$U(1)'$ extensions of the SM with generation-dependent couplings to quarks and leptons are a popular explanation of anomalies in rare B-decays, such as the $R_K$ and $R_K^*$ observables. However, a critical issue with such models is often ignored -- most configurations feature a Landau pole in the new gauge coupling that is well below the Planck scale. We present an ansatz that avoids such...
The ratios $R(D^{(*)})$ are defined as the $\tau/\mu$ ratio of the branching fractions of exclusive semileptonic $B \to D^{(*)}$ decays. They are a powerful test of Lepton Flavour Universality, one of the pillars of the SM, and are determined by the hadronic Form Factors (FFs) describing the $B \to D^{(*)}$ decays. Through the novel Dispersive Matrix (DM) approach, we describe these FFs...
Decays of B mesons that proceed through electroweak and radiative penguin amplitudes attract significant attention due to a number of observed discrepancies between the standard-model predictions and the results. Belle II is expected to perform measurements on channels closely related to those exhibiting anomalies and that are uniquely available to Belle II. We present recent results on...
Intriguing hints for lepton flavor universality violation have emerged from the measurement of B meson decays by LHCb. These B anomalies point to the new physics at the low TeV scale, exactly where we expect a solution to the hierarchy problem! In this talk, I will present an economical model that can address the two problems at once. The model is based on a SU(4)/SP(4) fundamental composite...
Dilepton tail distributions have proven to be a very good ground for limiting different New Physics (NP) scenarios. Assuming NP in left-handed particles,the operators $c^{(1)}_{ijk}(\bar{Q_i} \gamma^{\mu} Q_{j})(\bar{L_k}\gamma_\mu L_k)$ and $c^{(3)}_{ijk} (\bar{Q_i} \gamma^{\mu} \sigma_{a} Q_{j})(\bar{L_k}\gamma_\mu \sigma^{a} L_k) $ also induce a neutral-current interaction with neutrinos,...
Given the hints of lepton-flavour non-universality in semi-leptonic~$B$ decays, leptoquark (LQ) models with sizeable couplings to heavy quark flavours are enjoying a renaissance. While such models are subject to stringent constraints from low-energy experiments also bounds from non-resonant dilepton searches at the Large Hadron Collider~(LHC) turn out to be phenomenologically relevant. Based...
In 2016 the ATOMKI collaboration measured an anomaly in the angular distribution of the pair produced by the M1 transition of the isoscalar 1+ state on 8Be, which might be explained by creation and decay of a boson, the X17, with mass 16.7 MeV/c2.
The MEGII detector is thought to measure the cLFV decay mu to e and gamma, but it could measure the X17 anomaly with a different detection...
Lepton Flavor Violating (LFV) observables such as $\mu\to e\gamma$, $\mu\to 3e$ and $\mu N \to eN$ are among the best probes for new physics at the TeV scale. In the near future the bounds on these observables will improve by many orders of magnitude. In this work we use the SM EFT to understand the impact of these measurements. The precision reach on the measurement of these processes is such...
Axion-like Particles (ALPs) appear in many well-motivated extensions of the Standard Model. We present a detailed study of the phenomenology of ALPs with lepton flavor-violating couplings. Studied observables include rare LFV decays of muons and taus, non-decay experiments such as the electron and muon electric dipole moment (EDM) and muonium oscillations, and as well the effect...
We construct an effective theory (EFT) of an axion-like particle (ALP) for an additionally gauged $U(1)$ symmetry. The Z' is associated to a global current of the Standard Model (SM), such as baryon ($B$) or lepton ($L_i$) numbers. In order to fulfill anomaly conditions as well as a trace condition, new fermions have to be introduced. Integrating out these fermions induces a kinetic mixing...
We extend the Standard Model gauge group by $U(1)_{L_e-L_\mu}$ and introduce two scalars, a doublet and a singlet, that are charged under this new group and have lepton flavour violating couplings. Since in this model $\mu \to e$ processes can only be mediated by $\mu\to \tau\times \tau\to e$ interactions, bounds from $\mu\to e$ transitions can be avoided while allowing for accessible new...
The search for charged Lepton Flavour Violation (cLFV) in muon decays is a sensitive tool to probe the Standard Model (SM). In this context, the MEG II and Mu3e experiments at the Paul Scherrer Institut (PSI) search for $\mu^+ \to e^+\gamma$ and $\mu^+ \to e^+e^-e^+$, respectively. In addition to their main channels, both experiments appear to be competitive in searching for more exotic...
LHCb is playing a crucial role in the study of rare and forbidden decays of charm hadrons, which might reveal effects beyond the Standard Model. This talk aims in particular to motivate the search for lepton flavour violation (LFV) in charm decays. A short overview of recent LFV results in the charm sector at LHCb is presented, as well as the prospects for future measurements exploiting new...
$SU(2)_L$-invariance links charged dilepton $\bar{q}\,q^\prime\,\bar{\ell}\,\ell$ and dineutrino $\bar{q}\,q^\prime\,\bar{\nu}\,\nu$ couplings. This connection can be established using SMEFT and holds if only SM-like left-handed light neutrinos are present. It allows to perform complementary experimental tests of lepton universality and charged lepton flavor conservation with flavor-summed...
Mu3e is a dedicated experiment to search the rare charged lepton flavor violating (cLFV) decay $\mu^{+} \rightarrow{} e^{+}e^{-}e^{+}$ with a sensitivity down to $10^{-16}$ under construction at PSI. In the Standard Model, this decay is heavily suppressed with a branching fraction of $10^{-54}$. The Mu3e experiment will be able to reconstruct low momentum electrons and positrons from rare...
Mu3e is an experiment under construction at the Paul Serrer Institute to search for the charged lepton flavor violating $\mu^+ \rightarrow e^+e^-e^+$ decay at branching fractions above $10^{-16}$. As this decay is heavily suppressed in the Standard Model, its observation would unambiguously indicate the existence of new physics. Achieving such sensitivity requires a high rate of muons and a...
Heavy neutral leptons (HNLs) are hypothetical heavy states mixing with the Standard Model neutrinos. They were proposed to explain neutrino masses, dark matter and the baryon asymmetry of the Universe. HNLs have been searched for in numerous experiments, however most of them report their limits under the assumption of one HNL mixing with a single neutrino flavor. Although convenient, we will...
Lorentz and CPT invariance are among the most fundamental physical symmetries. Progress in recent years has led to stringent constraints on several coefficients parametrizing Lorentz and CPT violation coupled to the fields of the standard model and general relativity. However, nonrenormalizable operators inducing tree-level lepton-flavor-violating decays remain completely unexamined. In this...
