Marzia Bordone

The new measurement of the $$K^+\rightarrow \pi ^+\nu \bar{\nu }$$ K + → π + ν ν ¯ branching ratio by the NA62 collaboration represents an important milestone in precision flavour physics. Under different flavour assumptions on beyond the standard model physics, this decay mode can be related to many other observables with underlying flavour-changing currents, such as $$B\rightarrow K\nu \bar{\nu }$$ B → K ν ν ¯ or $$B_s\rightarrow \mu \mu $$ B s → μ μ , effectively providing an additional handle in our exploration of flavour-violating new physics effects. We study the impact of this new result, under the hypotheses that deviations from the standard model arise from modified Z couplings, or from effects in semileptonic interactions. Finally, we review the implications of our scenarios for the yet to be measured partner mode $$K_L\rightarrow \pi ^0\nu \bar{\nu }$$ K L → π 0 ν ν ¯ .

We present a study of the rare baryonic decay $$\Lambda _b \rightarrow \Lambda \tau ^+ \tau ^-$$ Λ b → Λ τ + τ - as a probe of new physics (NP) coupled preferentially to third-generation fermions. Within the Standard Model, we evaluate the branching ratio and the lepton-flavour-universality (LFU) ratio $$R_{\Lambda }^{\tau /\mu },$$ R Λ τ / μ , including both perturbative and long-distance charm contributions. We show that the LFU ratio can be predicted with an uncertainty below 10%. Possible NP effects arising from lepton non-universal dynamics are analysed within an effective field theory framework motivated by the current anomalies in $$b \rightarrow c\tau \nu $$ b → c τ ν and $$b \rightarrow s\mu ^+\mu ^-$$ b → s μ + μ - transitions. In this context, $$R_{\Lambda }^{\tau /\mu }$$ R Λ τ / μ can be enhanced by several orders of magnitude, offering a clear target for upcoming searches. The implications for the related mode $$\Lambda _b \rightarrow pK\tau ^+\tau ^-$$ Λ b → p K τ + τ - are also briefly discussed.

A bstract: Recent publications by three lattice QCD collaborations have provided an unprecedented wealth of theoretical predictions for the $$ {\overline{B}}_q\to {D}_q^{\left(\ast \right)} $$ B ¯ q → D q ∗ form factors, for spectator flavours q = u / d and q = s . We analyse these predictions within the framework of the heavy-quark expansion (HQE) to order α _s , 1/ m _b , and $$ 1/{m}_c^2 $$ 1 / m c 2 . For the first time, our analysis imposes unitarity bounds for all of the $$ {\overline{B}}_q^{\left(\ast \right)}\to {D}_q^{\left(\ast \right)} $$ B ¯ q ∗ → D q ∗ form factors; this includes newly identified tensor form factors arising in $$ {\overline{B}}_q^{\ast}\to {D}_q^{\left(\ast \right)} $$ B ¯ q ∗ → D q ∗ . This enables us to treat all form factors in the same fashion. At the level of our present analysis, the inclusion of the tensor bounds is not yet constraining the HQE parameter space. We find the lattice QCD results to be well compatible with each other in a joint HQE fit as well as with QCD sum rule estimates that were used in previous HQE analyses. This is in contrast to the strong variability of the posterior predictions, in particular of the form factors ratios R ₀ and R ₂ . Using the posterior distributions of our HQE analysis, we provide predictions for angular observables and LFU ratios in the $$ {\overline{B}}_q\to {D}_q^{\left(\ast \right)}{\ell}^{-}\overline{\nu} $$ B ¯ q → D q ∗ ℓ − ν ¯ decays.

The rare semi-leptonic decays $$B\rightarrow K^*\ell ^+\ell ^-$$ B → K ∗ ℓ + ℓ - , with $$\ell =e, \mu $$ ℓ = e , μ , are highly sensitive to new physics (NP) due to their suppression in the Standard Model (SM). Current LHCb measurements in the muon channel exhibit a significant tension with state-of-the-art SM theory predictions. The proposed tera-Z run at FCC-ee provides a unique opportunity to untangle the origin of this tension by producing a very large sample of B-mesons in a clean $$e^+e^-$$ e + e - environment. We explore the expected precision of $$B\rightarrow K^*\ell ^+\ell ^-$$ B → K ∗ ℓ + ℓ - ( $$\ell =e, \mu $$ ℓ = e , μ ) measurements at FCC-ee, complementing existing studies with $$\tau ^+\tau ^-$$ τ + τ - in the final state, and compare with HL-LHC projections. For the case of muons in the final state, we show that HL-LHC and FCC-ee are expected to deliver a similar number of events, while the latter performs much better in the case of final state electrons. Regardless of the lepton flavour, we expect the FCC-ee environment to be much cleaner than at HL-LHC, with subleading systematics. We also find that a significant reduction in theory uncertainties on the SM predictions is required to capitalize on the advantage going from HL-LHC to FCC-ee. We demonstrate the power of such measurements at FCC-ee to extract information on the long-distance contribution to these decays, and to reveal evidence for new physics even if no deviations are seen in electroweak precision tests.

We present an analysis of the exclusive semileptonic decay $$B\rightarrow D^*\ell \bar{\nu }_\ell $$ B → D ∗ ℓ ν ¯ ℓ based on the Belle and Belle II data made public in 2023, combined with recent lattice-QCD calculations of the hadronic transition form factors by FNAL/MILC, HPQCD and JLQCD. We also consider a new combination of the Belle and Belle II data sets by HFLAV. The analysis is based on the form-factor parameterisation by Boyd–Grinstein–Lebed (BGL), using Bayesian and frequentist statistics, for which we discuss novel strategies. We compare the results of an analysis where the BGL parameterisation is fit only to the lattice data with those from a simultaneous fit to lattice and experiment, and discuss the resulting predictions for the CKM-matrix element $${|V_{cb}|}$$ | V cb | , as well as other phenomenological observables, such as $$R^{\tau /\mu }(D^*)$$ R τ / μ ( D ∗ ) . We find tensions when comparing analyses based on different combinations of experimental or theoretical input, requiring the introduction of a systematic error for some of our results.

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.

We analyse in detail the QED corrections to the total decay width and the moments of the electron energy spectrum of the inclusive semi-leptonic B → X _c eν decay. Our calculation includes short-distance electroweak corrections, the complete 𝒪(α) partonic terms and leading-logarithmic QED effects up to $$ \mathcal{O}\left({\Lambda}_{\textrm{QCD}}^3/{m}_b^3\right) $$ O Λ QCD 3 / m b 3 . A comprehensive numerical comparison of our results against those obtained with the Monte Carlo (MC) tool PHOTOS is presented. While the comparison indicates good overall agreement, our computation contains QED effects not included in PHOTOS and should therefore better describe photon radiation to B → X _c eν as measured by the B-factories. Our calculations represent the first steps in the construction of a fully differential higher-order QED MC generator for inclusive semi-leptonic B decays.

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.