The LPM effect in sequential bremsstrahlung: nearly complete results for QCD
Peter Arnold, Tyler Gorda, Shahin Iqbal
备注：
82 pages total; 24 figures. Change from v1: An erratum to eq. (A37) for the pole piece of sequential diagrams is now provided in a "Note added" section at the end of the main text
The splitting processes of bremsstrahlung and pair production in a medium are coherent over large distances in the very high energy limit, which leads to a suppression known as the Landau-Pomeranchuk-Migdal (LPM) effect. We continue study of the case when the coherence lengths of two consecutive splitting processes overlap (which is important for understanding corrections to standard treatments of the LPM effect in QCD), avoiding soft-emission approximations. Previous work has computed overlap effects for double splitting $g \to gg \to ggg$. To make use of those results, one also needs calculations of related virtual loop corrections to single splitting $g \to gg$ in order to cancel severe (power-law) infrared (IR) divergences. This paper provides calculations of nearly all such processes involving gluons and discusses how to organize the results to demonstrate the cancellation. In the soft emission limit, our results reproduce the known double-log behavior of earlier authors who worked in leading-log approximation. We also present a first (albeit numerical and not yet analytic) investigation of sub-leading, single IR logarithms. Ultraviolet divergences appearing in our calculations correctly renormalize the coupling $α_{\rm s}$ in the usual LPM result for leading-order $g \to gg$.
Virtual photon bremsstrahlung in a magnetized quark-gluon-plasma
Institute of Theoretical Physics, University of Wrocław（弗罗茨瓦夫大学理论物理研究所）；Polish Academy of Sciences PAN（波兰科学院）；International Institute for Sustainability with Knotted Chiral Meta Matter (WPI-SKCM 2 ), Hiroshima University（广岛大学缠绕手性超物质可持续国际研究所）
University of Virginia（弗吉尼亚大学）；South China Normal University（华南师范大学）；École polytechnique（巴黎综合理工学院）；Institut Polytechnique de Paris（巴黎理工学院）；National Centre for Physics（国家物理中心）；CERN（欧洲核子研究中心）
Shear and Bulk Viscosities of a Gluon Plasma in Perturbative QCD: Comparison of Different Treatments for the gg<->ggg Process
在微扰QCD中，胶子等离子体的剪切和体粘滞系数η和η的首阶贡献包括gg -> gg（22）过程、gg <->ggg（23）过程和称为Landau-Pomeranchuk-Migdal（LPM）效应的多重散射过程。Arnold，Moore and Yaffe（AMY）和Arnold，Dogan and Moore（ADM）分别对η和η进行了完整的首阶计算，其中非弹性过程由有效的<->ggg胶子分裂计算。我们研究了如何互补计算与22和23过程和一个简单的处理来模拟LPM效应比较AMY和ADM的结果，我们发现，我们的结果与他们的误差范围内同意。通过研究23过程对η的贡献，我们发现在流体局域静止系中末态胶子间的最小夹角θ在θ \sim \sqrt{α_{s}}处有一个峰值分布，类似于AMY和ADM所断言的近共线分裂，但θ的平均值远大于其峰值，因为它的分布是倾斜的，有一个长尾.在质心系中，如果把23矩阵元取到软胶子韧致辐射极限，也可以看到同样的θ行为。这表明CM框架中的软胶子韧致辐射在流体局域静止框架中仍然具有某些近似共线的行为。我们还将结果推广到一般的SU（N_c）纯规范理论，并总结了目前QCD中粘滞系数的计算。
