Search for $CP$ violation and measurement of branching fractions and decay asymmetry parameters for ${\Lambda }_c^{+}\to \Lambda h^{+}$ and ${\Lambda }_c^{+}\to {\Sigma }^0{h}^{+}$ ($h=K,\pi$)

Abstract

We report a study of ${\Lambda }_c^{+}\to \Lambda h^{+}$ and ${\Lambda }_c^{+}\to {\Sigma }^0{h}^{+}$ ($h=K,\pi$) decays based on a data sample of 980~${\mathrm{fb}}^{-1}$ collected with the Belle detector at the KEKB energy-asymmetric $e^{+}{e}^{-}$ collider. The first results of direct $CP$ asymmetry in two-body singly Cabibbo-suppressed (SCS) decays of charmed baryons are measured, $A_{CP}^{\mathrm{dir}}({\Lambda }_c^{+}\to \Lambda K^{+})=+0.021\pm 0.026\pm 0.001$ and $A_{CP}^{\mathrm{dir}}({\Lambda }_c^{+}\to {\Sigma }^0{K}^{+})=+0.025\pm 0.054\pm 0.004$. We also make the most precise measurement of the decay asymmetry parameters ($\alpha$) for the four modes of interest and search for $CP$ violation via the $\alpha$-induced $CP$ asymmetry ($A_{CP}^{\alpha }$). We measure $A_{CP}^{\alpha }({\Lambda }_c^{+}\to \Lambda K^{+})=-0.023\pm 0.086\pm 0.071$ and $A_{CP}^{\alpha }({\Lambda }_c^{+}\to {\Sigma }^0{K}^{+})=+0.08\pm 0.35\pm 0.14$, which are the first $A_{CP}^{\alpha }$ results for SCS decays of charmed baryons. We search for $\Lambda$-hyperon $CP$ violation in ${\Lambda }_c^{+}\to (\Lambda ,{\Sigma }^0){\pi }^{+}$ and find $A_{CP}^{\alpha }(\Lambda \to p{\pi }^{-})=+0.013\pm 0.007\pm 0.011$. This is the first time that hyperon $CP$ violation has been measured via Cabibbo-favored charm decays. No evidence of baryon $CP$ violation is found. We also obtain the most precise branching fractions for two SCS ${\Lambda }_c^{+}$ decays, $\mathcal{B}({\Lambda }_c^{+}\to \Lambda K^{+})=(6.57\pm 0.17\pm 0.11\pm 0.35)\times 10^{-4}$ and $\mathcal{B}({\Lambda }_c^{+}\to {\Sigma }^0{K}^{+})=(3.58\pm 0.19\pm 0.06\pm 0.19)\times 10^{-4}$. The first uncertainties are statistical and the second systematic, while the third uncertainties come from the uncertainties on the world average branching fractions of ${\Lambda }_c^{+}\to (\Lambda ,{\Sigma }^0){\pi }^{+}$.

15 pages, 9 figures, accepted for publication as an article in the Science Bulletin

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