Search

Abstract We analyze data on $$B\rightarrow K\mu ^+\mu ^-$$ B → K μ + μ - and $$B\rightarrow K^*\mu ^+\mu ^-$$ B → K ∗ μ + μ - decays in the whole dilepton invariant mass spectrum with the aim of disentangling short- vs. long-distance contributions. The sizable long-distance amplitudes from $$c \overline{c}$$ c c ¯ narrow resonances are taken into account by employing a dispersive approach. For each available $$q^2=m^2_{\mu \mu }$$ q 2 = m μ μ 2 bin and each helicity amplitude an independent determination of the Wilson coefficient $$C_9$$ C 9 , describing $$b\rightarrow s\ell ^+\ell ^-$$ b → s ℓ + ℓ - transitions at short distances, is obtained. The consistency of the $$C_9$$ C 9 values thus obtained provides an a posteriori check of the absence of additional, sizable, long-distance contributions. The systematic difference of these values from the Standard Model expectation supports the hypothesis of a non-standard $$b\rightarrow s \mu ^+\mu ^-$$ b → s μ + μ - amplitude of short-distance origin.

Abstract Several new physics scenarios that address anomalies in B-physics predict an enhancement of $$b \rightarrow s \tau \tau $$ b → s τ τ with respect to its Standard Model prediction. Such scenarios necessarily imply modifications of the lifetime ratio $$\tau _{B_{s}}/\tau _{B_{d}}$$ τ B s / τ B d and the lifetime difference $$\Delta \Gamma _{s}$$ Δ Γ s . In this work, we explore indirect bounds provided by these observables over new physics scenarios. We also estimate future projections, showing that future experimental and theoretical improvements on both $$\tau _{B_{s}}/\tau _{B_{d}}$$ τ B s / τ B d and $$\Delta \Gamma _{s}$$ Δ Γ s have the potential to provide bounds competitive with those directly extracted from $$b\rightarrow s \tau \tau $$ b → s τ τ transitions. After performing a model-independent analysis, we apply our results to the particular case of leptoquark mediators proposed to address the $$R_{D^{(*)}}$$ R D ( ∗ ) anomalies.

Abstract We analyze in detail the angular distributions in $${\bar{B}}\rightarrow D^*\ell {{\bar{\nu }}}$$ B ¯ → D ∗ ℓ ν ¯ decays, with a focus on lepton-flavour non-universality. We investigate the minimal number of angular observables that fully describes current and upcoming datasets, and explore their sensitivity to physics beyond the Standard Model (BSM) in the most general weak effective theory. We apply our findings to the current datasets, extract the non-redundant set of angular observables from the data, and compare to precise SM predictions that include lepton-flavour universality violating mass effects. Our analysis shows that the number of independent angular observables that can be inferred from current experimental data is limited to only four. These are insufficient to extract the full set of relevant BSM parameters. We uncover a $$\sim 4\sigma $$ ∼ 4 σ tension between data and predictions that is hidden in the redundant presentation of the Belle 2018 data on $${\bar{B}}\rightarrow D^*\ell {{\bar{\nu }}}$$ B ¯ → D ∗ ℓ ν ¯ decays. This tension specifically involves observables that probe $$e-\mu $$ e - μ lepton-flavour universality. However, we find inconsistencies in these data, which renders results based on it suspicious. Nevertheless, we discuss which generic BSM scenarios could explain the tension, in the case that the inconsistencies do not affect the data materially. Our findings highlight that $$e-\mu $$ e - μ non-universality in the SM, introduced by the finite muon mass, is already significant in a subset of angular observables with respect to the experimental precision.

Abstract We discuss the possibility of performing precise tests of $$\mu /e$$ μ / e universality in $$B \rightarrow \pi \ell ^+\ell ^-$$ B → π ℓ + ℓ - decays. We show that in wide regions of the dilepton invariant mass spectrum the ratio between muonic and electronic decay widths can be predicted with high accuracy, both within and beyond the Standard Model. We present numerical expressions which can be used to extract precise information on short-distance dynamics if a deviation from universality is observed in the data.

Abstract Lepton flavour violation (LFV) naturally occurs in many new physics models, specifically in those explaining the B anomalies. While LFV has already been studied for mesonic decays, it is important to consider also baryonic decays mediated by the same quark transition. In this paper, we study LFV in the baryonic $$\Lambda _b \rightarrow \Lambda \ell _1 \ell _2$$ Λ b → Λ ℓ 1 ℓ 2 using for the first time a full basis of New Physics operators. We present expected bounds on the branching ratio in a model-independent framework and using two specific new physics models. Finally, we point out the interplay and orthogonality between the baryonic and mesonic LFV searches.

Abstract We provide updated predictions for the hadronic decays $$\bar{B}_s^0\rightarrow D_s^{(*)+} \pi ^-$$ B ¯ s 0 → D s ( ∗ ) + π - and $$\bar{B}^0\rightarrow D^{(*)+} K^-$$ B ¯ 0 → D ( ∗ ) + K - . They are based on $${\mathcal {O}}(\alpha _s^2)$$ O ( α s 2 ) results for the QCD factorization amplitudes at leading power and on recent results for the $$\bar{B}_{(s)} \rightarrow D_{(s)}^{(*)}$$ B ¯ ( s ) → D ( s ) ( ∗ ) form factors up to order $$\mathcal{O}(\Lambda _\mathrm{QCD}^2/m_c^2)$$ O ( Λ QCD 2 / m c 2 ) in the heavy-quark expansion. We give quantitative estimates of the matrix elements entering the hadronic decay amplitudes at order $$\mathcal{O}(\Lambda _\mathrm{QCD}/m_b)$$ O ( Λ QCD / m b ) for the first time. Our results are very precise, and uncover a substantial discrepancy between the theory predictions and the experimental measurements. We explore two possibilities for this discrepancy: non-factorizable contributions larger than predicted by the QCD factorization power counting, and contributions beyond the Standard Model. We determine the $$f_s/f_d$$ f s / f d fragmentation fraction for the CDF, D0 and LHCb experiments for both scenarios.

Abstract We carry out a comprehensive analysis of the full set of $${\bar{B}}_q \rightarrow D_q^{(*)}$$ B¯q→Dq(∗) form factors for spectator quarks $$q=u,d,s$$ q=u,d,s within the framework of the Heavy-Quark expansion (HQE) to order $${\mathcal {O}}\left( \alpha _s, 1/m_b, 1/m_c^2\right) $$ Oαs,1/mb,1/mc2 . In addition to the available lattice QCD calculations we make use of two new sets of theoretical constraints: we produce for the first time numerical predictions for the full set of $${\bar{B}}_s \rightarrow D_s^{(*)}$$ B¯s→Ds(∗) form factors using Light-Cone sum rules with $$B_s$$ Bs -meson distribution amplitudes. Furthermore, we reassess the QCD three-point sum rule results for the Isgur-Wise functions entering all our form factors for both $$q=u,d$$ q=u,d and $$q=s$$ q=s spectator quarks. These additional constraints allow us to go beyond the commonly used assumption of $$SU(3)_F$$ SU(3)F symmetry for the $${{\bar{B}}}_s\rightarrow D_s^{(*)}$$ B¯s→Ds(∗) form factors, especially in the unitarity constraints which we impose throughout our analysis. We find the coefficients of the IW functions emerging at $${\mathcal {O}}\left( 1/m_c^2\right) $$ O1/mc2 to be consistent with the naive $${\mathcal {O}}\left( 1\right) $$ O1 expectation, indicating a good convergence of the HQE. While we do not find significant SU(3) breaking, the explicit treatment of $$q=s$$ q=s as compared to a simple symmetry assumption renders the unitarity constraints more effective. We find that the (pseudo)scalar bounds are saturated to a large degree, which affects our theory predictions. We analyze the phenomenological consequences of our improved form factors by extracting $$|V_{cb}|$$ |Vcb| from $${\bar{B}}\rightarrow D^{(*)}\ell \nu $$ B¯→D(∗)ℓν decays and producing theoretical predictions for the lepton-flavour universality ratios R(D), $$R(D^*)$$ R(D∗) , $$R(D_s)$$ R(Ds) and $$R(D_s^*)$$ R(Ds∗) , as well as the $$\tau $$ τ - and $$D_q^*$$ Dq∗ polarization fractions for the $${\bar{B}}_q\rightarrow D_q^{(*)}\tau \nu $$ B¯q→Dq(∗)τν modes.

Abstract We carry out an analysis of the full set of ten $$\bar{B}\rightarrow D^{(*)}$$ B¯→D(∗) form factors within the framework of the Heavy-Quark Expansion (HQE) to order $$\mathcal {O}\left( \alpha _s,\,1/m_b,\,1/m_c^2\right) $$ Oαs,1/mb,1/mc2 , both with and without the use of experimental data. This becomes possible due to a recent calculation of these form factors at and beyond the maximal physical recoil using QCD light-cone sum rules, in combination with constraints from lattice QCD, QCD three-point sum rules and unitarity. We find good agreement amongst the various theoretical results, as well as between the theoretical results and the kinematical distributions in $$\bar{B}\rightarrow D^{(*)}\lbrace e^-,\mu ^-\rbrace \bar{\nu }$$ B¯→D(∗){e-,μ-}ν¯ measurements. The coefficients entering at the $$1/m_c^2$$ 1/mc2 level are found to be of $$\mathcal {O}(1)$$ O(1) , indicating convergence of the HQE. The phenomenological implications of our study include an updated exclusive determination of $$|V_{cb}|$$ |Vcb| in the HQE, which is compatible with both the exclusive determination using the BGL parametrization and with the inclusive determination. We also revisit predictions for the lepton-flavour universality ratios $$R_{D^{(*)}}$$ RD(∗) , the $$\tau $$ τ polarization observables $$P_\tau ^{D^{(*)}}$$ PτD(∗) , and the longitudinal polarization fraction $$F_L$$ FL . Posterior samples for the HQE parameters are provided as ancillary files, allowing for their use in subsequent studies.

Abstract We analyse the rare processes $$K\rightarrow \pi \nu \bar{\nu }$$ K → π ν ν ¯ in view of the recent hints of violations of lepton-flavour universality (LFU) observed in B meson decays. If, as suggested by present data, the new interactions responsible for LFU violations couple mainly to the third generation of left-handed fermions, $$K\rightarrow \pi \nu \bar{\nu }$$ K → π ν ν ¯ decays turn out to be particularly interesting: these are the only kaon decays with third-generation leptons (the $$\tau $$ τ neutrinos) in the final state. In order to relate B-physics anomalies and K decays we adopt an effective field theory approach, assuming that the new interactions satisfy an approximate $$U(2)_q\times U(2)_\ell $$ U ( 2 ) q × U ( 2 ) ℓ flavour symmetry. In this framework we show that O(1) deviations from the Standard Model predictions in $$K\rightarrow \pi \nu \bar{\nu }$$ K → π ν ν ¯ branching ratios, closely correlated to similar effects in $$B\rightarrow K^{(*)}\nu \bar{\nu }$$ B → K ( ∗ ) ν ν ¯ , are naturally expected. The correlation of $$\mathcal {B}(K \rightarrow \pi \nu \bar{\nu })$$ B ( K → π ν ν ¯ ) , $$\mathcal {B}(B\rightarrow K^{(*)}\nu \bar{\nu })$$ B ( B → K ( ∗ ) ν ν ¯ ) , and the LFU violations in B decays would provide a very valuable tool to shed more light on this interesting phenomenon.