A search is reported for charge-parity $CP$ violation in D$^0$$\to$ K$^0_\mathrm{S}$K$^0_\mathrm{S}$ decays, using data collected in proton-proton collisions at $\sqrt{s}$ = 13 TeV recorded by the CMS experiment in 2018. The analysis uses a dedicated data set that corresponds to an integrated luminosity of 41.6 fb$^{-1}$, which consists of about 10 billion events containing a pair of b hadrons, nearly all of which decay to charm hadrons. The flavor of the neutral D meson is determined by the pion charge in the reconstructed decays D$^{*+}$$\to$ D$^0\pi^+$ and D$^{*-}$$\to$ D$^0\pi^-$. The $CP$ asymmetry in D$^0$$\to$ K$^0_\mathrm{S}$K$^0_\mathrm{S}$ is measured to be $A_{CP}$(K$^0_\mathrm{S}$K$^0_\mathrm{S}$) = (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 D$^0$ $\to$ K$^0_\mathrm{S}\pi^+\pi^-$ 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.
The measured $CP$ asymmetry in $D^{0} \to K^{0}_{S} K^{0}_{S}$
The measured difference in the $CP$ asymmetries between $D^{0} \to K^{0}_{S} K^{0}_{S}$ and $D^{0} \to K^{0}_{S}\pi^{+}\pi^{-}$
The production of $D^{*\pm}$, $D^\pm$ and $D_s^\pm$ charmed mesons has been measured with the ATLAS detector in $pp$ collisions at $\sqrt{s}=7$ TeV at the LHC, using data corresponding to an integrated luminosity of $280\,$nb$^{-1}$. The charmed mesons have been reconstructed in the range of transverse momentum $3.5<p_{\rm T}(D)<100$ GeV and pseudorapidity $|\eta(D)|<2.1$. The differential cross sections as a function of transverse momentum and pseudorapidity were measured for $D^{*\pm}$ and $D^\pm$ production. The next-to-leading-order QCD predictions are consistent with the data in the visible kinematic region within the large theoretical uncertainties. Using the visible $D$ cross sections and an extrapolation to the full kinematic phase space, the strangeness-suppression factor in charm fragmentation, the fraction of charged non-strange $D$ mesons produced in a vector state, and the total cross section of charm production at $\sqrt{s}=7$ TeV were derived.
The visible low-$p_T$, $3.5<p_T(D)<20\rm{\ GeV}$, and high-$p_T$, $20<p_T(D)<100\rm{\ GeV}$, cross sections of $D^{*\pm}$, $D^\pm$ and $D^\pm_s$ production with $|\eta|<2.1$. The data uncertainties are the total uncertainties obtained as sums in quadrature of the statistical, systematic, luminosity and branching-fraction uncertainties.
The measured differential cross sections $\rm{d}\sigma/\rm{d}p_T$ of $D^{*\pm}$ and $D^\pm$ production with $|\eta|<2.1$. The first and second errors are the statistical and systematic uncertainties, respectively. The systematic uncertainties corresponding to the tracking ($\delta_2$) uncertainties (Table 2 of the paper) are strongly correlated. The fully correlated uncertainties linked with the luminosity measurement ($3.5\%$) and branching fractions ($1.5\%$ and $2.1\%$ for $D^{*\pm}$ and $D^\pm$, respectively) are not shown.
The measured differential cross sections $\rm{d}\sigma/\rm{d}|\eta|$ of $D^{*\pm}$ and $D^\pm$ production with $3.5<p_T<20\,$GeV. The first and second errors are the statistical and systematic uncertainties, respectively. The systematic uncertainty fractions corresponding to the tracking ($\delta_2$) uncertainties (Table 2 of the paper) are strongly correlated. The fully correlated uncertainties linked with the luminosity measurement ($3.5\%$) and branching fractions ($1.5\%$ and $2.1\%$ for $D^{*\pm}$ and $D^\pm$, respectively) are not shown.
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