Search for CP violation in D 0 → K S 0 K S 0 decays in proton-proton collisions at s = 13 Te V

Abstract

A search is reported for charge-parity $\mathrm{CP}$ violation in ${\text{D}}^0\to {\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0$ decays, using data collected in proton-proton collisions at $\sqrt{s}=13\text{Te}\text{V}$ recorded by the CMS experiment in 2018. The analysis uses a dedicated data set that corresponds to an integrated luminosity of 41.6 ${\text{fb}}^{-1}$ , 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 D meson is determined by the pion charge in the reconstructed decays $\text{D}_{}^{\ast +}\to {\text{D}}^0{\pi }^{+}$ and $\text{D}_{}^{\ast -}\to {\overline{\text{D}}}^0{\pi }^{-}$ . The $\mathrm{CP}$ asymmetry in ${\text{D}}^0\to {\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0$ is measured to be $A_{\mathrm{CP}}\left({\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0\right)=(6.2\pm 3.0\pm 0.2\pm 0.8)\%$ , where the three uncertainties represent the statistical uncertainty, the systematic uncertainty, and the uncertainty in the measurement of the $\mathrm{CP}$ asymmetry in the ${\text{D}}^0\to {\text{K}}_{\text{S}}^0{\pi }^{+}{\pi }^{-}$ decay. This is the first $\mathrm{CP}$ asymmetry measurement by CMS in the charm sector as well as the first to utilize a fully hadronic final state.

Fig. 1

The decay of neutral charm meson to two neutral kaons: exchange (upper) and penguin annihilation (lower) diagrams

Fig. 2

The ${\text{D}}^0{\mathrm{\pi }}^{+}$ (left) and ${\overline{\text{D}}}^0{\mathrm{\pi }}^{-}$ (right) invariant mass distributions for the ${\text{K}}_{\text{S}}^0{\mathrm{\pi }}^{+}{\mathrm{\pi }}^{-}$ channel, with the result of the fit to both distributions

Fig. 3

The invariant mass distributions for $\text{D}_{}^{\ast +}$ candidates (left) and $\text{D}_{}^{\ast -}$ candidates (right), with the $m(\text{D}{\mathrm{\pi }}^{\pm })$ distributions in the upper row and the $m({\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0)$ distributions in the lower row. Projections of the simultaneous 2D fit are also shown

Fig. 4

Results of the 2D fit to the $m(\text{D}{\mathrm{\pi }}^{\pm })\times m({\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0)$ for the signal channel, $\text{D}_{}^{\ast +}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m({\text{D}}^0{\mathrm{\pi }}^{+})$ in ranges of $m({\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0)$: left sideband (upper left), region of ${\text{D}}_{\text{s}}^{\pm }\to {\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0{\mathrm{\pi }}^{\pm }$ contamination (upper right), signal region of ${\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m({\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0)$ in ranges of $m({\text{D}}^0{\mathrm{\pi }}^{+})$: left sideband (left), signal region of ${\text{D}}^0{\mathrm{\pi }}^{+}$ (center), and right sideband (right)

Fig. 5

Results of the 2D fit to the $m(\text{D}{\mathrm{\pi }}^{\pm })\times m({\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0)$ for the signal channel, $\text{D}_{}^{\ast -}$ candidates. Upper and middle rows show 1D projections of the 2D fit on $m({\overline{\text{D}}}^0{\mathrm{\pi }}^{-})$ in ranges of $m({\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0)$: left sideband (upper left), region of ${\text{D}}_{\text{s}}^{\pm }\to {\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0{\mathrm{\pi }}^{\pm }$ contamination (upper right), signal region of ${\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0$ (middle left), and right sideband (middle right). Lower row shows 1D projections of the 2D fit on $m({\text{K}}_{\text{S}}^0{\text{K}}_{\text{S}}^0)$ in ranges of $m({\overline{\text{D}}}^0{\mathrm{\pi }}^{-})$: left sideband (left), signal region of ${\overline{\text{D}}}^0{\mathrm{\pi }}^{-}$ (center), and right sideband (right)