We assess the status of past and future experiments on lepton flavor violating (LFV) muon and tau decays into a light, invisible, axion-like particle (ALP), a. We propose a new experimental setup for MEG II, the MEGII-fwd, with a forward calorimeter placed downstream from the muon stopping target. Searching for μ → ea decays MEGII-fwd is maximally sensitive to LFV ALPs, if these have nonzero couplings to right-handed leptons. The experimental set-up suppresses the (left-handed) Standard Model background in the forward direction by controlling the polarization purity of the muon beam. The reach of MEGII-fwd is compared with the present constraints, the reach of Mu3e and the Belle-II reach from τ → ℓa decays. We show that a dedicated experimental campaign for LFV muon decays into ALPs at MEG II and Mu3e will be able to probe the ALP parameter space in an unexplored region well beyond the existing astrophysical constraints. We study the implications of these searches for representative LFV ALP models, where the presence of a light ALP is motivated by neutrino masses, the strong CP problem and/or the SM flavor puzzle. To this extent we discuss the majoron in low-scale seesaw setups and introduce the LFV QCD axion, the LFV axiflavon and the leptonic familon, paying particular attention to the cases where the LFV ALPs constitute cold dark matter.

In this work we assess the potential of discovering new physics by searching for lepton-flavour-violating (LFV) decays of the Z boson, $$Z\rightarrow \ell _i \ell _j$$Z → ℓ i ℓ j , at the proposed circular $$e^+e^-$$e + e - colliders CEPC and FCC-ee. Both projects plan to run at the Z-pole as a “Tera Z factory”, i.e., collecting $${\mathcal {O}}\left( 10^{12} \right) $$O 10 12 Z decays. In order to discuss the discovery potential in a model-independent way, we revisit the LFV Z decays in the context of the Standard Model effective field theory and study the indirect constraints from LFV $$\mu $$μ and $$\tau $$τ decays on the operators that can induce $$Z\rightarrow \ell _i \ell _j$$Z → ℓ i ℓ j . We find that, while the $$Z\rightarrow \mu e$$Z → μ e rates are beyond the expected sensitivities, a Tera Z factory is promising for $$Z\rightarrow \tau \ell $$Z → τ ℓ decays, probing New Physics at the same level of future low-energy LFV observables.

We study the phenomenology of simplified $Z^{\prime }$ models with a global $U(2{)}^3$ flavor symmetry in the quark sector, broken solely by the Standard Model Yukawa couplings. This flavor symmetry, known as less-minimal flavor violation, protects $\mathrm{\Delta }F=2$ processes from dangerously large new physics (NP) effects and at the same time provides a free complex phase in $b\to s$ transitions, allowing for an explanation of the hints for additional direct $CP$ violation in kaon decays (${\epsilon }^{\prime }/\epsilon$) and in hadronic $B$-decays ($B\to K\pi$ puzzle). Furthermore, including the couplings of the $Z^{\prime }$ boson to the leptons, it is possible to address the intriguing hints for NP (above the $5\sigma$ level) in $b\to s{\ell }^{+}{\ell }^{-}$ transitions. Taking into account all flavor observables in a global fit, we find that our model can (for the first time) provide a common explanation for ${\epsilon }^{\prime }/\epsilon$, the $B\to K\pi$ puzzle and $b\to s{\ell }^{+}{\ell }^{-}$ data. Sizeable $CP$ violation in $b\to s{\ell }^{+}{\ell }^{-}$ observables, in particular $A_8$, is predicted, which can be tested in the near future, and an explanation of the $B\to K\pi$ and ${\epsilon }^{\prime }/\epsilon$ puzzles leads to effects in dijet tails at the LHC that are not far below the current limits. If we require that also $b\to s{\ell }^{+}{\ell }^{-}$ anomalies are explained, cancellations in dimuon tails (possibly by a second $Z^{\prime }$) are needed to satisfy LHC data.

The stringent experimental bound on μ → eγ is compatible with a simultaneous and sizable new physics contribution to the electron and muon anomalous magnetic moments (g − 2)_ℓ (ℓ = e, μ), only if we assume a non-trivial flavor structure of the dipole operator coefficients. We propose a mechanism in which the realization of the (g − 2)_ℓ correction is manifestly related to the mass generation through a flavor symmetry. A radiative flavon correction to the fermion mass gives a contribution to the anomalous magnetic moment. In this framework, we introduce a chiral enhancement from a non-trivial $$ \mathcal{O} $$ O (1) quartic coupling of the scalar potential. We show that the muon and electron anomalies can be simultaneously explained in a vast region of the parameter space with predicted vector-like mediators of masses as large as Mχ ∈ [0.6, 2.5] TeV.

In light of the recent result of the muon $g-2$ experiment and the update on the test of lepton flavor universality $R_K$ published by the LHCb Collaboration, we systematically study for the first time a set of models with minimal field content that can simultaneously give (i) a thermal dark matter candidate; (ii) large loop contributions to $b\to s\ell \ell$ processes able to address $R_K$ and the other $B$ anomalies; (iii) a natural solution to the muon $g-2$ discrepancy through chirally enhanced contributions. Moreover, this type of model with heavy particles and chiral enhancement can evade the strong limits from direct searches but can be tested at present and future colliders and direct-detection searches.

Displaced vertices at colliders, arising from the production and decay of long-lived particles, probe dark matter candidates produced via freeze-in. If one assumes a standard cosmological history, these decays happen inside the detector only if the dark matter is very light because of the relic density constraint. Here, we argue how displaced events could very well point to freeze-in within a non-standard early universe history. Focusing on the cosmology of inflationary reheating, we explore the interplay between the reheating temperature and collider signatures for minimal freeze-in scenarios. Observing displaced events at the LHC would allow to set an upper bound on the reheating temperature and, in general, to gather indirect information on the early history of the universe.