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Search for CP violation in $\mathrm{D^0}\to\mathrm{K^0_S}\mathrm{K^0_S}$ decays in proton-proton collisions at $\sqrt{s} =$ 13 TeV

CMS Collaboration

19 May 2024

Eur. Phys. J. C 84 (2024) 1264

Abstract: A search is reported for charge-parity CP violation in $\mathrm{D^0}\to\mathrm{K^0_S}\mathrm{K^0_S}$ decays, using data collected in proton-proton collisions at $\sqrt{s} =$ 13 TeV recorded by the CMS experiment in 2018. The analysis uses a dedicated data set that corresponds to an integrated luminosity of 41.6 fb$^{-1}$, which consists of about 10 billion events containing a pair of b hadrons, nearly all of which decay to charm hadrons. The flavor of the neutral $\mathrm{D}$ meson is determined by the pion charge in the reconstructed decays $\mathrm{D}^{*+}\to\mathrm{D^0}\pi^{+}$ and $\mathrm{D}^{*-}\to\overline{\mathrm{D}}^{0}\pi^{-}$. The CP asymmetry in $\mathrm{D^0}\to\mathrm{K^0_S}\mathrm{K^0_S}$ is measured to be $A_{CP}(\mathrm{K^0_S}\mathrm{K^0_S}) =$ (6.2 $\pm$ 3.0 $\pm$ 0.2 $\pm$ 0.8)%, where the three uncertainties represent the statistical uncertainty, the systematic uncertainty, and the uncertainty in the measurement of the CP asymmetry in the $\mathrm{D^0}\to\mathrm{K^0_S}\pi^{+}\pi^{-}$ decay. This is the first CP asymmetry measurement by CMS in the charm sector as well as the first to utilize a fully hadronic final state.

Links: e-print arXiv:2405.11606 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures
png pdf	Figure 1: The decay of neutral charm meson to two neutral kaons: exchange (left) and penguin annihilation (right) diagrams.
png pdf	Figure 1-a: The decay of neutral charm meson to two neutral kaons: exchange (left) and penguin annihilation (right) diagrams.
png pdf	Figure 1-b: The decay of neutral charm meson to two neutral kaons: exchange (left) and penguin annihilation (right) diagrams.
png pdf	Figure 2: The $\mathrm{D^0}\pi^{+}$ (left) and $\overline{\mathrm{D}}^{0}\pi^{-}$ (right) invariant mass distributions for the $\mathrm{K^0_S}\pi^{+}\pi^{-}$ channel, with the result of the fit to both distributions.
png pdf	Figure 2-a: The $\mathrm{D^0}\pi^{+}$ (left) and $\overline{\mathrm{D}}^{0}\pi^{-}$ (right) invariant mass distributions for the $\mathrm{K^0_S}\pi^{+}\pi^{-}$ channel, with the result of the fit to both distributions.
png pdf	Figure 2-b: The $\mathrm{D^0}\pi^{+}$ (left) and $\overline{\mathrm{D}}^{0}\pi^{-}$ (right) invariant mass distributions for the $\mathrm{K^0_S}\pi^{+}\pi^{-}$ channel, with the result of the fit to both distributions.
png pdf	Figure 3: The invariant mass distributions for $\mathrm{D}^{*+}$ candidates (left) and $\mathrm{D}^{*-}$ candidates (right), with the $m(\mathrm{D}\pi^{\pm})$ distributions in the upper row and the $m(\mathrm{K^0_S}\mathrm{K^0_S})$ distributions in the lower row. Projections of the simultaneous 2D fit are also shown.
png pdf	Figure 3-a: The invariant mass distributions for $\mathrm{D}^{*+}$ candidates (left) and $\mathrm{D}^{*-}$ candidates (right), with the $m(\mathrm{D}\pi^{\pm})$ distributions in the upper row and the $m(\mathrm{K^0_S}\mathrm{K^0_S})$ distributions in the lower row. Projections of the simultaneous 2D fit are also shown.
png pdf	Figure 3-b: The invariant mass distributions for $\mathrm{D}^{*+}$ candidates (left) and $\mathrm{D}^{*-}$ candidates (right), with the $m(\mathrm{D}\pi^{\pm})$ distributions in the upper row and the $m(\mathrm{K^0_S}\mathrm{K^0_S})$ distributions in the lower row. Projections of the simultaneous 2D fit are also shown.
png pdf	Figure 3-c: The invariant mass distributions for $\mathrm{D}^{*+}$ candidates (left) and $\mathrm{D}^{*-}$ candidates (right), with the $m(\mathrm{D}\pi^{\pm})$ distributions in the upper row and the $m(\mathrm{K^0_S}\mathrm{K^0_S})$ distributions in the lower row. Projections of the simultaneous 2D fit are also shown.
png pdf	Figure 3-d: The invariant mass distributions for $\mathrm{D}^{*+}$ candidates (left) and $\mathrm{D}^{*-}$ candidates (right), with the $m(\mathrm{D}\pi^{\pm})$ distributions in the upper row and the $m(\mathrm{K^0_S}\mathrm{K^0_S})$ distributions in the lower row. Projections of the simultaneous 2D fit are also shown.
png pdf	Figure 4: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*+}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\mathrm{D^0}\pi^{+})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\mathrm{D^0}\pi^{+})$: left sideband (left), signal region of $\mathrm{D^0}\pi^{+}$ (center), and right sideband (right).
png pdf	Figure 4-a: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*+}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\mathrm{D^0}\pi^{+})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\mathrm{D^0}\pi^{+})$: left sideband (left), signal region of $\mathrm{D^0}\pi^{+}$ (center), and right sideband (right).
png pdf	Figure 4-b: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*+}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\mathrm{D^0}\pi^{+})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\mathrm{D^0}\pi^{+})$: left sideband (left), signal region of $\mathrm{D^0}\pi^{+}$ (center), and right sideband (right).
png pdf	Figure 4-c: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*+}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\mathrm{D^0}\pi^{+})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\mathrm{D^0}\pi^{+})$: left sideband (left), signal region of $\mathrm{D^0}\pi^{+}$ (center), and right sideband (right).
png pdf	Figure 4-d: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*+}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\mathrm{D^0}\pi^{+})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\mathrm{D^0}\pi^{+})$: left sideband (left), signal region of $\mathrm{D^0}\pi^{+}$ (center), and right sideband (right).
png pdf	Figure 4-e: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*+}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\mathrm{D^0}\pi^{+})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\mathrm{D^0}\pi^{+})$: left sideband (left), signal region of $\mathrm{D^0}\pi^{+}$ (center), and right sideband (right).
png pdf	Figure 4-f: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*+}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\mathrm{D^0}\pi^{+})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\mathrm{D^0}\pi^{+})$: left sideband (left), signal region of $\mathrm{D^0}\pi^{+}$ (center), and right sideband (right).
png pdf	Figure 4-g: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*+}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\mathrm{D^0}\pi^{+})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\mathrm{D^0}\pi^{+})$: left sideband (left), signal region of $\mathrm{D^0}\pi^{+}$ (center), and right sideband (right).
png pdf	Figure 5: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*-}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\overline{\mathrm{D}}^{0}\pi^{-})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\overline{\mathrm{D}}^{0}\pi^{-})$: left sideband (left), signal region of $\overline{\mathrm{D}}^{0}\pi^{-}$ (center), and right sideband (right).
png pdf	Figure 5-a: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*-}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\overline{\mathrm{D}}^{0}\pi^{-})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\overline{\mathrm{D}}^{0}\pi^{-})$: left sideband (left), signal region of $\overline{\mathrm{D}}^{0}\pi^{-}$ (center), and right sideband (right).
png pdf	Figure 5-b: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*-}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\overline{\mathrm{D}}^{0}\pi^{-})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\overline{\mathrm{D}}^{0}\pi^{-})$: left sideband (left), signal region of $\overline{\mathrm{D}}^{0}\pi^{-}$ (center), and right sideband (right).
png pdf	Figure 5-c: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*-}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\overline{\mathrm{D}}^{0}\pi^{-})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\overline{\mathrm{D}}^{0}\pi^{-})$: left sideband (left), signal region of $\overline{\mathrm{D}}^{0}\pi^{-}$ (center), and right sideband (right).
png pdf	Figure 5-d: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*-}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\overline{\mathrm{D}}^{0}\pi^{-})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\overline{\mathrm{D}}^{0}\pi^{-})$: left sideband (left), signal region of $\overline{\mathrm{D}}^{0}\pi^{-}$ (center), and right sideband (right).
png pdf	Figure 5-e: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*-}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\overline{\mathrm{D}}^{0}\pi^{-})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\overline{\mathrm{D}}^{0}\pi^{-})$: left sideband (left), signal region of $\overline{\mathrm{D}}^{0}\pi^{-}$ (center), and right sideband (right).
png pdf	Figure 5-f: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*-}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\overline{\mathrm{D}}^{0}\pi^{-})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\overline{\mathrm{D}}^{0}\pi^{-})$: left sideband (left), signal region of $\overline{\mathrm{D}}^{0}\pi^{-}$ (center), and right sideband (right).
png pdf	Figure 5-g: Results of the 2D fit to the $m(\mathrm{D}\pi^{\pm}) \times m(\mathrm{K^0_S}\mathrm{K^0_S})$ for the signal channel, $\mathrm{D}^{*-}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m(\overline{\mathrm{D}}^{0}\pi^{-})$ in ranges of $m(\mathrm{K^0_S}\mathrm{K^0_S})$: left sideband (upper left), region of $\mathrm{D}_{s}^{\pm}\to\mathrm{K^0_S}\mathrm{K^0_S}\pi^{\pm}$ contamination (upper right), signal region of $\mathrm{K^0_S}\mathrm{K^0_S}$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m(\mathrm{K^0_S}\mathrm{K^0_S})$ in ranges of $m(\overline{\mathrm{D}}^{0}\pi^{-})$: left sideband (left), signal region of $\overline{\mathrm{D}}^{0}\pi^{-}$ (center), and right sideband (right).

Tables
png pdf	Table 1: Optimized selection criteria in the signal channel $\mathrm{K^0_S}\mathrm{K^0_S}$. The requirements on the $\mathrm{K^0_S}$ candidates in the third and fourth lines are given first for the $\mathrm{K^0_S}$ with larger $p_{\mathrm{T}}$, then for the $\mathrm{K^0_S}$ with lower $p_{\mathrm{T}}$.
png pdf	Table 2: Results of the fit to the selected $\mathrm{D}^{*+}\to\mathrm{D^0}\pi^{+}$ and $\mathrm{D}^{*-}\to\overline{\mathrm{D}}^{0}\pi^{-}$ candidates, where $\mathrm{D^0}\,(\overline{\mathrm{D}}^{0}) \to\mathrm{K^0_S}\pi^{+}\pi^{-}$. The ${\mathrm{D}^{\ast}(2010)^{\pm}}$ signal yields $N$ given in the second column are used in the evaluation of $A_{CP}^{\text{raw}}$. The uncertainties are statistical only.
png pdf	Table 3: Results of the 2D fit to the selected $\mathrm{D}^{*+}\to\mathrm{D^0}\pi^{+}$ and $\mathrm{D}^{*-}\to\overline{\mathrm{D}}^{0}\pi^{-}$ candidates, where $\mathrm{D^0}\,(\overline{\mathrm{D}}^{0}) \to\mathrm{K^0_S}\mathrm{K^0_S}$. The ${\mathrm{D}^{\ast}(2010)^{\pm}}$ signal yields $N$ given in the second column are used in the evaluation of $A_{CP}^{\text{raw}}$. The $\chi^2$ corresponds to the fit projection with 100 bins in the $x = m(\mathrm{D}\pi^{\pm})$ axis and 90 bins in the $y = m(\mathrm{K^0_S}\mathrm{K^0_S})$ axis, as shown in Fig 3. The uncertainties are statistical only.
png pdf	Table 4: Absolute systematic uncertainties in the measurement of $\Delta A_{CP}$.

Summary

A measurement of CP violation in $\mathrm{D^0}$ decays is reported, using proton-proton collision data collected at $\sqrt{s} =$ 13 TeV with a novel high-rate data stream (B parking). These data correspond to an integrated luminosity of 41.6 fb$^{-1}$ and include about 10 billion events containing beauty hadron decays. The difference in the CP asymmetries between $\mathrm{D^0}\to\mathrm{K^0_S}\mathrm{K^0_S}$ and $\mathrm{D^0}\to\mathrm{K^0_S}\pi^{+}\pi^{-}$ is measured to be: $\Delta A_{CP} \equiv A_{CP}(\mathrm{K^0_S}\mathrm{K^0_S})-A_{CP}(\mathrm{K^0_S}\pi^{+}\pi^{-}) =$ (6.3 $\pm$ 3.0 (stat) $\pm$ 0.2 (syst) )%. Using the world-average value of $A_{CP}(\mathrm{K^0_S}\pi^{+}\pi^{-}) = (-$ 0.1 $\pm$ 0.8 $)%$ [18,35,4], we report the measurement, $A_{CP}(\mathrm{K^0_S}\mathrm{K^0_S}) =$(6.2 $\pm$ 3.0 $\pm$ 0.2 $\pm$ 0.8)%, where the three uncertainties represent the statistical uncertainty, the systematic uncertainty, and the uncertainty in the measurement of the CP asymmetry in the $\mathrm{D^0}\to\mathrm{K^0_S}\pi^{+}\pi^{-}$ decay. The measured value is consistent with no CP violation within 2.0 standard deviations. Likewise, it is consistent with the LHCb [16] and the Belle measurements [17] at the level of 2.7 and 1.8 standard deviations, respectively. Tabulated results are provided in the HEPData record for this analysis [36]. This is the first CMS search for CP violation in the charm sector, paving the way for future measurements with more data, using new techniques, and in other channels.

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