arXiv subjects

Davide Milillo

Publications and source records attributed to Davide Milillo.

4 recordsLinked to original sources

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 $B^- \to \ell^- {\bar \nu}_\ell$, $\bar B^0 \to \pi^+ \ell^- {\bar \nu}_\ell$, and $B^- \to \rho^0 \ell^- {\bar \nu}_\ell$ 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.

hep-ph↗

arXiv2026

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

We examine possible correlations between $b \to s \ell_1^- \ell_2^+$ transitions -- both in the lepton flavor conserving ($\ell_1=\ell_2$) and violating case ($\ell_1 \neq \ell_2$) -- and purely leptonic flavor violating decays within the ABCD model [1], a minimal abelian extension of the Standard Model (SM) introducing a new $\text{U}(1)'$ symmetry. The associated neutral $Z'$ boson has generation-dependent, flavor non-universal couplings to SM fermions, governed by three rational parameters $\epsilon_{1,2,3}$, which sum to zero to ensure gauge anomaly cancellation. Each $\epsilon_i$ is common to all fermions of a given generation, thus inducing correlations among quark and lepton observables. For lepton flavor conserving (LFC) processes, only small deviations from SM predictions were found [2], reflecting the mutual constraints between the quark and lepton sectors, which preclude large discrepancies. On the other hand, the model allows tree-level lepton flavor violating (LFV) decays, yielding correlations between LFV $b\to s$ transitions and charged lepton decays. The analysis of such correlations shows that the current experimental upper bounds for the rates of $\tau^- \to \mu^-\mu^+\mu^-$, \ $\mu^-\to e^- \gamma$, \ $\mu^- \to e^- e^+e^-$ and $\mu^- \to e^-$ conversion in nuclei constrain branching ratios of LFV $B_{(s)}$ decays in hierarchical order [2].

hep-ph↗

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 $\epsilon_{1,2,3}$ summing to zero by the condition of cancellation of the gauge anomalies. Each $\epsilon_i$ 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 $\tau^- \to \mu^- \mu^+ \mu^-$, $\mu^- \to e^- \gamma$, $\mu^- \to e^- e^+ e^-$ and the $ \mu^- \to e^-$ conversion in nuclei play a hierarchical role in constraining the branching fractions of lepton flavour violating $B$ and $B_s$ decays.

hep-ph↗

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 $\epsilon_{1,2,3}$ that sum to zero imposing the cancellation of the gauge anomalies. Each $\epsilon_i$ 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 $\tau^- \to \mu^- \mu^+ \mu^-$, $\mu^- \to e^- \gamma$, $\mu^- \to e^- e^+ e^-$ and the $ \mu^- \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 $\tau^- \to \mu^- \mu^+ \mu^-$ does not impose significant restrictions, the other three modes set progressively more stringent limits, an important information for the planned new experimental facilities.

hep-ph↗