One of the key assumptions of the standard model of particle physics is that the interactions of the charged leptons, namely electrons, muons and taus, differ only because of their different masses. Whereas precision tests comparing processes involving electrons and muons have not revealed any definite violation of this assumption, recent studies of B-meson decays involving the higher-mass tau lepton have resulted in observations that challenge lepton universality at the level of four standard deviations. A confirmation of these results would point to new particles or interactions, and could have profound implications for our understanding of particle physics. Prepared for submission to Nature March 4, 2023 A Challenge to Lepton Universality in B Meson Decays Gregory Ciezarek 1, Manuel Franco Sevilla 2, P .M. Hamilton3, Robert Kowalewski 4, Thomas Kuhr 5, Vera L¨uth6, Y utaro Sato7 One of the key assumptions of the Standard Model of fundamental particles is that the interactions of the charged leptons, namely electrons, muons, and taus, differ only because of their different masses. While precision tests comparing processes involving electrons and muons have not revealed any definite violation of this assumption, recent studies involving the higher-mass tau lepton have resulted in observations that challenge lepton universality at the level of four standard deviations. A confirmation of these results would point to new particles or interactions, and could have profound implications for our understanding of particle physics. Based on information ex- tracted from experiments, theorists have combined the theory of electroweak (EW) interactions with quantum chromodynamics (QCD), the theory of strong interactions, and experiments have validated this theory to an extraordinary degree. Any observation that is proven to be inconsistent with SM assumptions would suggest a new type of interaction or particle. In the framework of the SM of particle physics the fundamen- tal building blocks, quarks and leptons, are each grouped in three generations of two members each. The three charged leptons, the electron (e−), the muon ( µ−) and the tau ( τ−) are each paired with a very low mass, electrically neutral neutrino, νe, νµ , and ντ. The electron, a critical component of matter, was discovered by J.J. Thomson [3] in 1897. The discovery of the muon in cosmic rays by C. D. Anderson and S. H. Neddermeyer [4] in 1937 came as a surprise, similarly surprising was the first observation of τ +τ− pair production by M. Perl et al. [ 5] at the SPEAR e+e− storage ring in 1975. As far as we know, all leptons are point-like particles, i.e. they have no substructure. Studies of the origin of this puzzling difference are under- way [10]. They are aimed at a better understanding of the proton radius and structure, and may reveal details of the true impact of muons and electrons on these interactions. Recent studies of purely leptonic and semileptonic decays of B mesons of the form B−→ τ−ν τ and B→ D(∗)ℓ−ν ℓ, with ℓ = e, µ, or τ, have resulted in observations that seem to challenge lepton universality. Measurements of B−→ τ−ν τ decays are based on leptonic τ decays, τ−→ e−νeντ and τ−→ µ−ν µ ντ, and on semileptonic decays, τ−→ π−ντ and τ−→ π−π0ντ, which together account for 70% of all τ− decays. Thus, the signature for signal events is a single charged particle, either a charged lepton, a π−, or a π− accompanied by a π0, plus a Btag. The presence of multiple neutrinos precludes the use of kine- matic constraints to effectively suppress backgrounds from other B decays. Diagrams for non-SM decay processes: (a) B−→ ℓ−ν ℓ with a purely leptonic final state and (b) B→ D(∗)ℓ−ν ℓ, involving a charm meson and lepton pair and mediated by a spin-0 lepto-quark (LQ). In the SM, these B decays are mediated by a virtual charged vector boson, a particle of spin 1, usually referred to as the W− (as indicated in the diagram in Figure 1) which couples equally to all leptons. If a hitherto unknown virtual particle existed that interacted differently with leptons of higher mass like the τ−, this could change the B decay rates and their kinematics. Another feasible solution might be leptoquarks [ 50], hypo- thetical particles with both electric and color (strong) charges that allow transitions from quarks to leptons and vice versa, and offer a unified description of three generations of quarks and leptons. Among the ten different types of leptoquarks, six could contribute to B→ D(∗)τν decays [51]. A diagram of a spin-0 state mediat- ing quark-lepton transitions is shown in Figure 7 for the B decay modes under study. BABAR and Belle have studied the implications of these hy- pothetical particles in the context of specific models [28, 34]. The q2 spec- trum [28, 34] and the momentum distributions of the D(∗) and electron or muon Also here, more data are needed to enhance the significance of these measurements and find possible links to B decays involving τ leptons. If the currently observed excess in the ratios RD and RD∗ is confirmed, experimenters will use their large data samples to measure properties of signal events and learn about the na- ture of the new particles and interactions that contribute to these decays [72, 73]. In conclusion, we can expect much larger event samples from the upgraded LHCb and Belle experiments in the not too distant future. Thomson, Carriers of negative electricity, Nobel lecture in physics, Elsevier Publishing, Amsterdam (The Nether- lands) 1967. [4] S. H. Neddermeyer and C. D. Anderson. The nature of cosmic ray particles. Phys. Rev., 51:884–886, 1937. [5] M. L. Perl et al. Evidence for anomalous lepton production in e+ e- annihilation. Phys. Rev. Lett., 35:1489–1492, 1975. [6] M. Ablikim et al. Precision measurement of the mass of the τ lepton. Phys. Rev., D90:012001, 2014. [7] C. Lazzeroni et al. Precision measurement of the ratio of the charged kaon leptonic decay rates. Phys. Lett., B719:326– 336, 2013.