We present a comprehensive analysis of low-energy signals of hypothetical scalar leptoquark interactions in lepton and kaon transitions. We derive the most general effective four-fermion Lagrangian induced by tree-level scalar leptoquark exchange and identify the Wilson coefficients predicted by the five possible types of scalar leptoquarks. The current constraints on the leptoquark Yukawa couplings arising from lepton and kaon processes are worked out, including also loop-induced transitions with only leptons (or quarks) as external states. In the presence of scalar leptoquark interactions, we also derive the differential distributions for flavour-changing neutral-current transitions in semileptonic kaon modes, including all known effects within the Standard Model. Their interference with the new physics contributions could play a significant role in future improvements of those constraints that are currently hampered by poorly-determined non-perturbative parameters.

Motivated by the persistent anomalies reported in the $$ b\to c\tau \overline{v} $$ b → cτ v ¯ 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 $$ B\to {D}^{\ast}\tau \overline{v} $$ B → D ∗ τ v ¯ cannot be easily accommodated within its experimental 1σ range. A general analysis of the three-body $$ B\to D\tau \overline{v} $$ B → Dτ v ¯ and four-body $$ B\to {D}^{\ast}\left(\to D\pi \right)\tau \overline{v} $$ B → D ∗ → Dπ τ v ¯ 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.

Semileptonic flavor-changing neutral-current transitions with a pair of neutrinos in the final state are very accurately determined in the Standard Model (SM) and thus provide an accurate and sensitive probe for physics beyond the SM. Until recently, the poor tagging efficiency for the $B\to K^{(*)}\nu \overline{\nu }$ modes made them less advantageous as a probe of new physics (NP) compared to the charged lepton counterparts. The most recent Belle II result on $B\to K\nu \overline{\nu }$ uses an innovative inclusive tagging technique, resulting in a higher tagging efficiency; this together with previous BABAR and Belle results indicates a possible enhancement in the branching fraction of $B^{+}\to K^{+}\nu \overline{\nu }$. A reanalysis of the full Belle dataset together with the upcoming Belle II dataset is expected to result in a much more precise measurement of this mode. If the branching ratio is indeed found to be enhanced with improved measurements, this would provide an unambiguous signal of NP without uncertainties due to long-distance nonfactorizable effects or power corrections (in contrast to $B\to K^{(*)}\ell \ell$). We have explored the possibilities of such an enhancement as a signal of NP within several scenarios, which can also explain some of the other tensions observed in neutral- as well as charged-current $B$ decays. In an effective field theory approach, with the most general dimension-6 Hamiltonian including light right-handed neutrinos, we explore the viability of all scalar and vector leptoquarks, as well as the parameter space possible with a generic vector gauge boson $Z^{\prime }$ model, assuming minimal new particle content. While being consistent with all data, correlations between the observed intriguing discrepancies in $B$ decays are also obtained, which will discriminate between the various NP scenarios.

We investigate the Standard Model (SM) extended with a colored charged scalar, leptoquark, having fractional electromagnetic charge [equation]. We mostly focus on the decays of the leptoquark into...

We derive a QCD sum rule for the inverse moment of the B_s-meson light-cone distribution amplitude in HQET. Within this method, the SU(3)_{f l} symmetry violation is traced to the strange quark mass and to the difference between strange and nonstrange quark condensate densities. We predict the ratio of inverse moments λ_Bs/λ_B = 1.19 ± 0.14 which can be used in various applications of these distribution amplitudes to the analyses of B_s-meson decays, provided an accurate value of λ_B is available from other sources, such as the B → ℓν_ℓγ decay.

It is a challenging task to explain, in terms of a simple and compelling new physics scenario, the intriguing discrepancies between the standard model expectations and the data for the neutral-current observables $R_K$ and $R_{K^{*}}$, as well as the charged-current observables $R(D)$ and $R(D^{*})$. We show that this can be achieved in an effective theory with only two unknown parameters. In addition, this class of models predicts some interesting signatures in the context of both $B$ decays as well as high-energy collisions.