![]() In addition, there is a hint of LFUV in the difference in the forward–backward asymmetries (Δ A FB) in B → D *μν versus B → D * eν ( 27, 28). Here, there is a long-standing discrepancy in ( g − 2) μ of 4.2σ ( 24– 26), which can be considered a hint of LFUV because, when compared with ( g − 2) e, the bound from the latter on flavor-blind NP is much more stringent. 2 In addition, anomalous magnetic moments ( g − 2) ℓ (ℓ = e, μ, τ) of charged leptons are intrinsically related to LFUV because they are chirality-flipping quantities. ![]() 1 In particular, measurements of the ratios of branching ratios (BRs) R = Br/Br, where ℓ = μ or e ( 8– 10), and R = Br/Br ( 11– 13), deviate from the SM expectation by more than 3σ ( 14– 18) and 4σ ( 19– 22), respectively. Recent experimental tests of LFU have accumulated intriguing hints of physics effects not included in the SM (for a short review, see 4). Therefore, LFU violation (LFUV) implies interactions with different couplings to electrons, muons, and τ leptons (disregarding phase-space effects) that directly distinguish among the charged leptons at the Lagrangian level. However, the impact of the lepton masses, originating from the Higgs Yukawa couplings after electroweak (EW) symmetry breaking on the lifetimes of charged leptons, is enormous as a result of kinematic effects. As these couplings are very small (at most, of the order of 1% for the τ lepton), LFU is an approximate accidental symmetry of the SM (at the Lagrangian level). In the SM, the gauge interactions are the same for all flavors in other words, they respect lepton flavor universality (LFU), which in fact is broken only by the Higgs Yukawa couplings. Furthermore, the symmetries of the SM can be exploited to obtain more precise predictions, as in most cases theoretical and parametric uncertainties are reduced. In this way, such searches are very sensitive to NP that does not necessarily respect these symmetries and thus leads to sizable effects even if the mass scale is quite high. Concerning the latter, an especially promising avenue is to search for the violation of (approximate) symmetries of the SM. There are, in general, two ways to search for new particles and interactions: direct searches at high energy colliders (such as the LHC) and indirect searches for quantum effects in precision observables ( 3). Therefore, the search for physics beyond the SM (BSM) is a prime subject of current research. In addition to many theoretical arguments for the existence of new physics (NP), the SM, for instance, can account neither for the existence of dark matter (DM) or dark energy established at cosmological scales nor for neutrino masses or the existence of exactly three generations of fermions. However, it is clear that the SM cannot be the ultimate fundamental theory of nature. Its final missing ingredient, the Higgs boson, was discovered at the Large Hadron Collider (LHC) at CERN in 2012 ( 1, 2). The Standard Model (SM) of particle physics describes the known constituents of matter: the three generations (or flavors) of quarks and leptons as well as their interactions (excluding gravity).
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