Study on manipulation mechanism of polarized positrons in nonlinear Breit-Wheeler scattering process
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摘要: 极化正电子束是前沿科学研究所需的重要探针。利用强激光场中非线性Breit-Wheeler散射产生极化正电子是近年来备受关注的新方案。探究该过程中激光与伽马光子参数对正电子最终极化状态的调控机理。在强场量子电动力学理论框架下，完整保留了所有粒子的自旋极化自由度，并精确纳入了平面波激光场的有限脉冲包络结构。计算表明：当激光与高能伽马光子皆为线偏振时，产生的正电子极化度为零；当驱动激光和伽马光子有一个为圆偏振时，正电子极化由圆偏振光主导，并随激光强度增强或伽马光子能量增加而下降，线偏振光的影响很小；当激光与伽马光子均为圆偏振时，高能正电子的极化由伽马光子主导，而低能正电子的极化则由二者共同决定，且激光强度的调控作用尤为显著。揭示了激光强度和伽马光子能量等关键因素对极化正电子产生的影响机理，为未来利用强激光与高能伽马光对撞产生高品质极化正电子束的实验方案提供了关键理论依据。
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关键词:
- 极化正电子 /
- 非线性Breit-Wheeler过程 /
- 强场量子电动力学 /
- 强激光 /
- 正负电子对产生
Abstract:Background Polarized positron beams are vital probes in fundamental physics. Generating them via the nonlinear Breit-Wheeler process in laser fields is a promising new approach, but control over the positron polarization requires further understanding.Purpose This study investigates how laser and γ-photon parameters control the final polarization of positrons in this process.Methods Within strong-field QED, we fully include all particle spins and the laser pulse's finite envelope. Systematic calculations are performed across various laser intensities, γ-photon energies, and polarization configurations.Results Key findings are: (1) No positron polarization arises with linearly polarized lasers and γ-photons. (2) When only one is circularly polarized, it dominates the positron polarization, which decreases with higher laser intensity or γ-photon energy. (3) With both circularly polarized, γ-photons dominate high-energy positron polarization, while both sources co-determine low-energy positron polarization, with laser intensity playing a stronger regulatory role.Conclusions These results clarify the dominant factors for positron polarization, providing a key theoretical basis for designing optimized laser-driven polarized positron sources. -
图 1 在线偏振激光脉冲（对应不同强度）中，通过非线性Breit-Wheeler过程产生的电子能谱
Figure 1.
$ Energy spectrum of generated electrons via NBW process in LP laser pulses of different intensities
图 2 线偏振激光与线偏振伽马光子散射产生的正电子的极化度随正电子-伽马光子能量比的变化关系
$ Figure 2. Polarization degree of positrons generated from the scattering of LP laser and LP γ-photon, versus the ratio of positron energy to γ-photon energy
图 3 线偏振激光与线偏振伽马光子散射产生的正电子的角度分布
Figure 3. The angular distribution (
$ {\theta }_{x} $ and$ {\theta }_{y} $ ) of positron yield for LP laser and LP γ-photon
图 4 正电子在不同激光强度下极化度随正电子-伽马光子能量比的变化关系
Figure 4. Polarization degree of positrons under different laser intensities, versus the ratio of positron energy to γ-photon energy
图 5 正电子在不同伽马光子能量下极化度随正电子-伽马光子能量比的变化关系
Figure 5. Polarization degree of positrons under different γ-photon energies, versus the ratio of positron energy to γ-photon energy
图 6 圆偏振激光与线偏振伽马光子产生的正电子的角度分布
Figure 6. The angular distribution (
$ {\theta }_{x} $ and$ {\theta }_{y} $ ) of positron yield for CP laser and LP γ-photon
图 7 线偏振激光、圆偏振激光及圆偏振伽马光子散射产生的正电子的极化度随正电子-伽马光子能量比的变化关系
Figure 7. Polarization degree of positrons generated from the scattering of LP laser, CP laser, and CP γ-photon, versus the ratio of positron energy to γ-photon energy
图 8 线偏振伽马光子、圆偏振伽马光子及圆偏振激光散射产生的正电子的极化度随正电子-伽马光子能量比的变化关系
Figure 8. Polarization degree of positrons generated from the scattering of LP γ-photon, CP γ-photon, and CP laser, versus the ratio of positron energy to γ-photon energy
图 9 圆偏振伽马光子与圆偏振激光散射产生的正电子的极化度随正电子-伽马光子能量比的变化关系
Figure 9. Polarization degree of positrons generated form the scattering of CP laser and CP γ-photon, versus the ratio of positron energy to γ-photon energy
图 10 圆偏振伽马光子与圆偏振激光散射产生的正电子的极化度随正电子-伽马光子能量比的变化关系
Figure 10. Polarization degree of positrons generated form the scattering of CP laser and CP γ-photon, versus the ratio of positron energy to γ-photon energy
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