The quantum electrodynamic (QED) theory predicts the photon emission and pair creation involved in QED cascades occur mainly in a forward cone with finite angular spread Δθ∼1/γi along the momenta of incoming particles. This finite beaming effect has been assumed to be negligible because of the particles' ultra-relativistic Lorentz factor γi≫1 in laser-driven QED cascades. We develop an energy- and angularly resolved particle-tracking code, resolving both the energy spectra and the momentum profile of the outgoing particles in each QED event, which improves substantially the agreement between the simulation and exact QED results. We investigate QED cascades driven by two counter-propagating circularly polarized laser pulses, and show that the narrow beaming could be accumulated to effectively suppress the long-term growth of cascades, even though it can hardly affect the early formation of cascades. For QED cascades longer than 10 laser cycles, the finite beaming effect could decrease the final pair yield, especially at ultrahigh intensities ξ>600, by more than 10%.

中文翻译：

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对 QED 级联的有限光束效应


量子电动 （QED） 理论预测 QED 级联中涉及的光子发射和对的产生主要发生在沿入射粒子动量的有限角扩散 Δθ∼1/γi 的正锥中。由于激光驱动的 QED 级联中粒子的超相对论洛伦兹因子 γi≫1，这种有限光束效应被认为是可以忽略不计的。我们开发了一种能量和角度分辨的粒子跟踪代码，可以解析每个 QED 事件中出射粒子的能谱和动量分布，这大大提高了仿真和精确 QED 结果之间的一致性。我们研究了由两个反向传播圆偏振激光脉冲驱动的 QED 级联，并表明窄光束可以累积以有效抑制级联的长期生长，即使它几乎不会影响级联的早期形成。对于超过 10 个激光周期的 QED 级联，有限光束效应可能会使最终对产量降低 10% 以上，尤其是在超高强度 ξ>600 下。