Precision Studies and Searches for CP Asymmetries in the Inclusive Decay $Λ_{c}^{+}\to ΛX$

Abstract

Based on $e^+e^-$ annihilation data collected with the BESIII detector at center-of-mass energies from 4.600 to 4.699 GeV, corresponding to an integrated luminosity of 4.5 fb$^{-1}$, we present the first measurement of the longitudinal polarization of $Λ$ hyperons produced in the inclusive decay $Λ_c^+ \to ΛX$, where $X$ denotes any allowed final state. The polarizations are determined to be $\mathcal{P}_Λ = -0.393 \pm 0.055_{\mathrm{sta.}} \pm 0.020_{\mathrm{sys.}}$ and $\mathcal{P}_{\barΛ} = 0.288 \pm 0.056_{\mathrm{sta.}} \pm 0.017_{\mathrm{sys.}}$. We then search for CP violation using an asymmetry constructed from the $Λ$ polarization and the $Λ\to p π^-$ decay asymmetry parameters, and obtain $\mathcal{A}_{\mathrm{CP}}^{\mathrm{pol}} = 0.15 \pm 0.12_{\mathrm{sta.}} \pm 0.04_{\mathrm{sys.}}$. We also perform an updated measurement of the absolute branching fraction, resulted as $\mathcal{B}(Λ_c^+ \to ΛX) = (38.07 \pm 0.38_{\mathrm{sta.}} \pm 0.49_{\mathrm{sys.}})\%$, with precision improved by a factor of four relative to the current world average. A search for direct CP violation yields $\mathcal{A}_{\mathrm{CP}}^{\mathrm{dir}} = (1.5 \pm 1.0_{\mathrm{sta.}} \pm 1.0_{\mathrm{sys.}})\%$. No evidence for CP violation in inclusive charm baryon decays is observed.

Figures (12)

• Figure 1: Projections of the 2D simultaneous fit on the $M_{\Lambda}$ versus $M_{\rm{BC}}$ distributions for the data taken at $\sqrt s=4.682$ GeV, where the vertical axis is shown on a logarithmic scale. Points with error bars are data, the blue solid lines are the sum of the fitting functions, the red dashed lines are the signal, the purple dashed lines are the $\Lambda_{c}^{+}\bar{\Lambda}_{c}^{-}$ background, the brown dashed lines are the "hadron 1" background, the magenta dashed lines are the "hadron 2" background and the green dashed lines are the unmatched background.
• Figure 2: Angular distribution of the inclusive decay $\Lambda_{c}^{+} \to \Lambda X$ and its subsequent decay $\Lambda\to p\pi^{-}$.
• Figure 3: Distribution of $M_{\rm{BC}}$ versus $M_{\Lambda}$ in data taken at $\sqrt{s}=4.682\,\rm{GeV}$. The $\Lambda$ signal region is defined as (1.111, 1.121) $\,\rm{GeV}/\it{c}^{\rm{2}}$, with sidebands taken as (1.096, 1.106) $\cup$ (1.126, 1.136) $\,\rm{GeV}/\it{c}^{\rm{2}}$, while the $M_{\rm{BC}}$ signal and sideband regions are chosen as (2.275, 2.31) $\,\rm{GeV}/\it{c}^{\rm{2}}$ and (2.25, 2.27) $\,\rm{GeV}/\it{c}^{\rm{2}}$, respectively.
• Figure 4: The angular distributions for (left) $\Lambda_{c}^{+} \to \Lambda X$ and (right) $\bar{\Lambda}_{c}^{-} \to \bar{\Lambda} X$, obtained by summing over the data samples of all the energy points. Points with error bars are data, the red lines are the fitting models, and the blue shaded areas are the background events of the 2D sideband region in data.
• Figure 5: Projections of the 2D simultaneous fit on the $M_{\Lambda}$ versus $M_{\rm{BC}}$ distributions for the data taken at $\sqrt s=4.600$ GeV. Points with error bars are data, the blue solid lines are the sum of fitting functions, the red dashed lines are the $\Lambda_{c}^{+} \to \Lambda X$ signal, the purple dashed lines are the $\Lambda_{c}^{+}\bar{\Lambda}_{c}^{-}$ background, the brown dashed lines are the "hadron 1" background, the magenta dashed lines are the "hadron 2" background and the green dashed lines are the unmatched background.