Search for $CP$ violation in D${}^0$ $\to$ K${}_{\mathrm{S}}^0$K${}_{\mathrm{S}}^0$ decays in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A search is reported for charge-parity $CP$ violation in D${}^0$ $\to$ K${}_{\mathrm{S}}^0$K${}_{\mathrm{S}}^0$ decays, using data collected in proton-proton collisions at $\sqrt{s}$ = 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${}^{\ast +}$ $\to$ D${}^0{\pi }^{+}$ and D${}^{\ast -}$ $\to$ D${}^0{\pi }^{-}$. The $CP$ asymmetry in D${}^0$ $\to$ K${}_{\mathrm{S}}^0$K${}_{\mathrm{S}}^0$ is measured to be $A_{CP}$(K${}_{\mathrm{S}}^0$K${}_{\mathrm{S}}^0$) = (6.2 $\pm$ 3.0 $\pm$ 0.2 $\pm$ 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${}^0$ $\to$ K${}_{\mathrm{S}}^0{\pi }^{+}{\pi }^{-}$ 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.