Lepton-flavour non-universality of $${\bar{B}}\rightarrow D^*\ell {{\bar{\nu }}}$$ angular distributions in and beyond the Standard Model

We analyze in detail the angular distributions in $${\bar{B}}\rightarrow D^*\ell {{\bar{\nu }}}$$ B ¯ → D ∗ ℓ ν ¯ decays, with a focus on lepton-flavour non-universality. We investigate the minimal number of angular observables that fully describes current and upcoming datasets, and explore their sensitivity to physics beyond the Standard Model (BSM) in the most general weak effective theory. We apply our findings to the current datasets, extract the non-redundant set of angular observables from the data, and compare to precise SM predictions that include lepton-flavour universality violating mass effects. Our analysis shows that the number of independent angular observables that can be inferred from current experimental data is limited to only four. These are insufficient to extract the full set of relevant BSM parameters. We uncover a $$\sim 4\sigma $$ ∼ 4 σ tension between data and predictions that is hidden in the redundant presentation of the Belle 2018 data on $${\bar{B}}\rightarrow D^*\ell {{\bar{\nu }}}$$ B ¯ → D ∗ ℓ ν ¯ decays. This tension specifically involves observables that probe $$e-\mu $$ e - μ lepton-flavour universality. However, we find inconsistencies in these data, which renders results based on it suspicious. Nevertheless, we discuss which generic BSM scenarios could explain the tension, in the case that the inconsistencies do not affect the data materially. Our findings highlight that $$e-\mu $$ e - μ non-universality in the SM, introduced by the finite muon mass, is already significant in a subset of angular observables with respect to the experimental precision.

Naturalness and the muon magnetic moment

We study a predictive model for explaining the apparent deviation of the muon anomalous magnetic moment from the Standard Model expectation. There are no new scalars and hence no new hierarchy puzzles beyond those associated with the Higgs; the only new particles at the TeV scale are vector-like singlet and doublet leptons. Interestingly, this simple model provides a calculable example violating the Wilsonian notion of naturalness: despite the absence of any symmetries prohibiting its generation, the coefficient of the naively leading dimension-six operator for (g − 2) vanishes at one-loop. While effective field theorists interpret this either as a surprising UV cancellation of power divergences, or as a delicate cancellation between matching UV and calculable IR corrections to (g − 2) from parametrically separated scales, there is a simple explanation in the full theory: the loop integrand is a total derivative of a function vanishing in both the deep UV and IR. The leading contribution to (g − 2) arises from dimension-eight operators, and thus the required masses of new fermions are lower than naively expected, with a sizeable portion of parameter space already covered by direct searches at the LHC. The viable parameter space free of fine-tuning for the muon mass will be fully covered by future direct LHC searches, and all of the parameter space can be probed by precision measurements at planned future lepton colliders.

Muon anomalous magnetic moment and Higgs potential stability in the 331 model from SU(6)

We consider a $$SU(3)_c \times SU(3)_L \times U(1)_X$$ S U ( 3 ) c × S U ( 3 ) L × U ( 1 ) X model from a SU(6) Grand Unified Theory (GUT). In order to explain the anomalous magnetic moments of muon and electron, we introduce two new scalar triplets without vacuum expectation values (VEVs) so that the leading contributions to $$\Delta a_{\mu }$$ Δ a μ and $$\Delta a_{e}$$ Δ a e can avoid the suppression from small muon mass. In addition, the Higgs potential stability of this 331 model is studied by giving a set of sufficient conditions to ensure the boundedness from below of the potential.

Forbidden dark matter annihilations into Standard Model particles

We present kinematically forbidden dark matter annihilations into Standard Model leptons. This mechanism precisely selects the dark matter mass that gives the observed relic abundance. This is qualitatively different from existing models of thermal dark matter, where fixing the relic density typically leaves open orders of magnitude of viable dark matter masses. Forbidden annihilations require the dark matter to be close in mass to the particles that dominate its annihilation rate. We show examples where the dark matter mass is close to the muon mass, the tau mass, or the average of the tau and muon masses. We find that most of the relevant parameter space can be covered by the next generation of proposed beam-dump experiments and future high-luminosity electron positron colliders. Forbidden dark matter predicts large couplings to the Standard Model that can explain the observed value of (g − 2)μ.

Radiative muon mass models and (g − 2)μ

Recent measurements of the Higgs-muon coupling are directly probing muon mass generation for the first time. We classify minimal models with a one-loop radiative mass mechanism and show that benchmark models are consistent with current experimental results. We find that these models are best probed by measurements of (g − 2)μ, even when taking into account the precision of Higgs measurements expected at future colliders. The current (g − 2)μ anomaly, if confirmed, could therefore be a first hint that the muon mass has a radiative origin.

THE 2–3 SYMMETRY AND LARGE REACTOR MIXING ANGLE

We introduce a 2–3 symmetric structure of the charged lepton mass matrix except for one breaking by the muon mass. Symmetry breaking effects are provided both in the charged lepton and the neutrino sector to produce corrections to the leptonic mixing and explain the recent θ13 measurements. A model that extends the SM by three right-handed neutrinos, an extra Higgs doublet, and multi-singlet scalars is introduced to generate the leptonic mixing.

PION AND MUON MASS DIFFERENCE: A DETERMINING FACTOR IN ELEMENTARY PARTICLE MASS DISTRIBUTION

The most fundamental to the elementary particles is the mass they possess and it would be of importance to explore a possible relationship amongst their masses. Here, an attempt is made to investigate this important aspect irrespective of their nature or scheme of classification. We show that there exists a striking tendency for successive mass differences between elementary particles to be close integral/half integral multiple of the mass difference between a neutral pion and a muon. Thus indicating discreteness in the nature of the mass occurring at the elementary particle level. Furthermore, this mass difference of 29.318 MeV is found to be common to the mass spectra of leptons and baryons, implying thereby existence of a basic mechanism linking elementary particles responding to different interactions.