# Where does the expression "golden plate decay" come from?

The decay of $$B \to J/\psi K_{S,L}$$ is often referenced as "golden plate decay", for example in

If there is a B-decay into a CP-eigenstate, like the golden plate decay $$B \to J/\psi K_{S,L}$$, let [...]

Where does this expression come from?

The decay $$B_d \to J/\psi K^0_s$$ was targeted (successfully!) by the BaBar and Belle experiments that first demonstrated that CP violation occurred for B mesons, as predicted by Kobayashi and Maskawa. This is because it contained a single weak phase (so it could be interpreted unambiguously), had a relatively high branching ratio, and had a very clean experimental signature, both the $$J/\psi$$ and the $$K^0_s$$ giving sharp mass peaks as well as vertex information. Being so desirable it was therefore known to everybody as the 'gold plated' channel.( see for example the BaBar physics book, page 243 https://slac.stanford.edu/pubs/slacreports/reports19/slac-r-504.pdf)