the cosmological constraints on the effective number of relativistic degree of freedom is investigated.Comment: v2 new section for the LFV processes, minor corrections, main conclusion unchanged, version to appear in Physics Letters

Simple Left-Right Theory: Lepton Number Violation at the LHC

We propose a simple left-right symmetric theory where the neutrino masses are generated at the quantum level. In this context the neutrinos are Majorana fermions and the model has the minimal degrees of freedom in the scalar sector needed for symmetry breaking and mass generation. We discuss the lepton number violating signatures with two charged leptons of different flavor and missing energy at the Large Hadron Collider in order to understand the testability of the theory.Comment: to appear as a Rapid Communication in Physical Review

Global fit to b→cτνb \to c \tau \nu transitions

We perform a general model-independent analysis of $b \to c \tau \bar{\nu}_\tau$ transitions, including measurements of $\mathcal{R}_D$, $\mathcal{R}_{D^*}$, their $q^2$ differential distributions, the recently measured longitudinal $D^*$ polarization $F_L^{D^*}$, and constraints from the $B_c \to \tau \bar{\nu}_\tau$ lifetime, each of which has significant impact on the fit. A global fit to a general set of Wilson coefficients of an effective low-energy Hamiltonian is presented, the solutions of which are interpreted in terms of hypothetical new-physics mediators. From the obtained results we predict selected $b \to c\tau\bar\nu_\tau$ observables, such as the baryonic transition $\Lambda_b \to \Lambda_c \tau \bar{\nu}_\tau$, the ratio $\mathcal{R}_{J/\psi}$, the forward-backward asymmetries ${\cal A}_\text{FB}^{D^{(*)}}$, the $\tau$ polarization asymmetries $\mathcal{P}_\tau^{D^{(*)}}$, and the longitudinal $D^*$ polarization fraction $F_L^{D^*}$. The latter shows presently a slight tension with any new-physics model, such that an improved measurement could have an important impact. We also discuss the potential change due the very recently announced preliminary $\mathcal{R}_{D^{(*)}}$ measurement by the Belle collaboration.Comment: 47 pages, 12 figures. Version published at JHE

Dark Matter and the Seesaw Scale

We discuss the possibility of finding an upper bound on the seesaw scale using the cosmological bound on the cold dark matter relic density. We investigate a simple relation between the origin of neutrino masses and the properties of a dark matter candidate in a simple theory where the new symmetry breaking scale defines the seesaw scale. Imposing the cosmological bounds, we find an upper bound of order multi-TeV on the lepton number violation scale. We investigate the predictions for direct and indirect detection dark matter experiments and the possible signatures at the Large Hadron Collider

Phenomenological and cosmological aspects of electroweak models beyond the Standard Model

El model estàndard de física de partícules (SM) és la millor teoria que tenim hui en dia per a entendre la natura. No obstant això, malgrat ser capaç de descriure amb alta precisió la major part dels fenòmens que observem, hi ha evidències de física més enllà les quals, sostingudes per observacions experimentals, denoten que el SM no pot ser la teoria final i motiva la cerca d'una teoria que el complete: -L'origen de la massa dels neutrins i la seua natura. Al Capítol 1 es descriu l'estatus dels neutrins i s'introdueix els mecanismes principals que poden generar la seua massa, la connexió entre la seua natura i el número leptònic es discuteix al Capítol 5, mentre que a aquest capítol com al Capítol 6, s'estudien les interrelacions entre els neutrins i altres problemes oberts com la matèria fosca o les teories de gran unificació (GUTs). -L'existència de matèria fosca (DM): Al Capítol 2 introduïm la DM, on dos dels candidats més atractius es presenten: partícules massives d'interacció feble i axions. -L'asimetria entre matèria i antimatèria: una de les condicions requerides per a donar lloc a aquesta asimetria en l'evolució de l'univers és la violació del número bariònic. Al Capítol 3 i 4 introduïm teories on aquesta condició es pot entendre. -Les B-anomalies de sabor: Intrigats pel seu possible origen, al Capítol 9 realitzem un global fit a les transicions b to c incloent com a entrades les dades més actualitzades de les que disposem. Per tal cosa adoptarem una perspectiva de baix-dalt emprant teories efectives, presentada al Capítol 8. -Les simetries del SM: les tres forces del SM suggereixen un origen comú a altes energies. Al Capítol 6 i 7 presentem teories minimals realistes alternatives al model original de SU(5) i la seua connexió amb altres problemes del SM com la massa dels neutrins, l'existència de DM i el problema fort de CP. Al SM hi ha simetries accidentals com el número bariònic i leptònic la presència dels quals pot suggerir una simetria local. Aquests es discuteixen al Capítol 3, on mostrem que promoguent aquestes a simetries locals, apareixen connexions molt atractives entre la massa dels neutrins i la matèria fosca. Mostrem que la cosmologia força a la quota superior d'aquestes teories a estar a l'escala baixa, com a màxim a l'escala d'uns pocs TeVs, la qual cosa fa que la seua fenomenologia siga comprovable als col·lisionadors actuals a pròxims, o a experiments de detecció directa o indirecta de DM, com mostrem als Capítols 3, 4 i 5. -El problema fort de CP: Al capítol 2 discutim els axions i la seua incorporació a teories de gran unificació al Capítol 7, on mostrem que al seu context podem predir la massa de l'axió. Els tòpics mencionats poden estar fortament connectats. L'objectiu d'aquesta tesi és explorar les extensions del SM motivades per connectar distints problemes oberts dels que s'han presentat sota la guia de la simplicitat, predictibilitat i la capacitat de comprovació, adjectius que definiran la brúixola que seguim en aquesta tesi per explorar els territoris més enllà del SM.The Standard Model of particle physics (SM) is the best theory that we have nowadays to understand nature. However, although it describes with high accuracy most phenomena we observe, there is evidence of physics beyond it which, supported by experimental observations, denote that the SM cannot be the ultimate theory and motivates us to seek for a theory that completes it: -The origin of neutrino masses and their nature. We will describe their status and introduce the main mechanisms behind their mass generation in Chap. 1, their connection between their nature and the lepton number will be discussed in Chap. 5, while also in that chapter and in Chap. 6 we will study their interplay with other open problems such as dark matter or Grand Unified Theories (GUTs). -The existence of dark matter (DM): DM will be introduced in Chap. 2, where two of the most appealing candidates will be presented: weak interacting massive particles and axions. -Matter-antimatter asymmetry: One of the conditions required for this asymmetry to occur in the evolution of the universe is the violation of baryon number. In Chap. 3 and 4 we will introduce theories where such condition can be understood. -The flavour B-anomalies: Puzzled by their possible origin, in Chap. 9 we perform a global fit to b to c transitions including as input the most updated values available. For that, we will adopt a bottom-up approach from an effective field theory point of view, presented in Chap. 8. -The symmetries of the SM: The three forces in the SM suggest a common origin at high energies. In Chap. 6, as well as in Chap. 7, we present minimal realistic alternatives to the original SU(5) model and their interplay with other problems for the SM such as neutrino masses, the existence of DM and the strong CP problem. There are accidental symmetries in the SM such as baryon and lepton numbers, that could suggest a local symmetry behind them. Those will be discussed in Chap. 3. In there, we will show that, by gauging these symmetries, very appealing connections between neutrino masses and DM appear. We show that cosmology forces the upper bound of these theories to be at maximum at the multi-TeV scale, what makes their phenomenology testable at the current or near future stages of colliders or direct detection or indirect detection DM experiments, as we will shown in Chaps. 3, 4 and 5. -The strong CP problem: Axions will be discussed in Chap. 2 and their embedding in GUT theories in Chap. 7, where we will show that in their context the mass of the axion can be predicted. The aforementioned topics, as shown above, might be deeply connected. The aim of this thesis is to explore extensions of the SM motivated for connecting the different open problems presented under the guidance of simplicity, predictivity, and testability, which define the compass that we will take in this thesis to explore beyond the SM territories

On Anomaly-Free Dark Matter Models

We investigate the predictions of anomaly-free dark matter models for direct and indirect detection experiments. We focus on gauge theories where the existence of a fermionic dark matter candidate is predicted by anomaly cancellation, its mass is defined by the new symmetry breaking scale, and its stability is guaranteed by a remnant symmetry after the breaking of the gauge symmetry. We find an upper bound on the symmetry breaking scale by applying the relic density and perturbative constraints. The anomaly-free property of the theories allows us to perform a full study of the gamma lines from dark matter annihilation. We investigate the correlation between predictions for final radiation processes and gamma lines. Furthermore, we demonstrate that the latter can be distinguished from the continuum gamma ray spectrum.Comment: 21 pages, 9 figures. v2: minor changes to the text, references added, version to appear in PR

The role of right-handed neutrinos in b→cτνˉb \to c \tau \bar{\nu} anomalies

Motivated by the persistent anomalies reported in the $b\to c\tau\bar{\nu}$ data, we perform a general model-independent analysis of these transitions, in the presence of light right-handed neutrinos. We adopt an effective field theory approach and write a low-energy effective Hamiltonian, including all possible dimension-six operators. The corresponding Wilson coefficients are determined through a numerical fit to all available experimental data. In order to work with a manageable set of free parameters, we define eleven well-motivated scenarios, characterized by the different types of new physics that could mediate these transitions, and analyse which options seem to be preferred by the current measurements. The data exhibit a clear preference for new-physics contributions, and good fits to the data are obtained in several cases. However, the current measurement of the longitudinal $D^*$ polarization in $B\to D^*\tau \bar\nu$ cannot be easily accommodated within its experimental $1\sigma$ range. A general analysis of the three-body $B\to D \tau \bar\nu$ and four-body $B\to D^*(\to D\pi)\tau \bar\nu$ angular distributions is also presented. The accessible angular observables are studied in order to assess their sensitivity to the different new physics scenarios. Experimental information on these distributions would help to disentangle the dynamical origin of the current anomalies.Comment: 42 pages, 11 figures. Version published at JHE