Search for $CP$ violation in $Ξ_c^+\toΣ^+h^+h^-$ and $Λ_c^+\to ph^+h^-$ at Belle II

TL;DR

This study uses 428 fb$^{-1}$ of Belle II $e^+e^-$ data to measure decay-rate CP asymmetries in singly-Cabibbo-suppressed three-body decays of charmed baryons: $Ξ_c^+ \rightarrow Σ^+ h^+ h^-$ and $Λ_c^+ \rightarrow p h^+ h^-$ with $h=K,π$. CP asymmetries are extracted from yield asymmetries after removing production and detection effects via forward/backward averaging and control channels, employing sPlot-based weighting to equalize kinematic distributions. The results, shown with statistical and systematic uncertainties, are all consistent with CP symmetry, and the corresponding $U$-spin sums are also compatible with zero, marking the first such measurements for individual hadronic three-body charmed-baryon decays. The measurements are statistically limited, underscoring the potential of future Belle II data to probe CP violation in the charm sector more precisely.

Abstract

We report decay-rate $CP$ asymmetries of the singly-Cabibbo-suppressed decays $Ξ_c^+\toΣ^+h^+h^-$ and $Λ_c^+\to ph^+h^-$, with $h=K,π$, measured using 428 fb$^{-1}$ of $e^+e^-$ collisions collected by the Belle II experiment at the SuperKEKB collider. The results, \begin{equation} A_{CP}(Ξ_c^+\toΣ^+K^+K^-) = (3.7\pm6.6\pm0.6)\%, \end{equation} \begin{equation} A_{CP}(Ξ_c^+\toΣ^+π^+π^-) = (9.5\pm6.8\pm0.5)\%, \end{equation} \begin{equation} A_{CP}(Λ_c^+\to pK^+K^-) = (3.9\pm1.7\pm0.7)\%, \end{equation} \begin{equation} A_{CP}(Λ_c^+\to pπ^+π^-) = (0.3\pm1.0\pm0.2)\%, \end{equation} where the first uncertainties are statistical and the second systematic, agree with $CP$ symmetry. From these results we derive the sums \begin{equation} A_{CP}(Ξ_c^+\toΣ^+π^+π^-) \, + \, A_{CP}(Λ_c^+\to pK^+K^-) = (13.4 \pm 7.0\pm 0.9)\%, \end{equation} \begin{equation} A_{CP}(Ξ_c^+\toΣ^+K^+K^-) \, + \, A_{CP}(Λ_c^+\to pπ^+π^-) = (\phantom{0}4.0 \pm 6.6\pm 0.7)\%, \end{equation} which are consistent with the $U$-spin symmetry prediction of zero. These are the first measurements of $CP$ asymmetries for individual hadronic three-body charmed-baryon decays.

Figures (5)

• Figure 1: Weighted and unweighted correct-candidate proton momentum (left) and $\cos\theta$ (right) distributions for $焄\xspace^{+}_c\xspace\rightarrow\xspace 焆\xspace^{+}\xspace K\xspace\xspace^+\xspace K\xspace\xspace^-\xspace\xspace$ and $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace 焆\xspace^{+}\xspace K\xspace\xspace^+\xspace K\xspace\xspace^-\xspace\xspace$ (top) and $焄\xspace^{+}_c\xspace\rightarrow\xspace 焆\xspace^{+}\xspace\pi\xspace\xspace^+\xspace\pi\xspace\xspace^-\xspace\xspace$ and $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace 焆\xspace^{+}\xspace\pi\xspace\xspace^+\xspace\pi\xspace\xspace^-\xspace\xspace$ (bottom).
• Figure 2: Weighted and unweighted correct-candidate proton momentum (left) and $\cos\theta$ (right) distributions for $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace p\xspace K\xspace\xspace^+\xspace K\xspace\xspace^-\xspace\xspace$ and $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace p\xspace\pi\xspace\xspace^+\xspace K\xspace\xspace^-\xspace$ (top) and $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace p\xspace\pi\xspace\xspace^+\xspace K\xspace\xspace^-\xspace$ and $D\xspace\xspace^0\xspace\rightarrow\xspace\pi\xspace\xspace^+\xspace K\xspace\xspace^-\xspace\pi\xspace\xspace^+\pi\xspace\xspace^-\xspace$ (bottom) with selection criteria for $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace p\xspace K\xspace\xspace^+\xspace K\xspace\xspace^-\xspace\xspace$.
• Figure 3: Weighted and unweighted correct-candidate proton momentum (left) and $\cos\theta$ (right) distributions for $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace p\xspace\pi\xspace\xspace^+\xspace\pi\xspace\xspace^-\xspace\xspace$ and $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace p\xspace\pi\xspace\xspace^+\xspace K\xspace\xspace^-\xspace$ (top) and $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace p\xspace\pi\xspace\xspace^+\xspace K\xspace\xspace^-\xspace$ and $D\xspace\xspace^0\xspace\rightarrow\xspace\pi\xspace\xspace^+\xspace K\xspace\xspace^-\xspace\pi\xspace\xspace^+\pi\xspace\xspace^-\xspace$ (bottom) with selection criteria for $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace p\xspace\pi\xspace\xspace^+\xspace\pi\xspace\xspace^-\xspace\xspace$.
• Figure 4: Mass distributions for $焄\xspace^{+}_c\xspace\rightarrow\xspace 焆\xspace^{+}\xspace h^{+}h^{-}\xspace$ (top) and $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace p\xspace h^{+}h^{-}\xspace$ (bottom) candidates for $h=K$ (left) and $\pi$ (right) and the results of the fits, summing forward and backward contributions; and their averaged yield asymmetries as functions of mass, with fit projection overlaid.
• Figure 5: Mass distributions for $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace 焆\xspace^{+}\xspace h^{+}h^{-}\xspace$ (top), $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace p\xspace\pi\xspace\xspace^+\xspace K\xspace\xspace^-\xspace$ (middle), and $D\xspace\xspace^0\xspace\rightarrow\xspace\pi\xspace\xspace^+\xspace K\xspace\xspace^-\xspace\pi\xspace\xspace^+\pi\xspace\xspace^-\xspace$ (bottom) candidates with the selection criteria for $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace p\xspace K\xspace\xspace^+\xspace K\xspace\xspace^-\xspace\xspace$ (left) and $焃\xspace^+_c\xspace\xspace\xspace\rightarrow\xspace p\xspace\pi\xspace\xspace^+\xspace\pi\xspace\xspace^-\xspace\xspace$ (right) and the results of the fits, summing the forward and backward contributions; and their averaged yield asymmetries as functions of mass, with fit projection overlaid.