Charming synergies: the role of charm-threshold studies in the search for physics beyond the Standard Model

The flavour sector of the Standard Model (SM) of particle physics contains many of the open questions and free parameters of the theory. Why are there six quarks with such a wide span of masses? What determines the hierarchical structure of the Cabibbo-Kobayashi-Maskawa (CKM) matrix? What other sources of CP violation exist in nature in addition to that found in the SM, which is accommodated by a complex phase in the CKM matrix? In recent years, experimental flavour physics has been dominated by studies of beauty and charm hadrons at the LHCb experiment at CERN, with a growing contribution expected in these topics from the Belle II experiment at KEK, Japan. However, precise measurements just above the threshold for charm-meson production in e⁺e^- collisions, which are possible at the BESIII experiment on the BEPCII accelerator in Beijing, have a very important and complementary role to play in this programme. Recent results from BESIII, mapping out the strong-phase characteristics of certain charm-meson decays, make clear the strong synergies that exist between these diverse facilities. In particular, precise studies by BESIII of the strong-phase variation in the decay D⁰ → K_S⁰ π⁺π^- reported in Refs. [1,2] have been extremely valuable in enabling important measurements of CP violation and neutral-meson oscillations at LHCb.