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.
The semi-classical expansion and resurgence in gauge theories: new perturbative, instanton, bion, and renormalon effects
本文利用费米子周期边界条件下的圆紧化方法，研究了所有规范群伴随费米子的四维规范理论的微扰和非微扰动力学.有新的规范现象。我们表明，在微扰理论的所有订单，许多规范群Higgsed的规范完整性周围的圆的产品的阿贝尔和nonabel规范群因子。在非微扰情况下，有费米子零模的夸克瞬子和两种夸克反夸克分子，称为生物子。一种类型是“磁生物子”，它携带净磁荷，并引起规范波动的质量间隙。另一种类型是磁中性的“中性生物子”，对它们的理解需要将量子力学中的多瞬子技术--我们称之为博戈莫尼-津恩-贾斯汀（BZJ）处方--推广到紧致化场论。应用于生物-反生物拓扑分子的BZJ处方预测了Borel平面的正实轴上的奇点（即，在微扰理论中，大阶求和的发散），它比领先的4-d BPST瞬子-反瞬子奇点更接近原点N倍，其中N是规范群的秩。的bion-反bion奇异的位置，从而定性类似的4-D IR renormalon奇异，我们猜想，它们是连续相关的紧化半径的变化。利用跨级数和Ecalle的复活理论，我们证明了一类允许半经典展开的场论的非微扰连续统定义是可能的。
We investigate the resurgence structure in quantum mechanical models originating in 2d non-linear sigma models with emphasis on nearly supersymmetric and quasi-exactly solvable parameter regimes. By expanding the ground state energy in powers of a supersymmetry-breaking deformation parameter $δε$, we derive exact results for the expansion coefficients. In the class of models described by real multiplets, the ${\mathcal O}(δε)$ ground state energy has a non-Borel summable asymptotic series, which gives rise to imaginary ambiguities leading to rich resurgence structure. We discuss the sine-Gordo…
We investigate the implications of coupling a topological quantum field theory (TQFT) to Yang-Mills theory with $SU(N)$ gauge group in the context of the IR-renormalon problem. Coupling a TQFT to QFT does not change the local dynamics and perturbation theory, but it does change the bundle topology. Crucially, the configurations with integer topological charge but fractional action contribute to the path integral of the original theory. In the semi-classical regime, these are critical points at infinity, called neutral bions, and since ${\rm Arg}(g^2)=0$ is a Stokes line, their Lefschetz thimbl…
This paper considers the $1/ε$ problem, which is the divergent behavior of the ground state energy of asymmetric potential in quantum mechanics, which is calculated with semi-classical expansion and resurgence technique. Using resolvent method, It is shown that including not only one complex bion but multi-complex bion and multi-bounce contributions solves this problem. This result indicates the importance of summing all possible saddle points contribution and also the relationship between exact WKB and path integral formalism.
Resurgence and Trans-series in Quantum Field Theory: The CP(N-1) Model
这项工作是一个非微扰连续定义的量子场论（QFT）的一步，开始与渐近自由的二维非线性σ模型，使用最近的想法从数学和QFT。来自数学的想法是复苏理论，trans-series框架和Borel-Ecalle重申。QFT的思想利用了R^1 x S^1_L上的连续性，也就是说，当N \to无穷大时没有任何相变，或者有限N的快速交叉，以及小L弱耦合极限，使得半经典扇区是明确定义的和可计算的。我们分类的半经典配置与行动1/N（扭结瞬子），2/N（bions和bi-kinks），在单位的2d瞬子行动归一化为1。微扰理论具有IR-重正马龙歧义，这是由于高阶微扰级数（Gevrey-1型）的非Borel求和性而产生的，其中微观抵消机制是未知的。这种发散必然存在，因为相应的展开是在复杂的耦合常数平面上的奇异Stokes射线上，并且总和表现出穿过射线的Stokes现象。我们发现，也有一个非微扰的模糊性固有的某些中性拓扑分子（中性生物和生物反生物）的半经典扩展。我们找到一组“汇合方程”，编码的两种不同类型的模糊度的精确取消。我们表明，一个新的概念“分级复苏三角形”是必要的，以捕捉复苏的路径积分方法，分级复苏的基础是一个潜在的严格定义的一般QFT。用第一性原理计算了真空能的质量间隙和θ角依赖关系，与大N和晶格结果一致。
In Polyakov model, a non-perturbative mass gap is formed at leading order semi-classics by instanton effects. By using the notions of critical points at infinity, cluster expansion and Lefschetz thimbles, we show that a third order effect in semi-classics gives an imaginary ambiguous contribution to mass gap, which is supposed to be real and unambiguous. This is troublesome for the original analysis, and it is difficult to resolve this issue directly in QFT. However, we find a new compactification of Polyakov model to quantum mechanics, by using a background 't Hooft flux (or coupling to TQFT)…
It was conjectured that bions, semi-classical objects found in a compactified spacetime, are responsible for the cancellation of the so-called renormalon ambiguities. Contrary to the conjecture, we argue that the ambiguity due to the bion corresponds to the proliferation of Feynman diagrams. We point out that the amplitudes of almost all Feynman diagrams are enhanced due to modifications of the infrared structure of perturbation theory upon an $S^1$ compactification and twisted boundary conditions. Our findings clarify the role of the semi-classical object in resurgence structure, which has be…
We investigate non-BPS exact solutions in ${\mathbb C}P^{N-1}$ sigma models on ${\mathbb R}^1 \times S^{1}$ with twisted boundary conditions, by using the Din-Zakrzewski projection method. We focus on the relation of the non-BPS solutions to the ansatz of multi-instanton (bion) configurations and discuss their significance in the context of the resurgence theory. We find that the transition between seemingly distinct configurations of multi-instantons occur as moduli changes in the non-BPS solutions, and the simplest non-BPS exact solution corresponds to multi-bion configurations with fully-co…
It has been argued that many non-perturbative phenomena in quantum mechanics (QM) and quantum field theory (QFT) are determined by complex field configurations, and that these contributions should be understood in terms of of Picard-Lefschetz theory. In this work we compute the contribution from non-BPS multi-instanton configurations, such as instanton-anti-instanton $[{I}\bar{I}]$ pairs, and argue that these contributions should be interpreted as exact critical points at infinity. The Lefschetz thimbles associated with such critical points have a specific structure arising from the presence o…
