Impact of CPV phases on flavour violating HH and ZZ decays

Standard Model extensions via heavy neutral leptons lead to modifications in the lepton mixing matrix, including new Dirac and Majorana CP violating phases. Here we consider the role of the Majorana fermions and of new CP violating phases in Higgs and $Z$-boson lepton flavour violating decays, as well as in the corresponding CP-asymmetries. We confirm that these decays are sensitive to the presence of additional sterile states and show that the new CP violating phases may lead to both destructive and constructive interferences in the decay rates. Interestingly the $Z\to \mu^\pm\tau^\mp$ rates are within FCC-ee reach, with associated CP-asymmetries that can potentially reach up to 30%.Comment: Contribution to the 2023 Electroweak session of the 57th Rencontres de Moriond, also presented at Beauty 202

A combined explanation of the B-decay anomalies with a single vector leptoquark

International audienc

Anomalies in 8^8Be nuclear transitions and (g−2)e,μ(g-2)_{e,\mu}: towards a minimal combined explanation

International audienceMotivated by a simultaneous explanation of the apparent discrepancies in the light charged lepton anomalous magnetic dipole moments, and the anomalous internal pair creation in $^{8}$Be nuclear transitions, we explore a simple New Physics model, based on an extension of the Standard Model gauge group by a U(1)$_{B−L}$. The model further includes heavy vector-like fermion fields, as well as an extra scalar responsible for the low-scale breaking of U(1)$_{B−L}$, which gives rise to a light Z′ boson. The new fields and currents allow to explain the anomalous internal pair creation in $^{8}$Be while being consistent with various experimental constraints. Interestingly, we find that the contributions of the Z′ and the new U(1)$_{B−L}$-breaking scalar can also successfully account for both (g −2)$_{e,μ}$ anomalies; the strong phenomenological constraints on the model’s parameter space ultimately render the combined explanation of (g − 2)$_{e}$ and the anomalous internal pair creation in $^{8}$Be particularly predictive. The underlying idea of this minimal “prototype model” can be readily incorporated into other protophobic U(1) extensions of the Standard Model

A combined explanation of the BB-decay anomalies with a single vector leptoquark

International audienceMotivated by the recent experimental progress on the $R_{D^{(\ast)}}$ and$R_{K^{(\ast)}}$ anomalies in $B$-decays, we consider an extensionof the Standard Model by a single vector leptoquark field. We study how one can achieve the required lepton flavour non-universality,starting from a priori universal gauge couplings. While the unitary coupling flavour structure, induced by the mass misalignment of quarks and leptons after $SU(2)_L$ breaking, does not allow to comply with stringent bounds from lepton flavour violating processes, we find that effectively non-unitary couplings, due to mixings with heavy vector-like fermions, hold the key tosimultaneously address the $R_{K^{(\ast)}}$ and $R_{D^{(\ast)}}$anomalies. Furthermore, in the near future, the expected progress in the sensitivity on charged lepton flavour violating observables should allow to probe a large region of the preferred parameter space in this class of vector leptoquark models

Prospects for a flavour violating Z′Z^\prime explanation of Δaμ,e\Delta a_{\mu,e}

The apparent tensions emerging from the comparison of experimental data of the anomalous magnetic moments of the muon and electron to the Standard Model predictions ($\Delta a_{\mu,e}$) could be interpreted as a potential signal of New Physics. Models encompassing a light vector boson have been known to offer a satisfactory explanation to $\Delta a_{\mu}$, albeit subject to stringent experimental constraints. Here we explore a minimal extension of the Standard Model via a leptophilic vector boson $Z^\prime$, under the hypothesis of strictly flavour-violating couplings of the latter to leptons. The most constraining observables to this ad-hoc construction emerge from lepton flavour universality violation (in $Z$ and $\tau$ decays) and from rare charged lepton flavour violating transitions. Once these are accommodated, one can saturate the tensions in $\Delta a_{\mu}$, but $\Delta a_{e}$ is predicted to be Standard Model-like. We infer prospects for several observables, including leptonic $Z$ decays and several charged lepton flavour violating processes. We also discuss potential signatures of the considered $Z^\prime$ at a future muon collider, emphasising the role of the $\mu^+\mu^- \to\tau^+\tau^-$ forward-backward asymmetry as a key probe of the model

The role of leptonic CPV phases in cLFV observables

International audienceIn models where the Standard Model is extended by Majorana fermions, interference effects due to the presence of CP violating phases have been shown to play a crucial role in lepton number violating processes. However, important effects can also arise in lepton number conserving, but charged lepton flavour violating (cLFV) transitions and decays. Here we show that the presence of CP violating (Dirac and Majorana) phases can have a striking impact for the predicted rates of cLFV observables. We explore the interference effects in several cLFV observables, carrying for the first time a thorough analysis of the different observables and the implications for future observation. We discuss how the presence of leptonic CP violating phases might lead to a loss of correlation between observables (typically present in simple SM extensions via heavy sterile fermions), or even to the suppression of certain channels; these effects can be interpreted as suggestive of non-vanishing phases

Flavour and CP symmetries in the inverse seesaw

International audienceWe consider an inverse seesaw mechanism of neutrino mass generation in which the Standard Model is extended by $3+3$ (heavy) sterile states, and endowed with a flavour symmetry $G_f$, $G_f=\Delta (3 \, n^2)$ or $G_f=\Delta (6 \, n^2)$, and a CP symmetry. These symmetries are broken in a peculiar way, so that in the charged lepton sector a residual symmetry $G_\ell$ is preserved, while the neutral fermion sector remains invariant under the residual symmetry $G_\nu =Z_2 \times CP$. We study the concrete setup, where the Majorana mass term for three of the sterile states conserves $G_\nu$, while the remaining mass terms (i.e. couplings of left-handed leptons and heavy sterile states, as well as the Dirac-type couplings among the latter) do not break the flavour or CP symmetry. We perform a comprehensive analysis of lepton mixing for different classes of residual symmetries, giving examples for each of these, and study in detail the impact of the additional sterile states on the predictions for lepton mixing. We further confront our results with those obtained in the model-independent scenario, in which the light neutrino mass matrix leaves the residual symmetry $G_\nu$ intact. We consider the phenomenological impact of the inverse seesaw mechanism endowed with flavour and CP symmetries, in particular concerning effects of non-unitarity of the lepton mixing matrix (which strongly constrain the parameter space of the scenario), prospects for neutrinoless double beta decay and for charged lepton flavour violating processes

Prospects for a flavour violating Z′Z^\prime explanation of Δaμ,e\Delta a_{\mu,e}

The apparent tensions emerging from the comparison of experimental data of the anomalous magnetic moments of the muon and electron to the Standard Model predictions ($\Delta a_{\mu,e}$) could be interpreted as a potential signal of New Physics. Models encompassing a light vector boson have been known to offer a satisfactory explanation to $\Delta a_{\mu}$, albeit subject to stringent experimental constraints. Here we explore a minimal extension of the Standard Model via a leptophilic vector boson $Z^\prime$, under the hypothesis of strictly flavour-violating couplings of the latter to leptons. The most constraining observables to this ad-hoc construction emerge from lepton flavour universality violation (in $Z$ and $\tau$ decays) and from rare charged lepton flavour violating transitions. Once these are accommodated, one can saturate the tensions in $\Delta a_{\mu}$, but $\Delta a_{e}$ is predicted to be Standard Model-like. We infer prospects for several observables, including leptonic $Z$ decays and several charged lepton flavour violating processes. We also discuss potential signatures of the considered $Z^\prime$ at a future muon collider, emphasising the role of the $\mu^+\mu^- \to\tau^+\tau^-$ forward-backward asymmetry as a key probe of the model

A nonunitary interpretation for a single vector leptoquark combined explanation to the BB-decay anomalies

International audienceIn order to simultaneously account for both R_{D(}_{∗)} and R_{K (}_{∗)} anomalies in B-decays, we consider an extension of the Standard Model by a single vector leptoquark field, and study how one can achieve the required lepton flavour non-universality, starting from a priori universal gauge couplings. While the unitary quark-lepton mixing induced by SU(2)$_{L}$ breaking is insufficient, we find that effectively nonunitary mixings hold the key to simultaneously address the R_{K (}_{∗)} and R$_{D}$(∗) anomalies. As an intermediate step towards various UV-complete models, we show that the mixings of charged leptons with additional vector-like heavy leptons successfully provide a nonunitary framework to explain R_{K (}_{∗)} and R_{D(}_{∗)} . These realisations have a strong impact for electroweak precision observables and for flavour violating ones: isosinglet heavy lepton realisations are already excluded due to excessive contributions to lepton flavour violating Z -decays. Furthermore, in the near future, the expected progress in the sensitivity of charged lepton flavour violation experiments should allow to fully probe this class of vector leptoquark models