GIM mechanism ref:
(mostly words) prediction of charm quark with certain properties/phenomenology associated with
examples of necessary cancellation and orders of magnitude
More Historical Detail? Verification of prediction?
[ V u d V u s V c d V c s ] = [ c o s ( θ ) s i n ( θ ) − s i n ( θ ) c o s ( θ ) ] {\displaystyle {\begin{bmatrix}V_{ud}&V_{us}\\V_{cd}&V_{cs}\end{bmatrix}}={\begin{bmatrix}cos(\theta )&sin(\theta )\\-sin(\theta )&cos(\theta )\end{bmatrix}}}