arXiv subjects

Fulvia De Fazio

Publications and source records attributed to Fulvia De Fazio.

At least 19 recordsLinked to original sources

arXiv2026

Semileptonic $B_q$ decays to heavy tensor mesons

The semileptonic decays of $B_q$ mesons ($q=u,d,s$) to $J^P=2^+$ charmed mesons are investigated. The form factors parametrising the hadronic matrix elements of the weak vector and axial-vector quark currents are evaluated using light-cone QCD sum rules with external $B_q$ state, including the 3-particle contributions. The form factors of pseudoscalar and tensor quark currents occurring in extensions of the Standard Model are also determined. The relations expected in the heavy quark limit are tested, providing a hint about the size of finite heavy quark mass corrections for the various form factors. Results are presented for the semileptonic $B_q$ to heavy tensor meson decay rates in the Standard Model, and for the ratios testing Lepton Flavour Universality.

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arXiv2026

Hunting for new physics in $B$ meson $b \to u$ transitions: A fresh look with Belle II data

We present an analysis of the decays $B^- \to \ell^- {\bar ν}_\ell$, $\bar B^0 \to π^+ \ell^- {\bar ν}_\ell$, and $B^- \to ρ^0 \ell^- {\bar ν}_\ell$ leveraging the precise measurements of binned differential branching fractions recently reported by the Belle II Collaboration for the semileptonic modes. We adopt a generalized low-energy Hamiltonian that incorporates all dimension-six operators with left-handed neutrinos -- consistent with the Standard Model effective field theory (SMEFT) -- featuring complex, lepton-flavor-dependent Wilson coefficients. For three representative values of $|V_{ub}|$, we perform a fit to the measured $q^2$-binned observables to constrain the coupling parameter space and determine the corresponding confidence regions. Finally, we perform a global fit for the three decay channels, determining $|V_{ub}|$ together with the new physics couplings.

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arXiv2026

Two-lepton tales: Dalitz decays of heavy quarkonia

We study the Dalitz decays of heavy quarkonia, which result from the internal virtual photon conversion into an $\ell^+ \ell^-$ lepton pair. Heavy-quark symmetries allow us to establish systematic relations between transitions of different quarkonium states, and to precisely determine the branching fractions for several charmonium and bottomonium decay modes. For charmonium, existing data on $χ_{cJ}(1P)\to J/ψ\ell^+ \ell^-$ and $ψ(2S)\to χ_{cJ}(1P) \ell^+ \ell^-$ enable us to determine the parameters of the transition form factors and to predict the rates of yet-unobserved modes. The Dalitz transitions of $χ_{c1}(3872)$ are important, as they can help assessing the structure of this meson. For bottomonium, recent LHCb measurements allow us to predict the branching fractions of $χ_{bJ}(nP)\to Υ(1S)\ell^+ \ell^-$ and $h_b(nP)\to η_b(1S) \ell^+ \ell^-$ ($n=1,\,2)$. We also investigate the sensitivity of heavy quarkonia Dalitz modes to the contribution of a new light vector mediator, such as the putative $X(17)$.

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arXiv2026

Quark-lepton correlations in gauge anomaly free abelian extension of the Standard Model

We study $b \to s \ell_1^+ \ell_2^-$ transitions, both for the lepton flavour conserving $\ell_1=\ell_2$ and violating case $\ell_1 \neq \ell_2$, in a minimal extension of the Standard Model proposed in [1]. In this framework, the Standard Model (SM) gauge group is enlarged by a new $U(1)^\prime$ component. The fermion $U(1)^\prime$ charges are assigned in a generation-dependent way, and involve three rational parameters $ε_{1,2,3}$ summing to zero by the condition of cancellation of the gauge anomalies. Each $ε_i$ is common to all fermions in a generation, which produces correlations among quark and lepton observables. The new neutral gauge boson $Z^\prime$ has flavour violating couplings to quarks and leptons. For SM allowed processes, small deviations with respect to the SM predictions are found: this is a consequence of a feature of the model where quark and lepton sectors preclude each other large deviations from SM. Lepton flavour violating processes are allowed at tree-level. The experimental upper bounds for the rates of the processes $τ^- \to μ^- μ^+ μ^-$, $μ^- \to e^- γ$, $μ^- \to e^- e^+ e^-$ and the $ μ^- \to e^-$ conversion in nuclei play a hierarchical role in constraining the branching fractions of lepton flavour violating $B$ and $B_s$ decays.

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arXiv2025

Correlating lepton flavour violating $b \to s$ and leptonic decay modes in a minimal abelian extension of the Standard Model

We consider an abelian extension of the Standard Model (SM) comprising a new gauge group $U(1)^\prime$, with the neutral gauge boson $Z^\prime$ having flavour violating couplings to quarks and leptons. The fermion content is the same as in SM except for the addition of three right-handed neutrinos. The model, proposed in \cite{Aebischer:2019blw}, describes the couplings of $Z^\prime$ to fermions in terms of three rational parameters $ε_{1,2,3}$ that sum to zero imposing the cancellation of the gauge anomalies. Each $ε_i$ is common to all fermions in a generation, a feature producing correlations among quark and lepton observables. We focus on $b \to s \ell_1^- \ell_2^+$ transitions for the lepton flavour conserving $\ell_1=\ell_2$ and lepton flavour violating case $\ell_1 \neq \ell_2$. Small deviations with respect to the SM predictions are found in the first case, which reflects a feature of the model where quark and lepton sectors prevent each other to manifest large discrepancies with respect to SM. We investigate the correlations between rare $B$ and $B_s$ decays and the leptonic processes $τ^- \to μ^- μ^+ μ^-$, $μ^- \to e^- γ$, $μ^- \to e^- e^+ e^-$ and the $ μ^- \to e^-$ conversion in nuclei. We show that the current experimental upper bounds on these four channels play an increasingly important role in constraining the branching fractions of lepton flavour violating $B$ and $B_s$ decays. While the present bound on $τ^- \to μ^- μ^+ μ^-$ does not impose significant restrictions, the other three modes set progressively more stringent limits, an important information for the planned new experimental facilities.

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arXiv2025

The charming case of $X(3872)$ and $χ_{c1}(2P)$

More than twenty years have elapsed since the discovery of $χ_{c1}(3872)$ (previously denoted as $X(3872)$), and an impressive amount of theoretical and experimental studies has been devoted to its properties, decays and production mechanisms. Despite the extensive work, a full understanding of the nature of $χ_{c1}(3872)$ is still missing. In the present study we reconsider a theoretical framework based on the heavy quark large mass limit to analyse the radiative decays of heavy quarkonia, in particular the electric dipole transitions of $χ_{c1}(2P)$ to S-wave charmonia. The results favourably compare to recent measurements for $χ_{c1}(3872)$, obtained by the LHCb Collaboration, if this meson is identified with $χ_{c1}(2P)$.

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arXiv2025

Kaon Physics: A Cornerstone for Future Discoveries

The kaon physics programme, long heralded as a cutting-edge frontier by the European Strategy for Particle Physics, continues to stand at the intersection of discovery and innovation in high-energy physics (HEP). With its unparalleled capacity to explore new physics at the multi-TeV scale, kaon research is poised to unveil phenomena that could reshape our understanding of the Universe. This document highlights the compelling physics case, with emphasis on exciting new opportunities for advancing kaon physics not only in Europe but also on a global stage. As an important player in the future of HEP, the kaon programme promises to drive transformative breakthroughs, inviting exploration at the forefront of scientific discovery.

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arXiv2024

Constraining $ν$SMEFT coefficients: the case of the extra $\text{U}(1)^\prime$

We study the constraints on low-energy coefficients of the $ν$SMEFT generalization of the Standard Model effective theory in the simple case of a $\text{U}(1)^\prime$ enlargement of the Standard Model gauge group. In particular, we analyse the constraints imposed by the requirement that the extended theory remains free of gauge anomalies. We present the cases of explicit realisations, showing the obtained correlations among the coefficients of $d=6$ operators.

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arXiv2024

New Physics couplings from angular coefficient functions of $\bar B \to D^* (D π) \ell \bar ν_\ell$

The Belle Collaboration has recently measured the complete set of angular coefficient functions for the exclusive decays $\bar B \to D^* (D π) \ell \bar ν_\ell$, with $\ell=e,\,μ$, in four bins of the parameter $w=\displaystyle\frac{m_B^2+m_{D^*}^2-q^2}{2m_B m_{D^*}}$, with $q$ the lepton pair momentum. Under the assumption that physics beyond the Standard Model does not contribute to such modes, the measurements are useful to determine the hadronic form factors describing the $B \to D^*$ matrix elements of the Standard Model weak current, and to improve the determination of $|V_{cb}|$. On the other hand, they can be used to assess the impact of possible new physics contributions. In a bottom-up approach, we extend the Standard Model effective Hamiltonian governing this mode with the inclusion of the full set of Lorentz invariant d=6 operators compatible with the gauge symmetry of the theory. The measured angular coefficient functions can tightly constrain the couplings in the generalized Hamiltonian. We present the first results of this analysis, discussing how improvements can be achieved when more complete data on the angular coefficient functions will be available.

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arXiv2024

Flavour anomalies, correlations, hadronic uncertainties, and all that

We present a short overview of the so-called flavour anomalies, discussing their significance and the connections with QCD issues discussed at the HADRON 2023 conference.

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arXiv2023

Novelties from Flavour Physics

Our present understanding of elementary particle interactions is the synergic result of developments of theoretical ideas and of experimental advances that lead to the theory known as the Standard Model of particle physics. Despite the uncountable experimental confirmations, we believe that it is not yet the ultimate theory, for a number of reasons that I will briefly recall in this lecture. My main focus will be on the role that Flavour Physics has played in development of the theory in its present formulation, as well as on the opportunities that this sector offers to discover physics beyond the Standard Model.

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arXiv2023

Dalitz decays $D_{sJ}^{()} \to D_s^{()} \ell^+ \ell^- $

The Dalitz decays of the positive parity $D_{sJ}^{(*)}$ charmed mesons, $D_{sJ}^{(*)} \to D_s^{(*)} \ell^+ \ell^-$ with $J=0,1,2$ and $\ell=e, μ$, are important processes to investigate the nature of the $D_{sJ}^{(*)}$ states. We analyze the full set of decays, considering the four lightest $D_{sJ}^{(*)}$ mesons as belonging to the heavy quark spin doublets $\displaystyle s_\ell^P=\frac{1}{2}^+$ and $\displaystyle \frac{3}{2}^+$, with $s_\ell^P$ the spin-parity of the light degrees of freedom in mesons. The description implies relations among the observables in various modes. We study the decay distributions in the dilepton invariant mass squared and the distributions in the angle between the charged lepton momentum and the momentum of the produced meson, which are expressed in terms of universal form factors and of effective strong couplings. Such measurements are feasible at the present facilities.

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arXiv2023

331 Model Predictions for Rare $B$ and $K$ Decays, and $ΔF=2$ Processes: an Update

Motivated by the improved results from the HPQCD lattice collaboration on the hadronic matrix elements entering $ΔM_{s,d}$ in $B_{s,d}^0-\bar B_{s,d}^0$ mixings and the increase of the experimental branching ratio for $B_s\toμ^+μ^-$, we update our 2016 analysis of various flavour observables in four 331 models, M1, M3, M13 and M16 based on the gauge group $SU(3)_C\times SU(3)_L\times U(1)_X$. These four models, which are distinguished by the quantum numbers, are selected among 24 331 models through their consistency with the electroweak precision tests and simultaneously by the relation $C_9^\text{NP}=-b\, C_{10}^\text{NP}$ with $b\ge 2$, which after new result on $B_s\toμ^+μ^-$ from CMS is favoured over the popular relation $C_9^\text{NP}=- C_{10}^\text{NP}$ predicted by several leptoquark models. In this context we investigate in particular the dependence of various observables on $|V_{cb}|$, varying it in the broad range $[0.0386,\,0.043]$, that encompasses both its inclusive and exclusive determinations. Imposing the experimental constraints from $\varepsilon_K$, $ΔM_s$, $ΔM_d$ and the mixing induced CP asymmetries $S_{ψK_S}$ and $S_{ψK_S}$, we investigate for which values of $|V_{cb}|$ the four models can be made compatible with these data and what is the impact on $B$ and $K$ branching ratios. In particular we analyse NP contributions to the Wilson coefficients $C_9$ and $C_{10}$ and the decays $B_{s,d}\toμ^+μ^-$, $K^+\rightarrowπ^+ν\barν$ and $K_L\rightarrowπ^0ν\barν$. This allows us to illustrate how the value of $|V_{cb}|$ determined together with other parameters of these models is infected by NP contributions and compare it with the one obtained recently under the assumption of the absence of NP in $\varepsilon_K$, $ΔM_s$, $ΔM_d$ and $ S_{ψK_S}$.

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arXiv2022

Inclusive semileptonic $Λ_b$ decays in the Standard Model and beyond

Inclusive semileptonic decays of beauty baryons are studied using the heavy quark expansion to ${\cal O}(1/m_b^3)$, at leading order in $α_s$. The case of a polarized decaying baryon is examined, with reference to $Λ_b$. An extension of the Standard Model effective Hamiltonian inducing $b \to U \ell {\bar ν}_ \ell$ transitions ($U=u,\,c$ and $\ell=e,\,μ,\,τ$) is considered, which comprises the full set of D=6 semileptonic operators with left-handed neutrinos. The effects of the new operators in several observables are described.

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arXiv2021

$c \to u ν{\bar ν}$ transitions of $B_c$ mesons: 331 model facing Standard Model null tests

The Glashow-Iliopoulos-Maiani mechanism is extremely efficient to suppress the flavour-changing neutral current decays of charmed hadrons induced by the $c \to u$ transitions, making such processes particularly sensitive to phenomena beyond the Standard Model. In particular, $c \to u$ decays with a neutrino pair in the final state are theoretically appealing due to the small long-distance contributions. Moreover, in the framework of the Standard Model Effective Field Theory (SMEFT), the $SU(2)_L$ invariance allows to relate the Wilson coefficients in the effective Hamiltonian governing the $c \to u ν{\bar ν}$ decays to the coefficients in the $c \to u \ell^+ \ell^-$ Hamiltonian. We analyze the $B_c \to B^{(*)+} ν{\bar ν}$ decays, for which branching fractions of at most ${\cal O}(10^{-16})$ are predicted in the Standard Model including short- and long-distance contributions, so small that they can be considered as null tests. Using SMEFT and the relation to the $c \to u \ell^+ \ell^-$ processes we study the largest enhancement achievable in generic new physics scenarios. Then we focus on a particular extension of the Standard Model, the 331 model. SMEFT relations and the connection with $c \to u \ell^+ \ell^-$ imply that ${\cal B}(B_c \to B^{(*)+} ν{\bar ν})$ could even reach ${\cal O}(10^{-6})$, an extremely large enhancement. A less pronounced effect is found in the 331 model, with ${\cal O}(10^{-11})$ predicted branching fractions. Within the 331 model correlations exist among the $B_c \to B^{(*)+}ν\bar ν$ and $K\to πν\bar ν$, $B\to (X_s, K, K^*) ν\bar ν$ channels.

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arXiv2021

Probing New Physics with heavy hadron decays

The increasing number of flavour anomalies calls for the investigation of new processes where tensions similar to the observed ones could emerge. Observables sensitive to physics beyond the Standard Model need to be identified. We discuss the inclusive semileptonic decays of polarized beauty baryons, computed through the heavy quark expansion at ${\cal O}(1/m_b^3)$ and at the leading order in $α_s$. We account for New Physics interactions in a model-independent way, extending the Standard Model $b \to U \ell {\bar ν}_ \ell$ low energy Hamiltonian (with $U=u,\,c$ and $\ell=e,\,μ,\,τ$) with the inclusion of the full set of D=6 semileptonic operators with left-handed neutrinos. We identify a set of promising observables, the study of which can be included in the physics programmes of future facilities, such as FCC-e.

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arXiv2021

The charm of 331

We perform a detailed analysis of flavour changing neutral current processes in the charm sector in the context of 331 models. As pointed out recently, in the case of $Z^\prime$ contributions in these models there are no new free parameters beyond those already present in the $B_{d,s}$ and $K$ meson systems analyzed in the past. As a result, definite ranges for new Physics (NP) effects in various charm observables could be obtained. While generally NP effects turn out to be small, in a number of observables they are much larger than the tiny effects predicted within the Standard Model. In particular we find that the branching ratio of the mode $D^0 \to μ^+ μ^-$, despite remaining tiny, can be enhanced by 6 orders of magnitude with respect to the SM. We work out correlations between this mode and rare $B_{d,s}$ and $K$ decays. We also discuss neutral charm meson oscillations and CP violation in the charm system. In particular, we point out that 331 models provide new weak phases that are a necessary condition to have non-vanishing CP asymmetries. In the case of $ΔA_{CP}$, the difference between the CP asymmetries in $D^0 \to K^+ K^-$ and $D^0 \to π^+ π^-$, we find that agreement with experiment can be obtained provided that two conditions are verified: the phases in the ranges predicted in 331 models and large hadronic matrix elements.

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arXiv2019

Quark-Lepton Connections in $Z^\prime$ Mediated FCNC Processes: Gauge Anomaly Cancellations at Work

We consider scenarios with a heavy $Z^\prime$ gauge boson with flavour non-universal quark and lepton couplings with the goal to illustrate how the cancellation of gauge anomalies generated by the additional U(1)$^\prime$ gauge symmetry would imply correlations between FCNC processes in the quark sector, in the lepton sector and most interestingly between quark flavour and lepton flavour violating processes. We present scenarios with only left-handed flavour-violating $Z^\prime$ couplings and those with also right-handed flavour-violating couplings. These are characterized by a small number of free parameters but in contrast to gauge anomaly cancellation in the Standard Model, that takes place separately within each generation, in our scenarios anomaly cancellation involves simultaneously quarks and leptons of all three generations. Our models involve, beyond the ordinary quarks and leptons, three heavy right-handed neutrinos. The models with only left-handed FCNCs of $Z^\prime$ involve beyond $g_{Z^\prime}$ and $M_{Z^\prime}$ two real parameters characterizing the charges of all fermions under the U(1)$^\prime$ gauge symmetry and the CKM and PMNS ones in the quark and lepton sectors, respectively. The models with the right-handed FCNCs of $Z^\prime$ involve few additional parameters. Imposing constraints from well measured $ΔF=2$ observables we identify interesting correlations that involve e.g. $\varepsilon'/\varepsilon$, $B_{s,d}\toμ^+μ^-$, $B\to K(K^*) \ell^+\ell^-$, $K^+\rightarrowπ^+ν\barν$, $K_{L}\rightarrowπ^0ν\barν$ and purely lepton flavour violating decays like $μ\to eγ$, $μ\to 3 e$, $τ\to 3μ$ and $μ-e$ conversion} among others. Also $(g-2)_{μ,e}$ are considered. The impact of the experimental $μ\to eγ$, $μ\to 3 e$ and in particular $μ-e$ conversion bounds on rare $K$ and $B$ decays is emphasized.

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