Measurement of the time-integrated $CP$ asymmetry in $D^0 \to K^0_S K^0_S$ decays at LHCb

The $D^0 \to K^0_S K^0_S$ decay is a promising discovery channel for $CP$ violation in charm. A prediction based on the Standard Model gives an upper limit for the $CP$ asymmetry of about 1% and further enhancements could result from contributions from physics beyond the Standard Model. A preliminary measurement of the time-integrated $CP$ asymmetry in prompt $D^0 \to K^0_S K^0_S$ decays is presented, performed using data collected with the LHCb experiment in 2015 and 2016 at a 13TeV pp center-of-mass energy (Run 2). The $CP$ asymmetry is measured to be $A^{CP}$ ($D^0 \to K^0_S K^0_S$) = (0.042±0.034±0.010), where the first uncertainty is statistical and the second is systematic. This result represents a significant improvement with respect to the previous LHCb Run 1 measurement.