Measurement of the time-integrated $CP$ asymmetry in $D^0 \rightarrow K^0_{\rm S} K^0_{\rm S}$ decays with LHCb

$CP$ violation in charm has not yet been observed, although measurements of time-integrated $CP$ asymmetries in $D^0 \rightarrow K^+K^-$ and $D^0 \rightarrow \pi^+ \pi^-$ decays have reached a precision of $\mathcal{O}(10^{-3})$. The $D^0 \rightarrow K^0_{\rm S} K^0_{\rm S}$ 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 $D^0 \rightarrow K^0_{\rm S} K^0_{\rm S}$ decays is presented, performed using data collected with the LHCb experiment in 2015 and 2016 at a 13 TeV $pp$ center-of-mass energy (Run 2). The $CP$ asymmetry is measured to be\[A^{CP}(D^0 \rightarrow K^0_{\rm S} K^0_{\rm S})=(0.042 \pm 0.034 \pm 0.010),\]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.