arXiv 1907.011512019-07hep-phhep-ex

外磁场诱导超子与反超子的极化差异

Polarization difference between hyperons and anti-hyperons induced by external magnetic field

Hai-Bo Li, Xin-Xin Ma

发表信息：: Phys. Rev. D 100, 076007 (2019)
DOI：: 10.1103/PhysRevD.100.076007

摘要：

我们研究了在反应$e^{+}e^{-} \to J/ψ\to Λ\barΛ$中量子相关的$Λ\barΛ $生成。由于$Λ$或$\barΛ$具有非零磁矩，其自旋将在探测器的磁场中经历拉莫尔进动，例如BESIII实验。由于自旋进动，$Λ$和$\barΛ$的角分布略有改变。因此，我们得到了由于拉莫尔进动的影响而修正的角分布的相应项。我们还估计了它对$CP$破坏测量的潜在影响，以及$Λ$的衰变不对称参数和极化。$Λ$或$\barΛ$在生产顶点的偏振将围绕$B$场轴旋转一个角度，该角度取决于飞行长度in，但它仍然可以通过拟合校正的角分布来测量。重要的是，我们得出结论：在$e^{+}e^{-} \to J/ψ\to Λ\barΛ$过程中，如果忽略$Λ$和$\barΛ$的自旋旋进，将导致$10^{-4}$级的非零$CP$不对称性.这种$CP$不对称性的大小是标准模型在超子衰变中预测的几倍。虽然这种效应很小，但它将在未来的高精度实验中发挥重要作用，例如超级tau魅力工厂。

英文摘要：

We investigate the quantum correlated $Λ\barΛ $ production in the reaction $e^{+}e^{-} \to J/ψ\to Λ\barΛ$. Since the $Λ$ or $\barΛ$ has a nonzero magnetic moment, its spin will undergo a Larmor precession in the magnetic field of the detector, such as the BESIII experiment. Because of the spin precession, the angular distribution of the $Λ$ and $\barΛ$ is slightly modified. Therefore, we obtain the corresponding term of the modified angular distribution due to the effect of the Larmor precession. We also estimate its potential effect on the measurements of $CP$ violation, as well as the decay asymmetry parameter and polarization of $Λ$. The polarization of the $Λ$ or $\barΛ$ at the production vertex will rotate around the $B$-field axis, over an angle depending on the flight length in, but it still could be measured by fit to the corrected angular distribution. Of important, We conclude that a nonzero $CP$ asymmetry of order $10^{-4}$ will be caused once neglecting spin precession of the $Λ$ and $\barΛ$ in the $e^{+}e^{-} \to J/ψ\to Λ\barΛ$ process. The size of this $CP$ asymmetry is several times that of predicted within Standard Model in the hyperon decay. Although this effect is small, it will play an important role in future high precision experiments, such as the super-tau-charm factory.

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英文摘要：

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