We present the first detailed study of Standard Model (SM) neutrino tridents involving tau leptons at the near detectors of accelerator neutrino facilities. These processes were previously thought to be negligible, even at future facilities like DUNE, based on approximations that underestimated the tau trident cross sections. Our full $2\to 4$ calculation, including both coherent and incoherent scatterings, reveals that the DUNE near detector will actually get a non-negligible number of tau tridents, which is an important background to new physics searches. We identify promising kinematic features that may allow distinction of tau tridents from the usual neutrino charged-current background at DUNE, and thus could establish the observation of tau tridents for the first time. We also comment on the detection prospects at other accelerator and collider neutrino experiments.

Scalar leptoquarks (LQ) with masses between 2 TeV and 50 TeV are prime candidates to explain deviations between measurements and Standard-Model predictions in decay observables of b-flavored hadrons (“flavor anomalies”). Explanations of low-energy data often involve 𝒪 (1) LQ-quark-lepton Yukawa couplings, especially when collider bounds enforce a large LQ mass. This calls for the calculation of radiative corrections involving these couplings. Studying such corrections to LQ-mediated b → cτν and b → sℓ+ℓ− amplitudes, we find that they can be absorbed into finite renormalizations of the LQ Yukawa couplings. If one wants to use Yukawa couplings extracted from low-energy data for the prediction of on-shell LQ decay rates, one must convert the low-energy couplings to their high-energy counterparts, which subsume the corrections to the on-shell LQ-quark-lepton vertex. We present compact formulae for these correction factors and find that in scenarios with S1, R2, or S3 LQ the high-energy coupling is always smaller than the low-energy one, which weakens the impact of collider data on the determination of the allowed parameter spaces. For the R2 scenario addressing b → cτν, in which one of the two involved Yukawa coupling must be significantly larger than 1, we find this coupling reduced by 15 MS scheme for the Yukawa couplings of the S3 scenario.

We study the properties of vectorlike fermions that have the same gauge charges as the Standard Model lepton doublets, but opposite lepton number. These antileptons undergo decays mediated by heavier scalar leptoquarks, while the symmetries of this renormalizable model protect the vectorlike fermions and the leptoquarks from standard decays probed so far at colliders. If the new particles couple predominantly to second-generation quarks, then their collider probes involve multiple jets and two taus or neutrinos, and are hampered by large backgrounds. If couplings to third-generation quarks are large, then the collider signals involve top quarks, and can be probed more efficiently at the LHC. Even in that case, both the vectorlike fermion doublet and the leptoquarks remain more elusive than in models with standard decays.

Motivated by flavour symmetry models, we construct theories based on a low-energy limit featuring lepton flavour triality that have the flavour-violating decays $\tau^\pm \to \mu^\pm \mu^\pm e^\mp$ and $\tau^\pm \to e^\pm e^\pm \mu^\mp$ as the main phenomenological signatures of physics beyond the standard model. These decay modes are expected to be probed in the near future with increased sensitivity by the Belle II experiment at the SuperKEKB collider. The simple standard model extensions featured have doubly-charged scalars as the mediators of the above decay processes. The phenomenology of these extensions is studied here in detail.

We construct an extension of the Standard Model with a scalar leptoquark $\phi \sim (3,1,-\tfrac13)$ and the discrete flavour symmetry $G_f=D_{17}\times Z_{17}$ to explain anomalies observed in charged-current semi-leptonic $B$ meson decays and in the muon anomalous magnetic moment, together with the charged fermion masses and quark mixing. The symmetry $Z_{17}^{\rm diag}$, contained in $G_f$, remains preserved by the leptoquark couplings, at leading order, and efficiently suppresses couplings of the leptoquark to the first generation of quarks and/or electrons, thus avoiding many stringent experimental bounds. The strongest constraints on the parameter space are imposed by the radiative charged lepton flavour violating decays $\tau\to\mu\gamma$ and $\mu\to e\gamma$. A detailed analytical and numerical study demonstrates the feasibility to simultaneously explain the data on the lepton flavour universality ratios $R(D)$ and $R(D^\star)$ and the muon anomalous magnetic moment, while passing the experimental bounds from all other considered flavour observables.

We study the two scalar leptoquarks capable of generating chirally-enhanced, sign-dependent contributions to lepton magnetic dipole moments (MDMs) and electric dipole moments (EDMs), R2 ∼ (3,2, 7/6) and S1 ∼ (3,1,−1/3). We consider the case in which the electron and muon sectors are decoupled, and leptoquark couplings are assigned complex values. Adopting the coupling anzatz that the electron dipole operator is generated by charm-containing loops, and muon dipole operator by top-containing loops, we find that both minimal leptoquark models remain viable solutions for reconciling anomalies in the muon and electron MDMs, accounting for either of the two current (disparate) electron MDM results from Cs and Rb interferometry experiments. We also examine the correlated corrections to the muon and electron masses generated by these models, and argue that to minimise fine-tuning this introduces an upper bound on viable LQ φ masses, mφ . O(4) TeV. Similar arguments allow us to make a prediction for the upper bound of the muon EDM generated by these models, |dμ| . O(10−22) e cm, which could be within reach of upcoming experimental programs, including Muon g−2 at Fermilab (FNAL), and muEDM at Paul Scherrer Institut (PSI).

We construct an extension of the Standard Model with a scalar leptoquark ϕ∼ (3,1,-13) and the discrete flavour symmetry G_f=D_17× Z_17 to explain anomalies observed in charged-current semi-leptonic B meson decays and in the muon anomalous magnetic moment, together with the charged fermion masses and quark mixing. The symmetry Z_17^ diag, contained in G_f, remains preserved by the leptoquark couplings, at leading order, and efficiently suppresses couplings of the leptoquark to the first generation of quarks and/or electrons, thus avoiding many stringent experimental bounds. The strongest constraints on the parameter space are imposed by the radiative charged lepton flavour violating decays τ→μγ and μ→ eγ. A detailed analytical and numerical study demonstrates the feasibility to simultaneously explain the data on the lepton flavour universality ratios R(D) and R(D^⋆) and the muon anomalous magnetic moment, while passing the experimental bounds from all other considered flavour observables.

We study the two scalar leptoquarks capable of generating chirally enhanced, sign-dependent contributions to lepton magnetic dipole moments (MDMs) and electric dipole moments (EDMs), ${R}_{2}\ensuremath{\sim}(\mathbf{3},\mathbf{2},7/6)$ and ${S}_{1}\ensuremath{\sim}(\mathbf{3},\mathbf{1},\ensuremath{-}1/3)$. We consider the case in which the electron and muon sectors are decoupled, and leptoquark couplings are assigned complex values. Adopting the coupling ansatz that the electron dipole operator is generated by charm-containing loops, and muon dipole operator by top-containing loops, we find that both minimal leptoquark models remain viable solutions for reconciling anomalies in the muon and electron MDMs, accounting for either of the two current (disparate) electron MDM results from Cs and Rb interferometry experiments. We also examine the correlated corrections to the muon and electron masses generated by these models, and argue that to minimize fine-tuning this introduces an upper bound on viable leptoquark ($\ensuremath{\phi}$) masses, ${m}_{\ensuremath{\phi}}<\mathcal{O}(4)\text{ }\text{ }\mathrm{TeV}$. Similar arguments allow us to make a prediction for the upper bound of the muon EDM generated by these models, $|{d}_{\ensuremath{\mu}}|<\mathcal{O}({10}^{\ensuremath{-}22})e\text{ }\text{ }\mathrm{cm}$, which could be within reach of upcoming experimental programs, including Muon $g\ensuremath{-}2$ at Fermilab (FNAL), and muEDM at Paul Scherrer Institut (PSI).

We identify the two scalar leptoquarks capable of generating sign-dependent contributions to leptonic magnetic moments, R-2 similar to (3, 2, 7/6) and S-1 similar to (3,1, -1/3), as favored by current measurements. We consider the case in which the electron and muon sectors are decoupled, and real-valued Yukawa couplings are specified using an up-type quark mass-diagonal basis. Contributions to Delta a(e) arise from charm-containing loops and Delta a(e) from top-containing loops-hence avoiding dangerous LFV constraints, particularly from mu -> e gamma. The strongest constraints on these models arise from contributions to the Z leptonic decay widths, high-p(T) leptonic tails at the LHC, and from (semi)leptonic kwon decays. To be a comprehensive solution to the (g - 2)(e/mu) puzzle we find that the mass of either leptoquark must be less than or similar to 65 TeV. This analysis can be embedded within broader flavor anomaly studies, including those of hierarchical leptoquark coupling structures. It can also be straightforwardly adapted to accommodate future measurements of leptonic magnetic moments, such as those expected from the Muon g - 2 collaboration in the near future.

A bstract We introduce two scalar leptoquarks, the SU(2) L isosinglet denoted ϕ ∼ ( 3 , 1 , − 1 / 3) and the isotriplet φ ∼ ( 3 , 3 , − 1 / 3), to explain observed deviations from the standard model in semi-leptonic B -meson decays. We explore the regions of parameter space in which this model accommodates the persistent tensions in the decay observables R D ( ∗ ), R K ( ∗ ) , and angular observables in b → sμμ transitions. Additionally, we exploit the role of these exotics in existing models for one-loop neutrino mass generation derived from ∆ L = 2 effective operators. Introducing the vector-like quark χ ∼ ( 3 , 2 , − 5 / 6) necessary for lepton-number violation, we consider the contribution of both leptoquarks to the generation of radiative neutrino mass. We find that constraints permit simultaneously accommodating the flavour anomalies while also explaining the relative smallness of neutrino mass without the need for cancellation between leptoquark contributions. A characteristic prediction of our model is a rate of muon-electron conversion in nuclei fixed by the anoma- lies in b → sμμ and neutrino mass; the COMET and Mu2e experiments will thus test and potentially falsify our scenario. The model also predicts signatures that will be tested at the LHC and Belle II.