Search for CP violation in D⁰ K⁰SK⁰S decays in proton-proton collisions at s = 13 TeV

A search is reported for charge-parity CP violation in D⁰ K⁰SK⁰S decays, using data collected in proton-proton collisions at s = 13 TeV recorded by the CMS experiment in 2018. The analysis uses a dedicated data set that corresponds to an integrated luminosity of 41. 6 fb^-1, which consists of about 10 billion events containing a pair of b hadrons, nearly all of which decay to charm hadrons. The flavor of the neutral D meson is determined by the pion charge in the reconstructed decays D^*+ D⁰^+ and D^*- D⁰^-. The CP asymmetry in D⁰ K⁰SK⁰S is measured to be A₂₏ (K⁰SK⁰S) = (6. 2 3. 0 0. 2 0. 8) %, where the three uncertainties represent the statistical uncertainty, the systematic uncertainty, and the uncertainty in the measurement of the CP asymmetry in the D⁰ K⁰S^+^- decay. This is the first CP asymmetry measurement by CMS in the charm sector as well as the first to utilize a fully hadronic final state.