The Curious Early History of the Cabibbo–Kobayashi–Maskawa Matrix: Miracles Happen!

Abstract

Abstract The 1973 Kobayashi–Maskawa paper proposed a compelling link between Cabibbo’s flavor mixing scheme and ${\mathcal {C}}$${\mathcal {P}}$ violation but, since it required the existence of six quarks at a time when the physics community was happy with only three, it received zero attention. However, two years after the paper appeared—at which time it had received a grand total of two citations—the charmed quark was discovered and it finally got some notice and acceptance. After this stumbling start, it subsequently emerged as the focal point of an enormous amount of experimental and theoretical research activity. In an invited talk at a KEK symposium to celebrate the 50th anniversary of the KM paper, I reviewed some of the less well known circumstances that occurred in the years preceding and following the paper’s appearance. Some spoilers: (i) Kobayashi and Maskawa (and a number of other Japanese physicists) were convinced about the existence of the charmed quark nearly three years before its “discovery” at Brookhaven and SLAC. (ii) The matrix provided in their seminal 1973 paper was mathematically incorrect. Another version that was in common use for the following 12 years was technically correct, but not really a rotation matrix. (iii) The CKM matrix ${\mathcal {C}}$${\mathcal {P}}$ phase was only measurable because of the very specific hierarchy of the flavor mixing angles and meson masses. (iv) Similarly, the neutrino mixing discovery, and the PMNS matrix measurability were only possible because of favorable values of the neutrino mass differences and mixing angles. In addition I include some speculations about what may be in store for the future.