ADDENDUM I TO P326 Continuation of the physics programme of the NA62 experiment

The NA62 experiment took data successfully in 2016-2018, and has proven that the decay-in- flight technique employed for ultra-rare kaon decays works. The NA62 Collaboration proposes to continue the data taking of the experiment during the period after CERN Accelerators Long Shutdown 2 (LS2) and before Long Shutdown 3 (LS3). The continuation will allow NA62 to perform a measurement of the branching ratio of the ultra-rare kaon decay K+ → π+νν ̄ with significantly improved accuracy, to substantially increase its sensitivity on several rare and forbidden kaon decays, and to reach unprecedented sensitivity in the investigation of several Standard Model (SM) extensions involving faintly interacting long-lived particles.