The symmetry approach to quark and lepton masses and mixing

The Standard Model lacks an organizing principle to describe quark and lepton “flavours”. We review the impact of neutrino oscillation experiments, which show that leptons mix very differently from quarks, placing a major challenge, but also providing a key input to the flavour puzzle. We briefly sketch the seesaw and “scotogenic” approaches to neutrino mass, the latter including also WIMP dark matter. We discuss the limitations of popular neutrino mixing patterns and examine the possibility that they arise from symmetry, giving a bottom-up approach to residual flavour and CP symmetries. We show how family and/or CP symmetries can generate novel viable and predictive mixing patterns. We review the model-independent ways to predict lepton mixing and test both mixing predictions as well as mass sum rules. We also discuss UV-complete flavour theories in four and more space-time dimensions, and their predictions. Benchmarks given include an A_4 scotogenic construction with trimaximal mixing pattern TM2. Higher-dimensional completions are also reviewed, such as 5-D warped flavordynamics. We present a T^' warped flavordynamics theory with TM1 mixing pattern, detectable neutrinoless double beta decay rates and providing a very good fit of flavour observables, including quarks. We also review how 6-D orbifolds offer a way to determine the structure of the 4-D family symmetry from the symmetries between the extra-D branes. We describe a scotogenic A_4 orbifold predicting the “golden” quark-lepton mass relation, large neutrino mass with normal ordering, higher atmospheric octant, restricted reactor angle, and an excellent global flavour fit, including quark observables. Finally, we discuss promising recent progress in tackling the flavor issue through the use of modular symmetries.