The previous chapters have revealed the standard model as a remarkable theory which successfully describes the vast majority of all of modern particle physics. Better yet, its implications are very robust since it is the most general theory which is consistent with general principles (like Lorentz invariance and unitarity) plus the two assumptions: (i) renormalizability and (ii) the given particle content – all of which are seen in experiments (modulo confinement for the case of quarks and gluons) save the as-yet-undiscovered Higgs scalar.

In this chapter we encounter the first (and, as of this writing, only) known case where there is good evidence that the standard model does not provide a good description: the phenomenon of neutrino oscillations. After describing the conceptual issues and the evidence for the standard model's failure, we briefly describe what can be said at present about which of the two core assumptions must be relinquished.

The standard-model prediction which has gone sour – encountered in Subsection 2.5.2 – asserts the separate conservations (up to negligible corrections involving the electroweak anomaly) of the three lepton numbers, Le, Lµ, and Lτ. Even considering the electroweak anomaly, the quantities Le − Lµ and Lµ − Lτ are anomaly free and so should be exactly conserved. As a consequence the theory predicts exactly massless and stable neutrinos, νe, νµ, ντ, with νℓ only participating in charged-current weak interactions together with its corresponding charged lepton, ℓ−.