A search is reported for charge-parity CP violation in D 0 → K S 0 K S 0 decays, using data collected in proton-proton collisions at s = 13 Te V 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 ∗ + → D 0 π + and D ∗ - → D ¯ 0 π - . The CP asymmetry in D 0 → K S 0 K S 0 is measured to be A CP ( K S 0 K S 0 ) = ( 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 D 0 → K S 0 π + 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.

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