Searching for new physics in bottom-quark decays

In recent years, small deviations between experimental measurements and predictions from the Standard Model (SM) have been observed in bottom-quark decays, which indicate that Lepton Flavour Universality (LFU) may be violated. These small deviations, the so-called b-anomalies, have been observed in the two decay channels b → cℓ ̄ν and b → sℓℓ. To ensure that the b-anomalies are not an underestimation of the systematic uncertainties, we aim in this thesis to improve the precision of flavour physics prediction while also providing insight on the tension between theory prediction and experimental measurement by incorporating New Physics (NP) effects and their impact on observables in the bottom-quark decay.
This dissertation is written in the form of a cumulative work based on our six articles, four of which have been published in international peer-reviewed journals and the remaining two are being prepared for publication. First, we investigate the background signals of the inclusive decay B → Xcℓ ̄ν that is relevant for the extraction of |Vcb|. In order to increase precision, we argue that removing the background signals from Monte-Carlo simulation data should be avoided because it introduces uncertainties. Instead, we can precisely compute the background signals using Heavy Quark Expansion (HQE).
In addition, we investigate the LFU symmetry of the SM in the semileptonic B → Xcℓ ̄ν decays. Ratios of branching fractions between decays are used to probe the LFU hypothesis, where our calculation of the LFU ratios for the SM takes into account the mass effects in the total decay rate. We provide updated results for the branching ratio of B → Xcτ ̄ν. If there is a difference between SM predictions and experimental data, this could indicate the presence of NP impacts.
We also investigate the possibility of NP effect in the inclusive semileptonic B → Xcℓ ̄ν decay. The calculations rely on the HQE and use non-perturbative parameters extracted from decay spectra. The extraction of HQE parameters is done assuming the SM, but we explore the idea that NP effects might be hidden in the HQE parameters. The primary goal is to lay the groundwork for a global fit analysis that includes the full basis of NP operators, allowing for the extraction of HQE parameters and consequently an updated result for |Vcb| with NP effects.
In the baryonic decay channel we investigate the possibility of the Lepton Flavour Violation (LFV) for the exclusive decay Λb → Λℓ−ℓ+ using a full basis of NP operators for the first time. We investigate the branching ratio and forward-backward symmetry of the decay quantitatively using both a model-independent and model-dependent approach. We emphasize that the baryonic decay constrains the NP Wilson coefficients differently from the mesonic decay, which has the potential to further constrain the allowed parameter space for NP models. We can also improve the constraint of the NP models by reducing the hadronic uncertainties caused by the ten independent local form factors of Λb → Λ. To improve the control of the uncertainties, we introduce a new parametrization for the local form factors in which the form factor parameters are bounded due to orthonormal polynomials that diagonalize the form factor contribution within their respective dispersive bounds. We show, using a Bayesian analysis of available lattice QCD data, that our model provides excellent control over systematic uncertainty when extrapolating to the region of large hadronic recoil.
As part of this thesis’s final project, we investigate the light-cone distribution amplitude (LCDA) of the B meson. We are particularly interested in three-particle LCDAs, which can be found in higher dimensional vacuum-to-meson matrix elements. These matrix elements can be parametrized in terms of two parameters. To estimate the parameters, we propose alternative diagonal QCD sum rules. The sum rules of our new approach have the advantage of being positive definite, which means that we expect the quark-hadron duality to be more accurate than the previous studied sum rules.