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arXiv 1902.09098hep-exhep-ph
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$e^+e^-\toΛ\barΛ$中$Λ$超子在BESIII的自旋极化

Spin polarization of $Λ$ hyperons in $e^+e^-\toΛ\barΛ$ at BESIII

Cui Li, Karin Schoenning

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1 Talk at BEACH 2018
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摘要：

BESIII合作报告了在两个不同过程中产生的极化$Λ$和$\barΛ$超子的首次观测：i）共振$e^+e^- \to J/ψ\toΛ\barΛ$，使用1.31 $\times$ 10 $^9$ $J/ψ$事件的数据样本; ii）非共振$e^+e^-\to γ^* \to Λ\barΛ$，使用在$\sqrt{s} =$ 2.396 GeV收集的66.9 pb $^{-1}$数据样本。在$e^+e^-\to J/ψ\toΛ\barΛ$中，首次测量电振幅和磁振幅之间的相位为$42.3^{\mathrm{o}} \pm 0.6^{\mathrm{o}} \pm 0.5^{\mathrm{o}}$。多维分析使得能够独立于模型测量$Λ\to pπ^-$（$α_-$）、$\barΛ\to\bar{p}π^+$（$α_+$）和$\barΛ\to\bar{n}π^0$（$\barα_0$）的衰变参数。所得值$α_-=0.750\pm0.009\pm0.004$与PDG值相差约5 PDMATHBNE。该值与测量$α_+=-0.758\pm0.010\pm0.007$一起允许迄今为止$Λ$衰变中CP破坏的最精确测试：$-0.006\pm0.012\pm0.007$的$A_{CP} = (α_- + α_+)/(α_- - α_+)$。首次测量了衰变不对称性$\barα_0 = -0.692\pm0.016\pm0.006$。在$\sqrt{s} =$ 2.396 GeV的$e^+e^- \to Λ\barΛ$反应，使第一次完整的测量的任何重子的类时的电和磁的形状因子的比例$R=|G_E/G_M|$和相对相位$ΔΦ=Φ_E-Φ_M$的模量。利用来自$J/ψ$数据的衰变不对称性参数，获得的值为$R=0.96\pm0.14\pm0.02$和$ΔΦ= 37^{\mathrm{o}} \pm 12^{\mathrm{o}} \pm 6^{\mathrm{o}}$。此外，横截面已经以前所未有的精度测量为$σ= 119.0\pm 5.3\pm5.1$ pb，其对应于$|G|=0.123 \pm 0.003 \pm 0.003$的有效形状因子。

英文摘要：

The BESIII collaboration here reports the first observation of polarized $Λ$ and $\barΛ$ hyperons produced in two different processes: i) the resonant $e^+e^- \to J/ψ\toΛ\barΛ$, using a data sample of 1.31 $\times$ 10$^9$ $J/ψ$ events and ii) the non-resonant $e^+e^-\to γ^* \to Λ\barΛ$, using a 66.9 pb$^{-1}$ data sample collected at $\sqrt{s} =$ 2.396 GeV. In $e^+e^-\to J/ψ\toΛ\barΛ$, the phase between the electric and the magnetic amplitude is measured for the first time to be $42.3^{\mathrm{o}} \pm 0.6^{\mathrm{o}} \pm 0.5^{\mathrm{o}}$. The multi-dimensional analysis enables a model-independent measurement of the decay parameters for $Λ\to pπ^-$ ($α_-$), $\barΛ\to\bar{p}π^+$ ($α_+$) and $\barΛ\to\bar{n}π^0$ ($\barα_0$). The obtained value $α_-=0.750\pm0.009\pm0.004$ differs with ~5$σ$ from the PDG value. This value, together with the measurement $α_+=-0.758\pm0.010\pm0.007$ allow for the most precise test of CP violation in $Λ$ decays so far: $A_{CP} = (α_- + α_+)/(α_- - α_+)$ of $-0.006\pm0.012\pm0.007$. The decay asymmetry $\barα_0 = -0.692\pm0.016\pm0.006$ is measured for the first time. The $e^+e^- \to Λ\barΛ$ reaction at $\sqrt{s} =$ 2.396 GeV enables a first complete measurement of the time-like electric and magnetic form factor of any baryon, of the modulus of the ratio $R=|G_E/G_M|$ and of the relative phase $ΔΦ=Φ_E-Φ_M$. With the decay asymmetry parameters from the $J/ψ$ data, the obtained values are $R=0.96\pm0.14\pm0.02$ and $ΔΦ= 37^{\mathrm{o}} \pm 12^{\mathrm{o}} \pm 6^{\mathrm{o}}$. In addition, the cross section has been measured with unprecedented precision to be $σ= 119.0\pm 5.3\pm5.1$ pb, which corresponds to an effective form factor of $|G|=0.123 \pm 0.003 \pm 0.003$.

机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。

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