2 We have searched for CP violation in the charmed meson decays D + ( s ) → K 0 $_{S}$π + and D + ( s ) → K 0 $_{S}$K + using 673 fb − 1 of data collected with the Belle detector at the KEKB asymmetric-energy e $^{+}$e − collider. No evidence for CP violation is observed. We report the most sensitive CP asymmetry measurements to date for these decays: A D $^{+}$→ K 0 $_{S}$π + CP = ( − 0 . 71 ± 0 . 19 ± 0 . 20)%, A D + s → K 0 $_{S}$π + CP = (+5 . 45 ± 2 . 50 ± 0 . 33)%, A D $^{+}$→ K 0 $_{S}$K + CP = ( − 0 . 16 ± 0 . 58 ± 0 . 25)%, and A D + s → K 0 $_{S}$K + CP = (+0 . 12 ± 0 . 36 ± 0 . 22)%, where the first uncertainties are statistical and the second are systematic. PACS numbers: 11.30.Er, 13.25.Ft, 14.40.Lb Violation of the combined Charge-conjugation and Parity symmetries ( CP ) in the standard model (SM) is produced by a non-vanishing phase in the Cabibbo- Kobayashi-Maskawa flavor-mixing matrix [1]. For charged meson decays this may be observed as a non- zero CP asymmetry, defined as scribed above are expected. Observing A$_{CP}$ values of O (1)% or larger in the decays considered in this Letter would represent strong evidence for processes involving physics beyond the SM [5]. The data used in this analysis were recorded at or near the Υ(4 S ) resonance with the Belle detector [6] at the e $^{+}$e − asymmetric-energy collider KEKB [7]. The sample corresponds to an integrated luminosity of 673 fb − $^{1}$. where Γ is the partial decay width, X denotes a charged meson, and f is a final state. In the SM, the charmed particle processes for which a significant non-vanishing CP violation is expected are singly Cabibbo-suppressed (SCS) decays in which there is both interference between two different decay amplitudes and a strong phase shift from final state interactions. In the SM, CP violation in SCS charmed meson decays is predicted to occur at the level of O (0 . 1)% or lower [2]. The SM also predicts a CP asymmetry in the final states containing a neutral kaon that is produced via K 0 − ¯ K 0 mixing even if no CP violating phase exists in the charm decay amplitudes. The expected magnitude for this type of asymmetry is (0 . 332 ± 0 . 006)% from K 0 L semileptonic decay [3]. Searches for CP violation in charmed mesons are complementary to those in B and K mesons, since the former tests the CP violating couplings of the up -type quarks while the latter those of the down -type quarks. In this Letter we report results from searches for CP violation in the D + s → K 0 $_{S}$π $^{+}$, D + → K 0 $_{S}$K $^{+}$, D + → K 0 $_{S}$π $^{+}$, and D + s → K 0 $_{S}$K + decay processes [4]. The for- mer two channels are SCS decays and the latter two are mixtures of Cabibbo-favored (CF) and doubly Cabibbo- suppressed (DCS) decays, where SM CP violations de- where N$_{rec}$ is the number of reconstructed decays and h is a charged hadron. The measured asymmetry in Eq. (2) includes two contributions other than A$_{CP}$ . One is the forward-backward asymmetry ( A$_{FB}$ ) due to γ $^{∗}$− Z 0 interference in e $^{+}$e − → c ¯ c and the other is a detec- tion efficiency asymmetry between positively and nega- tively charged tracks ( A h + ϵ ). Since K 0 S mesons are recon- structed from a π $^{+}$π − pair, there is no detection asym- We determine the quantities A X $^{+}$→ K 0 $_{S}$h + CP defined in Eq. (1) by measuring the signal yield asymmetry D + and D + s candidates are reconstructed using the same requirements used in the measurement of the branching ratios for these same decays reported in Ref. [8]. Figure 1 shows the reconstructed K 0 $_{S}$π ± and K 0 $_{S}$K ± invariant mass distributions. All signals are pa- rameterized as a sum of two Gaussian distributions ex- cept for D + s → K 0 $_{S}$π + in which a single Gaussian is used for the signal parameterization. The parameterizations of the random combinatorial background and the peaking background due to K/π misidentification are described in detail in Ref. [8]. A X $^{+}$→ K 0 $_{S}$h + rec = N X $^{+}$→ K 0 $_{S}$h + rec − N X $^{−}$→ K 0 $_{S}$h − rec N X $^{+}$→ K 0 $_{S}$h + rec + N X − → K 0 $_{S}$h − rec , (2) A$_{CP}$ = Γ( X + → f $^{+}$) − Γ( X − → f $^{−}$) Γ( X + f $^{+}$) + Γ( X − f $^{−}$) (1) $^{35}$RIKEN BNL Research Center, Upton, New York 11973 $^{36}$University of Science and Technology of China, Hefei $^{37}$Seoul National University, Seoul $^{38}$Sungkyunkwan University, Suwon $^{39}$School of Physics, University of Sydney, NSW 2006 $^{40}$Tata Institute of Fundamental Research, Mumbai $^{41}$Excellence Cluster Universe, Technische Universit¨ at M¨ unchen, Garching $^{42}$Toho University, Funabashi $^{43}$Tohoku Gakuin University, Tagajo $^{44}$Tohoku University, Sendai $^{45}$Department of Physics, University of Tokyo, Tokyo $^{46}$Tokyo Metropolitan University, Tokyo $^{47}$Tokyo University of Agriculture and Technology, Tokyo $^{48}$IPNAS, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 $^{49}$Yonsei University, Seoul