Measurement of the time-integrated $CP$ asymmetry in $D^0 \to K^0_{\rm S} K^0_{\rm S}$ decays using opposite-side flavor tagging at Belle and Belle II

TL;DR

This work measures the time-integrated CP asymmetry $A_{C\!P}$ for the charm decay $D^0 \to K^0_{\rm S} K^0_{\rm S}$ using an independent opposite-side flavor tagging approach (CFT) with data from Belle and Belle II. The analysis employs a two-dimensional unbinned maximum-likelihood fit to the $D^0$ mass and the CFT dilution variable, with data-driven PDFs and calibration of the per-candidate dilution from $D^0 \to K^- \pi^+$ decays, achieving tagging powers of $23.52\%$ and $32.71\%$ for Belle and Belle II, respectively. Signal yields of $N_{sig}=14490\pm340$ (Belle) and $5180\pm120$ (Belle II) lead to $A_{C\!P}=(2.5\pm2.7)\%$ and $(-0.1\pm3.0)\%$ in the two samples, which combine to $A_{C\!P}=(1.3\pm2.0\pm0.2)\%$ when Belle and Belle II results are merged. The measurement is consistent with CP symmetry and, when combined with prior $D^{*+}$-tagged results, provides the most precise determination to date, reinforcing the bounds on new physics in up-type quark decays.

Abstract

We measure the time-integrated $CP$ asymmetry in $D^0 \to K^0_{\rm S} K^0_{\rm S}$ decays reconstructed in $e^+e^-\to c{\overline c}$ events collected by the Belle and Belle II experiments. The corresponding data samples have integrated luminosities of 980 and 428 fb${}^{-1}$, respectively. To infer the flavor of the $D^0$ meson, we exploit the correlation between the flavor of the reconstructed decay and the electric charges of particles reconstructed in the rest of the $e^+e^-\to c{\overline c}$ event. This results in a sample which is independent from any other previously used at Belle or Belle II. The result, $A_{CP}(D^0 \to K^0_{\rm S} K^0_{\rm S}) = (1.3 \pm 2.0 \pm 0.2)\%$, where the first uncertainty is statistical and the second systematic, is consistent with previous determinations and with $CP$ symmetry.

Figures (3)

• Figure 1: Distributions of the BDT output for the training samples of (green) signal and (red) background candidates in (top) Belle and (bottom) Belle II.
• Figure 2: True dilution as a function of the predicted dilution for $D\xspace\xspace^0\xspace\rightarrow\xspace K\xspace\xspace^-\xspace\pi\xspace\xspace^+\xspace$ and $\overline{ D\xspace}{}\xspace^0\xspace\rightarrow\xspace K\xspace\xspace^+\xspace\pi\xspace\xspace^-\xspace$ decays in (top) Belle and (bottom) Belle II data with projections of the calibration fit overlaid. The bisector of the plane (red dotted line) represents the expected relation for perfectly calibrated predicted dilution.
• Figure 3: Distributions of (left) $m(K\xspace\xspace^0_{\rm S}\xspace K\xspace\xspace^0_{\rm S}\xspace)$ and (right) $qr$ for $D\xspace\xspace^0\xspace\rightarrow\xspace K\xspace\xspace^0_{\rm S}\xspace K\xspace\xspace^0_{\rm S}\xspace$ candidates in (top) Belle and (bottom) Belle II data, with fit projections overlaid. The $qr$ distributions are only for candidates in the $m(K\xspace\xspace^0_{\rm S}\xspace K\xspace\xspace^0_{\rm S}\xspace)$ signal regions indicated by the vertical lines.