Measurement of the time-integrated $CP$ asymmetry in $D^0 rightarrow K^0_S K^0_S$ decays

A measurement of the time-integrated $CP$ asymmetry in $D^0rightarrow K^0_S K^0_S$ decays is reported. The data correspond to an integrated luminosity of about $2$ fb$^{-1}$ collected in 2015-2016 by the LHCb collaboration in $pp$ collisions at a centre-of-mass energy of $13$ TeV. The $D^0$ candidate is required to originate from a $D^{ast +} rightarrow D^0 pi^+$ decay, allowing the determination of the flavour of the $D^0$ meson using the pion charge. The $D^0 rightarrow K^{+}K^{-}$ decay, which has a well measured $CP$ asymmetry, is used as a calibration channel. The $CP$ asymmetry for $D^0rightarrow K^0_S K^0_S$ is measured to be begin{equation*} mathcal{A}^{CP}(D^0rightarrow K^0_S K^0_S) = (4.3pm 3.4pm 1.0)%, end{equation*} where the first uncertainty is statistical and the second is systematic. This result is combined with the previous LHCb measurement at lower centre-of-mass energies to obtain begin{equation*} mathcal{A}^{CP}(D^0rightarrow K^0_S K^0_S) = (2.3pm 2.8pm 0.9)%. end{equation*}