Testing Lepton Flavor Universality with Pion, Kaon, Tau, and Beta Decays

Abstract

We present an overview of searches for violation of lepton flavor universality with focus on low energy precision probes using pions, kaons, tau leptons, and nuclear beta decays. The current experimental results are reviewed, the theoretical status within the context of the Standard Model is summarized, and future prospects (both experimental and theoretical) are discussed. We review the implications of these measurements for physics beyond the Standard Model by performing a global modelindependent fit to modified W couplings to leptons and four-fermion operators. We also discuss new physics in the context of simplified models and review Standard Model extensions with focus on those that can explain a possible deviation from unitarity of the Cabibbo-Kobayashi-Maskawa quark mixing matrix.

One-loop determination of τ → π(K)ντ [γ] branching ratios and new physics tests

“…The couplings appearing in the F π,K V /A (W 2 , k 2 ) form factors are not restricted by chiral symmetry. 5 Fortunately, some of them are predicted by demanding the behaviour given by the operator product expansion of QCD to the relevant Green functions [19-21, 36, 48-51].…”

“…So far, such anomalies are not statistically significant enough to claim evidence of new physics (NP), but these intriguing data have drawn the interest of the particle physics community suggesting several possible explanations of NP. Then, one of the most active tasks in both the theoretical and experimental particle physics program is the study and review of all the different observables testing LU, including low-energy JHEP02(2022)173 precision probes using pion, kaons and tau leptons [5]. In this work, we test LU between the second and third families of charged leptons through the ratio (P = π, K) [6][7][8][9][10][11] R τ /P ≡ Γ (τ…”

Hyperon production in quasielastic $\barν_τ-$nucleon scattering

“…the origins of LFUV. Further hints for the LFUV [43,44] have been inferred from the recent measurements of the anomalous magnetic moment (g − 2) of the muons, at the Fermilab [45], as well as the anomalies reported in the recent measurements of the Cabibbo angle [46] and production of lepton pairs through the process of q q −→ l l (l = e and µ) in pp collisions at CERN [47]. In the case of b −→ sl l decays, LHCb [39] collaboration has reported the ratio of the branching fractions in l = µ mode to l = e mode to be 2.6 σ lower than the SM prediction.…”

One-loop determination of $τ\to π(K) ν_τ[γ]$ branching ratios and new physics tests

“…So far, such anomalies are not statistically significant enough to claim evidence of new physics (NP), but these intriguing data have drawn the interest of the particle physics community suggesting several possible explanations of NP. Then, one of the most active tasks in both the theoretical and experimental particle physics program is the study and review of all the different observables testing LU, including low-energy precision probes using pion, kaons and tau leptons [3]. In this work, we test LU between the second and third families of charged leptons through the ratio (P = π, K) [4][5][6] R τ /P ≡ Γ (τ…”