QFT和QCD中的新方法：从大N轨道等价到Bions和复苏
New Methods in QFT and QCD: From Large-N Orbifold Equivalence to Bions and Resurgence
摘要：
我们提出了一个广泛的概念介绍非微扰QFT的一些新的想法。大- $N$ orbifold-orientifold等价将QCD的自然大- $N$极限与带有伴随费米子的QCD联系起来。具有周期边界条件和Yang-Mills双迹形变的QCD（adj）满足大- $N$体积无关性，这是一种轨道等价.在$N=\infty$处满足体积独立性的某些QFT在有限$N$处表现出绝热连续性，并且还变得在小$\mathbb R^3 \times S^1$上半经典可计算。我们讨论的作用，自旋瞬子，磁和中性bion鞍连接到质量间隙，中心和手征对称性实现。中性的bions也提供了一个弱耦合的红外重正马龙的半经典实现。这些考虑有助于激发半经典分析中路径积分（Picard-Lefschetz理论）的复化的必要性，并突出了隐藏拓扑角的重要性。最后，我们简要回顾了复活计划，这可能提供了一个新的非微扰连续定义QFT。所有这些想法都是不断连接的。
英文摘要：
We present a broad conceptual introduction to some new ideas in non-perturbative QFT. The large-$N$ orbifold-orientifold equivalence connects a natural large-$N$ limit of QCD to QCD with adjoint fermions. QCD(adj) with periodic boundary conditions and double-trace deformation of Yang-Mills theory satisfy large-$N$ volume independence, a type of orbifold equivalence. Certain QFTs that satisfy volume independence at $N=\infty$ exhibit adiabatic continuity at finite-$N$, and also become semi-classically calculable on small $\mathbb R^3 \times S^1$. We discuss the role of monopole-instantons, and magnetic and neutral bion saddles in connection to mass gap, and center and chiral symmetry realizations. Neutral bions also provide a weak coupling semiclassical realization of infrared-renormalons. These considerations help motivate the necessity of complexification of path integrals (Picard-Lefschetz theory) in semi-classical analysis, and highlights the importance of hidden topological angles. Finally, we briefly review the resurgence program, which potentially provides a novel non-perturbative continuum definition of QFT. All these ideas are continuously connected.
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