First measurement of time-dependent $CP$ violation in the flavor-changing neutral-current decay $B^{0}\rightarrow K_{S}^{0}μ^{+}μ^{-}$

Abstract

A flavor-tagged time-dependent analysis of $B^{0}\rightarrow K_{S}^{0}μ^{+}μ^{-}$ decays is performed across the full dimuon mass range excluding the $J/ψ$ and $ψ(2S)$ resonance regions. The analysis uses proton-proton collision data collected by the LHCb experiment in 2011--2018 at center-of-mass energies of 7, 8 and 13TeV, corresponding to an integrated luminosity of 9$fb^{-1}$. The CP violation parameters are determined to be $C=-0.13 \pm 0.32 \pm 0.04$ and $S= +0.82\pm 0.29 \pm 0.05$, where the first uncertainties are statistical and the second are systematic.The results are consistent with the Standard Model prediction. This is the first experimental study of time-dependent CP violation in $b\rightarrow sl^{+}l^{-}$ processes.

Figures (4)

• Figure 1: Mass distribution of the selected $\mathinner{{{\mathrm{B}\xspace}\xspace^0}\xspace\!\rightarrow\xspace {{\mathrm{K}\xspace}\xspace^0_{\mathrm{S}}}\xspace {\upmu\xspace^+}\xspace{\upmu\xspace^-}\xspace}$ candidates, with the fit result also shown.
• Figure 2: Left: background-subtracted decay-time distributions for ${{\overline{ \mathrm{B}\xspace}}\xspace{}^0}\xspace$ and ${{\mathrm{B}\xspace}\xspace^0}\xspace$ candidates identified through flavor tagging, where the fit result is also shown. The difference between ${{\overline{ \mathrm{B}\xspace}}\xspace{}^0}\xspace$ and ${{\mathrm{B}\xspace}\xspace^0}\xspace$ distributions reveals the $C\!P$ asymmetry. Right: time-dependent $C\!P$ asymmetry. The data points show the binned signal yield asymmetry, with tagging dilution effects corrected for. The result of the maximum-likelihood fit is shown as the solid curve.
• Figure 3: Two-dimensional confidence regions for the parameters $C$ and $S$ from profile likelihood scans, without including systematic uncertainties. The green dash-dotted, blue dashed and red solid contours represent the $68.3\%$ confidence regions for the low-$q^2$, high-$q^2$ and total regions, respectively. The black marker with error bars represents the fit results in the total sample, while the magenta marker stands for the SM prediction.
• Figure 4: Two-dimensional confidence regions for $C$ and $S$ based on a likelihood-ratio ordering technique, corresponding to the total $q^2$ range. The red solid, green dashed and blue dash-dotted contours represent the 68.3%, 95.5%, and 99.7% confidence intervals given the physical constraint $C^2 + S^2 \leq 1$. The black marker with error bands represents the fit results for $C$ and $S$. The magenta diamond marker stands for the SM prediction.