Search for CP Violation in D⁰ → K_S π⁺ π^-

Introduction

While CP violation is well established for B mesons and kaons, this is not the case for charm mesons. First evidence for CP violation in D⁰ → π⁺ π^- and D⁰ → K⁺ K^- was recently reported by LHCb. Whether this is a hint for new physics is not yet clear. Therefore it is important to search for and measure CP violation in further charm meson decays.

In a data sample corresponding to an integrated luminosity of 6.0 fb^-1 we search for time integrated CP asymmetries in the resonant substructure of the three-body decay D⁰ → K_S π⁺ π^- decays. As the standard model expectation of these CP asymmetries is of the order 10^-6 and thus well below the experimantal sensitivity, an observation of CP violation would be a clear hint for new physics. The production flavor of the D⁰ is determined by the charge of the pion from the strong decay D^*(2010)⁺ → D⁰ π⁺. For the first time at a hadron collider, the Dalitz plot technique is applied for the description of the dynamic decay structure. Two complementary approaches are used, namely a full Dalitz fit employing the Isobar model for the involved resonances and a model-independent bin-by-bin comparison of the D⁰ and D⁰bar Dalitz plots.

Results

A_CP = -0.0005 ± 0.0057 (stat.) ± 0.0054 (syst.)

List of Figures

• Figure 1: The M(K_S π⁺ π^-) (eps) (png) and M(K_S π⁺ π^- π⁺) - M(K_S π⁺ π^-) (eps) (png) mass distributions
• Figure 2: Dalitz plot (eps) (png)
• Figure 3: Projections of efficiency parametrization on M²(K_S π(RS)) (eps) (png), M²(π⁺ π^-) (eps) (png), and M²(K_S π(WS)) (eps) (png)
• Figure 4: Dalitz fit projections on M²(K_S π(RS)) (eps) (png), M²(π⁺ π^-), and (eps) (png), M²(K_S π(WS)) (eps) (png)
• Figure 5: Asymmetry between D^*+ and D^*- yields (eps) (png)
• Figure 6: Dalitz plot (eps) (png) and histogram (eps) (png) of asymmetry significances