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

TL;DR

This paper reports a measurement of the time-integrated $CP$ asymmetry in $D^0 ightarrow K^0_S K^0_S$ decays using LHCb data, providing results consistent with no $CP$ violation within uncertainties.

Contribution

First measurement of $CP$ asymmetry in $D^0 ightarrow K^0_S K^0_S$ decays at 13 TeV, combining new data with previous results for improved precision.

Findings

Measured $CP$ asymmetry: (4.3 ± 3.4 ± 1.0)%

Combined $CP$ asymmetry: (2.3 ± 2.8 ± 0.9)%

Results are consistent with no $CP$ violation.

Abstract

A measurement of the time-integrated $CP$ asymmetry in $D^0\rightarrow K^0_S K^0_S$ 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 $D^0\rightarrow K^0_S K^0_S$ is measured to be \begin{equation*} \mathcal{A}^{CP}(D^0\rightarrow K^0_S K^0_S) = (4.3\pm 3.4\pm 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^0\rightarrow K^0_S K^0_S) = (2.3\pm 2.8\pm 0.9)\%. \end{equation*}