arXiv subjects

Nico Gubernari

Publications and source records attributed to Nico Gubernari.

At least 19 recordsLinked to original sources

arXiv2026

Decay constants of the $B_c$ mesons with vector and tensor currents

We calculate the decay constants of the lowest-lying $B_c$ mesons in the spin-parity channels $J^P = 0^-,\,0^+,\,1^-,\,1^+$, commonly referred to as the $B_c,\, B_{c0}^*,\, B_c^*,\, B_{c1}$ mesons, respectively. Within the framework of QCD sum rules, we consider the decay constants associated with both the (axial-)vector and (axial-)tensor interpolating currents. The decay constants with (axial-)vector currents have already been studied in the literature. We refine previous QCD sum rule results by including higher-order power corrections and performing a comprehensive uncertainty analysis. Furthermore, we provide the first determination of the (axial-)tensor decay constants of the $B_c^*$ and $B_{c1}$ mesons. These new results will not only improve theoretical predictions for purely leptonic $B_c$ decays, but also strengthen unitarity constraints on $b \to c$ form factors, thereby improving the precision of predictions for $\bar{B} \to D^{(*)} \ell \bar\nu$ decays and the determination of $|V_{cb}|$.

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arXiv2026

Three-particle di-light-cone distribution amplitudes of the $B$-meson in heavy-quark effective theory

We present a systematic study of the three-particle di-light-cone distribution amplitudes (DLCDAs) of the $B$-meson. They are defined through $B$-meson--to--vacuum matrix elements of trilocal HQET operators, in which the light antiquark and the gluon field-strength tensor are located on two back-to-back light rays. In this sense, the DLCDAs generalise the conventional $B$-meson light-cone distribution amplitudes to configurations where soft fields couple to collinear degrees of freedom in two distinct directions. As such, they parametrise the non-perturbative dynamics associated with non-factorisable soft-gluon contributions in rare and non-leptonic exclusive $B$-meson decays. We derive the complete Lorentz decomposition of the matrix elements of generic trilocal operators, identify eight independent DLCDAs, and organise them in a basis of definite twist. Using local operator identities and equations-of-motion constraints, we obtain tree-level relations for their normalisation integrals and first moments in terms of a minimal set of hadronic parameters. These relations allow us to construct simple momentum-space models for all independent DLCDAs. For the leading-twist distribution, we further incorporate the perturbative radiative tail at order $\alpha_s$ and discuss its impact on the resulting parametrisation.

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arXiv2026

Unitarity bounds and form-factor predictions for $B$-meson decays

This paper is organized around three main objectives. First, I review in a pedagogical way the unitarity bounds for form factors in $B$-meson decays, together with the parametrizations most commonly used in phenomenological analyses. These include BGL, BCL, CLN, and the Dispersive Matrix (DM) method. I also clarify the relation between BGL and DM, showing that they are two equivalent implementations of the same unitarity information. Second, I demonstrate that the standard BGL and DM constructions are strictly rigorous only when no subthreshold cuts are present. For $B$-meson decays, this requirement is fulfilled exclusively by the $B\to\pi$ FFs. To treat the generic case, I fully develop the GG parametrization introduced in previous work and show how the same logic extends to a DM-like construction. Third, I perform three combined analyses and obtain form-factor predictions over the full semileptonic region: one for $B\to\pi$ and $B_s\to K^{(*)}$, one for $B\to K^{(*)}$ and $B_s\to \phi$, and one for $B\to D^{(*)}$ and $B_s\to D_s^{(*)}$. All numerical results, posterior samples, analysis files, and plots are provided in the supplementary material (https://github.com/gubernari/suppl-unitb).

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arXiv2025

Correlator with tensor currents and two masses at two loops

We calculate the vacuum-to-vacuum correlator of two quark tensor currents with two massive quarks, retaining full momentum dependence. For the first time, we include perturbative corrections up to next-to-leading order. Our fully analytical expressions are provided in machine-readable form. Furthermore, we present numerical results for various input parameters, including an estimate of the scale uncertainties. Our results are essential input for applications of dispersive methods, including unitarity bounds and QCD sum rules.

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arXiv2025

Challenging $\bar{B}_{(s)}\to D_{(s)}^{(*)}$ Form Factors with the Heavy Quark Expansion

Recent publications by three lattice QCD collaborations have provided an unprecedented wealth of theoretical predictions for the $\bar{B}_q \to D_q^{(*)}$ 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 $\alpha_s$, $1/m_b$, and $1/m_c^2$. For the first time, our analysis imposes unitarity bounds for all of the $\bar{B}_q^{(*)} \to D_q^{(*)}$ form factors; this includes newly identified tensor form factors arising in $\bar{B}_q^*\to D_q^{(*)}$. 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 $\bar{B}_q \to D_q^{(*)}\ell^-\bar{\nu}$ decays.

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arXiv2025

Impact on $L$-observables of a new combined analysis of $B_{d,s}\to K^{(*)}$ form factors

We explore the impact of a combined analysis of $B_{d,s}\to K^{(*)}$ form factors on a set of $L$-observables. The $L$-observables are constructed from ratios of branching fractions in $B_{s}\to VV,PP,PV$ versus $B_d \to VV,PP,PV$ decays with $P=K^0,\bar{K}^0$ and $V=K^{*0},\bar{K}^{*0}$, thereby partially reducing their hadronic uncertainties. We show the change of the Standard Model predictions of the $L$-observables under different determinations of the ratio of the relevant form factors (with correlations) including lattice QCD data and a novel light-cone sum rule analysis. In addition, we provide precise results for all $B_{d,s} \to K^{(*)}$ form factors in machine-readable files. We find that the inclusion of our up-to-date results, as well as the use or omission of lattice QCD data for the form factors, has a significant impact on the $L$-observables. We also discuss how the New Physics interpretation is affected by the updated form factors and present revised predictions for the mechanism identified in our analysis of $B \to VP$ decays, now employing more suitable new experimental observables defined in this paper.

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arXiv2024

Unitarity bounds with subthreshold and anomalous cuts for $b$-hadron decays

We derive a generalisation of the Boyd-Grinstein-Lebed (BGL) parametrization. Most form factors (FFs) in $b$-hadron decays exhibit additional branch cuts -- namely subthreshold and anomalous branch cuts -- beyond the ``standard'' unitarity cut. These additional cuts cannot be adequately accounted for by the BGL parametrization. For instance, these cuts arise in the FFs for $B\to D^{(*)}$, $B\to K^{(*)}$, and $\Lambda_b\to \Lambda$ processes, which are particularly relevant from a phenomenological standpoint. We demonstrate how to parametrize such FFs and derive unitarity bounds in the presence of subthreshold and/or anomalous branch cuts. Our work paves the way for a wide range of new FF analyses based solely on first principles, thereby minimising systematic uncertainties.

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arXiv2024

Malaphoric $Z'$ models for $b \rightarrow s \ell^+ \ell^-$ anomalies

We study some phenomenological effects of kinetic mixing between the hypercharge field and a new $U(1)$ gauge field in specific $Z^\prime$ models that ameliorate the tensions between measurements and Standard Model predictions in $b \rightarrow s\ell^+ \ell^-$ decays. To this end, we rederive the dimension-6 SMEFT coefficients resulting from integrating out the kinetically-mixed (`malaphoric') $Z^\prime$ field. The kinetic mixing provides a family-universal component to the couplings of the $Z^\prime$ field, which can improve fits to lepton flavour universality observables. We show how kinetic mixing improves the best fit of the $B_3-L_2$ model to $b\rightarrow s$ data by 7.0 units of $\chi^2$ while remaining compatible with other relevant data sets such as electroweak precision observables and measurements of $e^+ e^- \rightarrow \ell^+ \ell^-$ at LEP2.

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arXiv2024

Theoretical predictions for $b\to s \mu^+ \mu^-$ decays

I review the state of the art of the theoretical calculations for decays mediated by $b\to s \ell^+ \ell^-$ transitions, for $\ell=e,\mu$. I focus on the predictions of observables in $B\to K \mu^+\mu^-$, $B\to K^* \mu^+\mu^-$, and $B_s\to \phi \mu^+\mu^-$ decays, as many of these predictions are in tension with the corresponding experimental measurements. I also briefly discuss the $\Lambda_b\to \Lambda \mu^+\mu^-$ decay and present a new calculation for this channel. Special emphasis is placed on the non-local contributions, as they are the largest systematic uncertainties in these decays. The current theoretical calculations for $b\to s \mu^+ \mu^-$ decays are not able to explain the tensions with the experimental measurements.

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arXiv2023

Non-factorisable Contributions of Strong-Penguin Operators in $\Lambda_b \to \Lambda \ell^+\ell^-$ Decays

We investigate for the first time a certain class of non-factorisable contributions of the four-quark operators ${\cal O}_{3-6}$ in the weak effective Hamiltonian to the $\Lambda_b \to \Lambda \ell^+\ell^-$ decay amplitude. We focus on the case where a virtual photon is radiated from one of the light constituents of the $\Lambda_b$ baryon, in the kinematic situation of large hadronic recoil with an energetic $\Lambda$ baryon in the final state. The effect on the suitably defined ``non-local form factors'' is calculated using the light-cone sum rule approach for a correlator with an interpolating current for the light $\Lambda$ baryon. We find that this approach requires the introduction of new soft functions that generalise the standard light-cone distribution amplitudes (LCDAs) for the heavy $\Lambda_b$ baryon. We give a heuristic discussion of their properties and a model that relates them to the standard LCDAs. Within this framework, we provide numerical results for the size of the non-local form factors considered.

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arXiv2023

$B\to D_0^$ and $B_s\to D_{s0}^$ form factors from QCD light-cone sum rules

We present the first application of QCD light-cone sum rules (LCSRs) with $B_{(s)}$-meson distribution amplitudes to the $B_{(s)}\!\to\! D_{(s)0}^*$ form factors, where $D_{(s)0}^*$ is a charmed scalar meson. We consider two scenarios for the $D_0^*$ spectrum. In the first one, we follow the Particle Data Group and consider a single broad resonance $D_0^*(2300)$. In the second one, we assume the existence of two scalar resonances, $D_0^*(2105)$ and $D_0^*(2451)$, as follows from a recent theoretically motivated analysis of $B\to D\pi\pi$ decays. The $B\!\to\! D_0^*$ form factors are calculated in both scenarios, also taking into account the large total width of $D_0^*(2300)$. Furthermore, we calculate the $B_s\!\to\! D_{s0}^*$ form factors, considering in this case only the one-resonance scenario with $D_{s0}(2317)$. In this LCSRs calculation, the $c$-quark mass is kept finite and the $s$-quark mass is taken into account. We also include contributions of the two- and three-particle distribution amplitudes up to twist-four. Our predictions for semileptonic $B\!\to\! D_0^*\ell\nu_\ell$ and $B_s\!\to\! D_{s0}^*\ell\nu_\ell$ branching ratios are compared with the available data and HQET-based predictions. As a byproduct, we also obtain the $D_0^*$- and $D_{s0}^*$-meson decay constants and predict the lepton flavour universality ratios $R(D_0^*)$ and $R(D_{s0}^*)$.

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arXiv2023

Dispersive Analysis of $B\to K^{(*)}$ and $B_s\to \phi$ Form Factors

We perform the first simultaneous dispersive analysis of the $B\to K$, $B\to K^*$, and $B_s\to \phi$ form factors. By means of an improved parametrization, we take into account the form factors' below-threshold branch cuts arising from on-shell $\bar{B}_s \pi^0$ and $\bar{B}_s \pi^0 \pi^0$ 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 $\bar{B}\to \bar{K}^{(*)}$ and $\bar{B}_s\to \phi$ form factors in and beyond the entire semileptonic phase space.

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arXiv2022

On the contribution of the electromagnetic dipole operator ${\cal O}_7$ to the $\bar B_s \to μ^+μ^-$ decay amplitude

We construct a factorization theorem that allows to systematically include QCD corrections to the contribution of the electromagnetic dipole operator in the effective weak Hamiltonian to the $\bar B_s \to μ^+μ^-$ decay amplitude. We first rederive the known result for the leading-order QED box diagram, which features a double-logarithmic enhancement associated to the different rapidities of the light quark in the $\bar B_s$ meson and the energetic muons in the final state. We provide a detailed analysis of the cancellation of the related endpoint divergences appearing in individual momentum regions, and show how the rapidity logarithms can be isolated by suitable subtractions applied to the corresponding bare factorization theorem. This allows us to include in a straightforward manner the QCD corrections arising from the renormalization-group running of the hard matching coefficient of the electromagnetic dipole operator in soft-collinear effective theory, the hard-collinear scattering kernel, and the $B_s$-meson distribution amplitude. Focusing on the contribution from the double endpoint logarithms, we derive a compact formula that resums the leading-logarithmic QCD corrections.

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arXiv2022

Improved Theory Predictions and Global Analysis of Exclusive $\boldsymbol{b\to sμ^+μ^-}$ Processes

We provide improved Standard Model theory predictions for the exclusive rare semimuonic processes $B\to K^{(*)}μ^+μ^-$ and $B_s\toϕμ^+μ^-$. Our results are based on a novel parametrization of the non-local form factors, which manifestly respects a recently developed dispersive bound. We critically compare our predictions to those obtained in the framework of QCD factorization. Our predictions provide, for the first time, parametric estimates of the systematic uncertainties due to non-local contributions. Comparing our predictions within the Standard Model to available experimental data, we find a large tension for $B\to Kμ^+μ^-$. A simple model-independent analysis of potential effects beyond the Standard Model yields results compatible with other approaches, albeit with larger uncertainties for the $B\to K^*μ^+μ^-$ and $B_s\to ϕμ^+μ^-$ decays. Our approach yields systematically improvable predictions, and we look forward to its application in further analyses beyond the Standard Model.

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arXiv2022

$B\to D_1(2420)$ and $B\to D_1'(2430)$ form factors from QCD light-cone sum rules

We perform the first calculation of form factors in the semileptonic decays $B\!\to\! D_1(2420)\ellν_\ell$ and $B \to D_1^\prime (2430)\ell ν_\ell$ using QCD light-cone sum rules (LCSRs) with $B$-meson distribution amplitudes. In this calculation the $c$-quark mass is finite. Analytical expressions for two-particle contributions up to twist four are obtained. To disentangle the $D_1$ and $D_1^\prime$ contributions in the LCSRs, we suggest a novel approach that introduces a combination of two interpolating currents for these charmed mesons. To fix all the parameters in the LCSRs, we use the two-point QCD sum rules for the decay constants of $D_1$ and $D_1^\prime$ mesons augmented by a single experimental input, that is the $B \to D_1(2420)\ellν_\ell$ decay width. We provide numerical results for all $B\to D_1$ and $B\to D_1^\prime$ form factors. As a byproduct, we also obtain the $D_1$- and $D_1'$-meson decay constants and predict the lepton-flavour universality ratios $R(D_1)$ and $R(D_1')$.

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arXiv2021

EOS -- A Software for Flavor Physics Phenomenology

EOS is an open-source software for a variety of computational tasks in flavor physics. Its use cases include theory predictions within and beyond the Standard Model of particle physics, Bayesian inference of theory parameters from experimental and theoretical likelihoods, and simulation of pseudo events for a number of signal processes. EOS ensures high-performance computations through a C++ back-end and ease of usability through a Python front-end. To achieve this flexibility, EOS enables the user to select from a variety of implementations of the relevant decay processes and hadronic matrix elements at run time. In this article, we describe the general structure of the software framework and provide basic examples. Further details and in-depth interactive examples are provided as part of the EOS online documentation.

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arXiv2021

Non-local matrix elements in $B_{(s)}\to \{K^{(*)},ϕ\}\ell^+\ell^-$

We revisit the theoretical predictions and the parametrization of non-local matrix elements in rare $\bar{B}_{(s)}\to \lbrace \bar{K}^{(*)},ϕ\rbrace\ell^+\ell^-$ and $\bar{B}_{(s)}\to \lbrace \bar{K}^{*}, ϕ\rbrace γ$ decays. We improve upon the current state of these matrix elements in two ways. First, we recalculate the hadronic matrix elements needed at subleading power in the light-cone OPE using $B$-meson light-cone sum rules. Our analytical results supersede those in the literature. We discuss the origin of our improvements and provide numerical results for the processes under consideration. Second, we derive the first dispersive bound on the non-local matrix elements. It provides a parametric handle on the truncation error in extrapolations of the matrix elements to large timelike momentum transfer using the $z$ expansion. We illustrate the power of the dispersive bound at the hand of a simple phenomenological application. As a side result of our work, we also provide numerical results for the $B_s \to ϕ$ form factors from $B$-meson light-cone sum rules.

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arXiv2021

Lepton-flavour non-universality of $\bar{B}\to D^*\ell \barν$ angular distributions in and beyond the Standard Model

We analyze in detail the angular distributions in $\bar{B}\to D^*\ell \barν$ 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 current presentation of the experimental data is not ideal and prohibits the extraction of the full set of relevant BSM parameters, since the number of independent angular observables that can be inferred from data is limited to only four. We uncover a $\sim4σ$ tension between data and predictions that is hidden in the redundant presentation of the Belle 2018 data on $\bar{B}\to D^*\ell \barν$ decays. This tension specifically involves observables that probe $e-μ$ 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 $e-μ$ 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.

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