EOS -- A Software for Flavor Physics Phenomenology

I present EOS, an open-source software dedicated to a variety of tasks in the processing of flavor physics observables. EOS is written in C++ and offers both a C++ and a Python interface. It is developed for three main tasks, the production of theoretical predictions for flavor physics observables; the inference of theoretical parameters from an extensible database of likelihoods; and the production of Monte Carlo samples of flavor processes for sensitivity studies.Comment: 9 pages, 4 figure

Dispersive analysis of Λb→Λ(1520)\Lambda_b \to \Lambda(1520) local form factors

We perform an analysis of $\Lambda_b\to\Lambda(1520)$ local form factors. We use dispersive techniques to provide a model-independent parametrisation of the form factors that can be used in the whole kinematic region. We use lattice QCD data to constrain the free parameters in the form factors expansion, which is further constrained by endpoint relations, dispersive bounds, and SCET relations. We analyse different scenarios, where we expand the form factors up to different orders, and their viability. Finally, we use our results to obtain predictions for some observables in $\Lambda_b\to\Lambda(1520)\ell^+\ell^-$ decays, as the differential branching ratio, the forward-backwards lepton asymmetry and the branching ratio of $\Lambda_b\to\Lambda(1520)\gamma$. Finally, we provide a python notebook based on the software EOS to reproduce our result.Comment: 21 pages, 4 figures, 1 ancillary file, v2: correction of typos, v3: version accepted in JHE

Testable likelihoods for beyond-the-standard model fits

Studying potential BSM effects at the precision frontier requires accurate transfer of information from low-energy measurements to high-energy BSM models. We propose to use normalising flows to construct likelihood functions that achieve this transfer. Likelihood functions constructed in this way provide the means to generate additional samples and admit a “trivial” goodness-of-fit test in form of a χ2 test statistic. Here, we study a particular form of normalising flow, apply it to a multi-modal and non-Gaussian example, and quantify the accuracy of the likelihood function and its test statistic

Prospects for searches of b→sννˉb \to s \nu \bar{\nu} decays at FCC-ee

We investigate the physics reach and potential for the study of various decays involving a $b \to s \nu \bar{\nu}$ transition at the Future Circular Collider running electron-positron collisions at the $Z$-pole (FCC-ee). Signal and background candidates, which involve inclusive $Z$ contributions from $b\bar{b}$, $c\bar{c}$ and $uds$ final states, are simulated for a proposed multi-purpose detector. Signal candidates are selected using two Boosted Decision Tree algorithms. We determine expected relative sensitivities of $0.53\%$, $1.20\%$, $3.37\%$ and $9.86\%$ for the branching fractions of the $B^{0} \to K^{*0} \nu \bar{\nu}$, $B^{0}_{s} \to \phi \nu \bar{\nu}$, $B^{0} \to K^{0}_{S} \nu \bar{\nu}$ and $\Lambda_{b}^{0} \to \Lambda^{0} \nu \bar{\nu}$ decays, respectively. In addition, we investigate the impact of detector design choices related to particle-identification and vertex resolution. The phenomenological impact of such measurements on the extraction of Standard Model and new physics parameters is also studied

Prospects for searches of b → sν ν ¯ decays at FCC-ee

We investigate the physics reach and potential for the study of various decays involving a b→sνν¯ transition at the Future Circular Collider running electron-positron collisions at the Z-pole (FCC-ee). Signal and background candidates, which involve inclusive Z contributions from bb¯, cc¯ and uds final states, are simulated for a proposed multi-purpose detector. Signal candidates are selected using two Boosted Decision Tree algorithms. We determine expected relative sensitivities of 0.53%, 1.20%, 3.37% and 9.86% for the branching fractions of the B0→K∗0νν¯, Bs0→ϕνν¯, B0→KS0νν¯ and Λb0→Λνν¯ decays, respectively. In addition, we investigate the impact of detector design choices related to particle-identification and vertex resolution. The phenomenological impact of such measurements on the extraction of Standard Model and new physics parameters is also studied

Analysis of the ψ ( 3770 ) resonance in line with unitarity and analyticity constraints

We study the inclusive and exclusive cross sections of e+e-→hadrons for center-of-mass energies between 3.70 and 3.83GeV to infer the mass, width, and couplings of the ψ(3770) resonance. By using a coupled-channel K-matrix approach, we setup our analysis to respect unitarity and the analyticity properties of the underlying scattering amplitudes. We fit several models to the full dataset and identify our nominal results through a statistical model comparison. We find that, accounting for the interplay between the ψ(2S) and the ψ(3770), no further pole is required to describe the ψ(3770) line shape. In particular we derive from the pole location Mψ(3770)=3778.8±0.3MeV and Γψ(3770)=25.0±0.5MeV. Moreover, we find the decay to D+D- and D0D¯0 to be consistent with isospin symmetry and derive an upper bound on the branching ratio B(ψ(3770)→non-DD¯)<6% at 90% probability

Dispersive analysis of B → K (*) and B s → ϕ form factors

We propose a stronger formulation of the dispersive (or unitarity) bounds à la Boyd-Grinstein-Lebed (BGL), which are commonly applied in analyses of the hadronic form factors for B decays. In our approach, the existing bounds are split into several new bounds, thereby disentangling form factors that are jointly bounded in the common approach. This leads to stronger constraints for these objects, to a significant simplification of our numerical analysis, and to the removal of spurious correlations among the form factors. We apply these novel bounds to B¯→K¯∗ and B¯s→ϕ form factors by fitting them to purely theoretical constraints. Using a suitable parametrization, we take into account the form factors’ below-threshold branch cuts arising from on-shell B¯sπ0 and B¯sπ0π0 states, which so-far have been ignored in the literature. In this way, we eliminate a source of hard-to-quantify systematic uncertainties. We provide machine readable files to obtain the full set of the B¯→K¯∗ and B¯s→ϕ form factors in and beyond the entire semileptonic phase space

Toward a complete description of b → uℓ− v within the Weak Effective Theory decays

We fit the available data on exclusive semileptonic b → uℓ −ν¯ decays within the Standard Model and in the Weak Effective Theory. Assuming Standard Model dynamics,we find |Vub| = 3.59+0.13−0.12 × 10−3. Lifting this assumption, we obtain stringent constraintson the coefficients of the ub`ν sector of the Weak Effective Theory. Performing a Bayesian model comparison, we find that a beyond the Standard Model interpretation is favoured over a Standard Model interpretation of the available data. We provide a Gaussian mixturemodel that enables the efficient use of our fit results in subsequent analyses beyond the Standard Model, within and beyond the framework of the Standard Model Effective Field Theory

Bs to l+ l- gamma as a Test of Lepton Flavor Universality

We discuss a number of strategies to reduce the $\mathcal B(B^0_s \to \ell^{+} \ell^{-} \gamma)$ theoretical error, and make such a measurement a new probe of the interactions that are interesting in the light of present-day flavor discrepancies. In particular, for low di-lepton invariant mass we propose to exploit the close parenthood between $\mathcal B(B^0_s \to \ell^{+} \ell^{-} \gamma)$ and the measured $\mathcal B(B^0_s \to \phi (\to K^+ K^-) \gamma)$. For high $q^2$, conversely, we exploit the fact that the decay is dominated by two form-factor combinations, plus contributions from broad charmonium that we model accordingly. We construct the ratio $R_\gamma$, akin to $R_K$ and likewise sensitive to lepton-universality violation. Provided the two rates in this ratio are integrated in a suitable region that minimises bremsstrahlung contributions while maximising statistics, the ratio is very close to unity and the form-factor dependence cancels to an extent that makes it a new valuable probe of lepton-universality violating contributions in the effective Hamiltonian. We finally speculate on additional ideas to extract short-distance information from resonance regions, which are theoretically interesting but statistically limited at present.Comment: 21 pages, 4 figures. v4: in appendix removed equation already present in main tex

Les écarts dans les désintégrations semileptoniques de b → s : une étude complète des aspects de construction de modèles, la définition de nouvelles observables, à leur mesure avec le détecteur LHCb.

Although the Standard Model (SM) of particle physics gives an overall excellent description of the observations, a few results, mainly obtained by the LHCb detector at CERN, point towards deviations in the transitions from quark b to quarks s and c. If confirmed, these anomalies would give a clear signal of physics beyond the SM, as they violate lepton flavour universality (LFUV). In this context, new measurements and their theoretical predictions are crucial to define possible new physics scenarios. This thesis concerns both aspects and is therefore divided into two parts. The first one is devoted to theoretical considerations on the b to s transition as well as models explaining the anomalies. The second part presents a new measurement, namely the search for the decays of Bs mesons to two muons and a photon at LHCb.Radiative leptonic decays are promising to test the SM because the additional photon not only enlarges the branching ratio by lifting the chiral suppression factor, but also offers a sensitivity to other operators. Using the language of effective field theory (EFT), the decay of the Bs meson into two leptons and a photon is studied and new methods to reduce the theoretical uncertainty on its branching ratio are proposed. Besides, the behavior of this decay at large dilepton mass gives the possibility of an indirect measurement where the total branching ratio is measured as a background of the corresponding non-radiative channel. Furthermore, if LFUV is experimentally confirmed, one also expects violation of lepton flavour. Measuring such violation, which would be an undeniable sign of new physics, is yet very challenging. Radiative decays can however support this search by offering additional channels with potentially larger branching ratios.The interpretation of the B anomalies in term of shifts in the EFT coefficients put a few scenarios forward. These scenarios can then be interpreted in term of new physics models. One such model is based on the consideration of an additional symmetry group acting horizontally between the SM generations. This model is severely constrained by low energy observables, such as meson mixings and leptonic decays, but allowing for a mass degeneracy between the new group's bosons explains all b to s anomalies while passing other experimental constraints. The absence of new physics in low energy observables can also be interpreted as the presence of leptoquarks, for which the interaction between two quark- or two lepton-currents only arises at the one loop level. A model based on a vector leptoquark can for example give an explanation to both b to s and b to c anomalies. Interestingly, ultraviolet completions of these models contain natural Dark Matter candidates, hence relating two outstanding problems of particle physics.The analysis presented in the second part is particularly challenging because the probed decay is both very rare and radiative. The main difficulty lies in the presence of a large combinatorial background due to light meson decays. Tackling it while keeping a high efficiency on the signal selection requires the use of two successive multivariate analyses. The signal is then normalised to a similar decay of the Bs where no new physics is expected. Measuring a ratio of yields instead of an unique branching ratio allows for a partial cancellation of experimental uncertainties. On the other hand, this procedure requires a good knowledge of each selection step efficiencies for the two channels. These efficiencies are extracted from Monte-Carlo simulations or, when possible, directly from the data. Although no significant excess is found, this analysis allows the first limit on the total branching ratio to be set.Bien que le Modèle Standard (SM) de la physique des particules permette une excellente description des observations, quelques résultats, principalement obtenus par le détecteur LHCb du CERN, montrent des signes de déviations dans les transitions du quark b aux quarks s et c. Si ces anomalies sont confirmées, elles donneront un signal clair de physique au-delà du SM car elles violent l'universalité de la saveur leptonique. Dans ce contexte, de nouvelles mesures et leurs prédictions théoriques sont cruciales pour définir de potentiels scénarios de nouvelle physique (NP). Cette thèse s'intéresse à tous ces aspects et est divisée en deux parties. La première propose des considérations théoriques sur la transition de b en s ainsi que des modèles expliquant les anomalies. La seconde partie présente une nouvelle mesure, la recherche de désintégrations de mésons Bs en deux muons et un photon à LHCb.Les désintégrations leptoniques radiatives sont particulièrement prometteuses pour tester le SM, car l'ajout du photon n'entraîne pas seulement un agrandissement du rapport de branchement (BR), mais offre aussi une sensibilité à de nouveaux opérateurs. La désintégration du Bs en deux leptons et un photon est alors étudiée en utilisant une théorie effective des champs, et de nouvelles méthodes permettant de réduire l'incertitude théorique sur son BR sont proposées. Le comportement de cette désintégration lorsque la masse du dilepton est grande donne par ailleurs la possibilité de réaliser une mesure indirecte, où le BR est mesuré comme bruit de fond de son équivalent non-radiatif. En outre, si la violation de l'universalité leptonique est confirmée, on s'attend aussi à une violation de la saveur leptonique. La mesure de cette dernière, signe indéniable de NP, est néanmoins très difficile. Les désintégrations radiatives permettent cependant de simplifier cette recherche en offrant des canaux supplémentaires.L'interprétation des anomalies en termes de modification des coefficients de la théorie effective met en avant quelques scénarios. Ceux-ci peuvent alors être interprétés en terme de modèles de NP. L'un d'eux se base sur un nouveau groupe de symétrie qui agit horizontalement entre les générations du SM. Ce modèle est fortement contraint par des observables de basse énergie, mais attribuer différentes masses aux bosons du nouveau groupe permet d'expliquer les anomalies de b en s tout en satisfaisant les contraintes expérimentales. L'absence de NP dans les observables de basse énergie peut en outre suggérer la présence de leptoquarks, pour lesquels l'interaction entre deux courants de quarks ou de leptons n'arrive qu'à une boucle. Un modèle basé sur un leptoquark vecteur explique par exemple à la fois les anomalies de b en s et en c. Il est intéressant de noter que les complétions ultraviolettes de ces différents modèles contiennent des candidats pour la Matière Noire, reliant ainsi deux problèmes prépondérants de la physique des particules.L'analyse présentée dans la seconde partie est rendue particulièrement difficile par la rareté et le caractère radiatif de la désintégration étudiée. La difficulté principale réside dans la présence d'un important bruit de fond combinatoire dû à la désintégration de mésons légers. Le prendre en compte tout en gardant une grande efficacité sur la sélection du signal nécessite d'utiliser deux analyses multivariées successives. Le signal est alors normalisé à une désintégration similaire du Bs où aucune NP est attendue. Mesurer le rapport des efficacités au lieu d'un unique BR permet une suppression partielle des incertitudes expérimentales. En revanche, cette procédure nécessite une bonne connaissance des efficacités de chaque étape de la sélection. Celles-ci sont extraites de simulation Monte-Carlo ou, quand c'est possible, directement des données. Bien qu'aucun excès significatif ne soit observé, cette analyse permet d'établir la première limite sur le BR total