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$^{0}$ → K$_{S}^{0}$ K$_{S}^{0}$ 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$^{*+}$ → D$^{0}$π$^{+}$ decay, allowing the determination of the flavour of the D$^{0}$ meson using the pion charge. The D$^{0}$ → K$^{+}$K$^{−}$ decay, which has a well measured CP asymmetry, is used as a calibration channel. The CP asymmetryfor D$^{0}$ → K$_{S}^{0}$ K$_{S}^{0}$ is measured to be $ {\mathcal{A}}^{CP}\left({D}^0\to {K}_S^0{K}_S^0\right)=\left(4.3\pm 3.4\pm 1.0\right)\%, $ 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 $ {\mathcal{A}}^{CP}\left({D}^0\to {K}_S^0{K}_S^0\right)=\left(2.3\pm 2.8\pm 0.9\right)\%. $