More than 70 years of particle physics research have led to an elegant and concise theory of particle interactions at the subnuclear level, commonly referred to as the standard model1,2. On the basis of information extracted from experiments, theorists have combined the theory of electroweak interactions with quantum chromodynamics, the theory of strong interactions, and experiments have validated this theory to an extraordinary degree. Any observation that is proven to be inconsistent with standard model assumptions would suggest a new type of interaction or particle.

In the framework of the standard model of particle physics, the fundamental building blocks, quarks and leptons, are each grouped in three generations of two members each. The three generations of charged leptons—the electron (e−), the muon (μ−) and the tau (τ−)—are each paired with an electrically neutral lepton, a very low mass neutrino, ν e , ν μ and ν τ , respectively. The electron, a critical component of matter, was discovered by Thomson3 in 1897. The discovery of the muon in cosmic rays by Anderson and Neddermeyer4 in 1937 came as a surprise. Similarly surprising was the first observation of τ+τ− pair production by Perl et al.5 at the SPEAR e+e− storage ring in 1975. As far as we know, all leptons are point-like particles, that is, they have no substructure.

The three generations are ordered by the mass of the charged lepton, which ranges from 0.511 MeV for e± to 105 MeV for μ± and to 1,777 MeV for τ± (ref. 6). These different masses lead to vastly different lifetimes, from the stable electron to 2.2 μs for muons, and to 0.29 ps for taus. Charged leptons participate in electromagnetic and weak interactions, but not in strong interactions, whereas neutrinos only undergo weak interactions. The standard model assumes that these interactions of the charged and neutral leptons are universal, that is, the same for the three generations.

Precision tests of lepton universality have been performed over many years by many experiments. To date no definite violation of lepton universality has been observed. Among the most precise tests is a comparison of the decay rates of K mesons, that is, versus (ref. 7). (Unless stated otherwise, the inclusion of charged-conjugate states and decay modes is implied here and in the following.) Furthermore, taking into account precision measurements of the tau and muon masses and lifetimes and the decay rates and , the equality of the weak coupling strengths of the tau and muon was confirmed6. On the other hand, a recent determination of the proton (p) radius, derived from very precise measurements of the Lamb shift in muonic hydrogen atoms8, differs by about 4% from measurements of normal hydrogen atoms and e–p scattering data. Studies of the origin of this puzzling difference are underway9. 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 have focused on purely leptonic decays of B mesons of the form and semileptonic B decays such as , with ℓ = e, μ or τ, and where D(*) refers to a low-mass charm meson, D or D*. These studies have resulted in observations that seem to challenge lepton universality. These weak decays involving leptons are well understood in the framework of the standard model, and therefore offer a unique opportunity to search for unknown phenomena and processes involving new particles: for instance, a yet undiscovered charged partner of the Higgs boson10. Such searches have been performed on data collected by three different experiments: the LHCb experiment at the pp collider at CERN in Europe, and the BaBar and Belle experiments at e+e− colliders in the USA and in Japan, respectively.

Measurements by these three experiments favour larger than expected rates for semileptonic B decays involving τ leptons. Currently, the combined significance of these results is at the level of four standard deviations, and the fact that all three experiments report an unexpected enhancement has drawn considerable attention. A confirmation of this violation of lepton universality and an explanation in terms of new-physics processes would be very exciting. In the following, we present details of the experimental techniques and preliminary studies to understand the observed effects, along with prospects of improved sensitivity and complementary measurements at current and future facilities.