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*+ → D⁰π⁺ and D*- → D⁰π⁻. The $CP$ asymmetry in D⁰ → K⁰SK⁰S is measured to be ACP(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 → K⁰Sπ⁺π⁻ 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.