Michael C. Birse
This lecture provides an introduction to the renormalisation group as applied to scattering of two nonrelativistic particles. As well as forming a framework for constructing effective theories of few-nucleon systems, these ideas also provide a simple example which illustrates general features of the renormalisation group.
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An introductory review, based on a series of lectures delivered at the XXXI SERC School, Kalyani University, 9--18 January 2017.
RENORMALISATION GROUP AIDED FINITE TEMPERATURE REDUCTION OF QUANTUM FIELD THEORIES
有限温度量子场论的降维可以借助于连续重整化群步骤得到改进。该方法适用于集成的最低非静态（$n=\pm 1$）模式的有限温度$Φ^4$理论。第二个重要的物理应用是德拜屏蔽的${\bf A_0}(x)$静态标量势在规范SU（2）Higgs模型中的积分。
In recent years three-, four- and five-loop beta functions have been computed for various phenomenologically interesting models. However, most of these results have not been implemented in easy to use software packages. $\texttt{RGE++}$ bridges this gap by providing a flexible, template-based, $\texttt{C++}$ library to solve renormalisation group equations. Furthermore, we implement the available beta functions for the Standard Model, the minimal supersymmetric extension of the Standard Model and two-Higgs-doublet models, as well as right-handed neutrino extensions of the former two.
We consider the initial value problem and its renormalisation in the framework of the two-particle-irreducible (2PI) effective action. We argue that in the case of appropriately chosen self-consistent initial conditions, the counterterms needed to renormalise the system in equilibrium are also sufficient to renormalise its time evolution. In this way we improve on Gaussian initial conditions which have the disadvantage of generically not showing a continuum limit. For a more detailed discussion see arXiv:0809.0496.
There exist renormalisation schemes that explicitly preserve the scale invariance of a theory at the quantum level. Imposing a scale invariant renormalisation breaks renormalisability and induces new non-trivial operators in the theory. In this work, we study the effects of such scale invariant renormalisation procedures. On the one hand, an explicitly quantum scale invariant theory can emerge from the scale invariant renormalisation of a scale invariant Lagrangian. On the other hand, we show how a quantum scale invariant theory can equally emerge from a Lagrangian visibly breaking scale invar…
The $θ$-vacuum from functional renormalisation
本文通过功能重整化群方法研究具有U(1)对称性的量子系统，探讨真空能量结构和拓扑Susceptibility，并利用复化流方程计算拓扑扇区的有效势。
We review the asymptotic safety scenario for quantum gravity and the role and implications of an underlying ultraviolet fixed point. We discuss renormalisation group techniques employed in the fixed point search, analyse the main picture at the example of the Einstein-Hilbert theory, and provide an overview of the key results in four and higher dimensions. We also compare findings with recent lattice simulations and evaluate phenomenological implications for collider experiments.
These lecture notes introduce exact Wilsonian renormalisation, and describe its technical approach, from an intuitive implementation to more advanced realisations. The methods and concepts are explained with a scalar theory, and their extension to quantum gravity is discussed as an application.
Dynamic equations for quantum fields far from equilibrium are derived by use of functional renormalisation group techniques. The obtained equations are non-perturbative and lead substantially beyond mean-field and quantum Boltzmann type approximations. The approach is based on a regularised version of the generating functional for correlation functions where times greater than a chosen cutoff time are suppressed. As a central result, a time evolution equation for the non-equilibrium effective action is derived, and the time-evolution of the Green functions is computed within a vertex expansion…
Worldline holography states that within the framework of the worldline approach to quantum field theory, sources of a quantum field theory over Mink$_4$ naturally form a field theory over AdS$_5$ {\sl to all orders} in the elementary fields and in the sources of arbitrary spin. (Such correspondences are also available for other pairs of spacetimes, not only Mink$_4\leftrightarrow\mathrm{AdS}_5$.) Schwinger's proper time of the worldline formalism is automatically grouped with the physical four spacetime dimensions into an AdS$_5$ geometry. We show that the worldline holographic effective actio…
Renormalisation Group Equations for BRST-Restored Chiral Theory in Dimensional Renormalisation: Application to Two-Loop Chiral-QED
本文在BMHV方案下利用代数重整化框架构建BRST恢复手征理论的重整化群方程，并以无质量右手QED为例与乘法重整化方法对比，计算了两圈β函数和反常维度。
We study a $ϕ^4$-theory at finite temperature in a finite volume. Quantum, thermal and volume fluctuations are treated with the functional renormalisation group. Specifically, we focus on the interplay of temperature and length scales driving the system. We find that thermodynamical observables at finite volume such as the pressure approach the infinite volume limit similarly to that of the vanishing temperature limit. We also advance the functional renormalisation group method at finite volume. In particular, we identify requirements for suitable regulators that admit the exponential thermal …
We compute the two-loop $β$-function of scalar and spinorial quantum electrodynamics as well as pure Yang-Mills and quantum chromodynamics using the background field method in a fully quadridimensional setup using Implicit Regularization (IREG). Moreover, a thorough comparison with dimensional approaches such as conventional dimensional regularization (CDR) and dimensional reduction (DRED) is presented. Subtleties related to Lorentz algebra contractions/symmetric integrations inside divergent integrals as well as renormalisation schemes are carefully discussed within IREG where the renormalisa…
We present a new method for renormalisation group improvement of the effective potential of a quantum field theory with an arbitrary number of scalar fields. The method amounts to solving the renormalisation group equation for the effective potential with the boundary conditions chosen on the hypersurface where quantum corrections vanish. This hypersurface is defined through a suitable choice of a field-dependent value for the renormalisation scale. The method can be applied to any order in perturbation theory and it is a generalisation of the standard procedure valid for the one-field case. I…
We present the initial release of ARGES, a toolkit for obtaining renormalisation group equations in perturbation theory. As such, ARGES can handle any perturbatively renormalisable four-dimensional quantum field theory. Notable further features include a symbolic rather than numeric computation, input of unconventional scalar and Yukawa sectors, an interactive evaluation and disentanglement as well as capabilities to inject algebraic simplification rules. We provide a conceptual and practical introduction into ARGES, and highlight similarities and differences with complementary packages.
We study the non-equilibrium dynamics of a system of coupled scalar fields in a Friedmann-Robertson-Walker (FRW) universe. We consider the evolution of spatially homogeneous "classical" fields and of their quantum fluctuations including the quantum backreaction in the one-loop approximation. We discuss in particular the dimensional regularisation of the coupled system and a special subtraction procedure in order to obtain the renormalised equations of motion and the renormalised energy-momentum tensor and ensure that the energy is well-defined and covariantly conserved. These results represent…
Exact Renormalisation Group Evolution of the Inflation Dynamics: Reconciling $α$-Attractors with ACT
本文提出基于泛函重整化群的非微扰框架，推导Friedmann-RG耦合流方程，发现量子修正使α吸引子暴胀轨迹失稳并提前结束慢滚，从而与最新ACT数据完全吻合。
We study the optimisation of exact renormalisation group (ERG) flows. We explain why the convergence of approximate solutions towards the physical theory is optimised by appropriate choices of the regularisation. We consider specific optimised regulators for bosonic and fermionic fields and compare the optimised ERG flows with generic ones. This is done up to second order in the derivative expansion at both vanishing and non-vanishing temperature. We find that optimised flows at finite temperature factorise. This corresponds to the disentangling of thermal and quantum fluctuations. A similar f…
Fermions and the Renormalisation Group at Large N
本文在大N极限下用泛函重整化群研究费米子理论，证明其有效作用量具有准定域精确解，并给出U(N)对称理论的定域势流方程，确定了各类相互作用下的共形不动点与标度维度。
Four Loop Renormalisation Group Equations in general Gross-Neveu-Yukawa Theories
本研究计算了含费米子与标量场的任意可重整化量子场论在$\u0001overline{\text{MS}}$方案下的四圈$\beta$函数与反常维数模板公式，提取了希格斯与顶夸克耦合的新贡献，解决了四圈维数延拓的相关挑战。
Renormalised spectral flows
本文提出一种洛伦兹不变的谱泛函重整化群方法，通过流动重整化条件推导有限泛函流方程，使非微扰截断和渐近安全理论也能进行谱处理。
We study the phase diagram of two-color Quantum Chromodynamics at finite temperature and chemical potential. This is done within an effective low-energy description in terms of quarks, mesons and diquarks. Quantum, thermal and density fluctuations are taken into account with the functional renormalisation group approach. In particular, we establish the phenomenon of pre-condensation, affecting the location of the phase boundary to Bose-Einstein condensation. We also discuss the Silver Blaze property in the context of the functional renormalisation group.
Self-consistent graviton spectral function in Lorentzian quantum gravity
本研究在物理质壳重整化方案下，利用引力谱重整化群方法首次完成量子引力中引力子谱函数的完全自洽计算，得到带无质量单引力子峰的正谱函数，验证其满足渐近态求和规则，为散射过程和渐近安全引力的幺正性研究提供支撑。
We discuss structural aspects of the functional renormalisation group. Flows for a general class of correlation functions are derived, and it is shown how symmetry relations of the underlying theory are lifted to the regularised theory. A simple equation for the flow of these relations is provided. The setting includes general flows in the presence of composite operators and their relation to standard flows, an important example being NPI quantities. We discuss optimisation and derive a functional optimisation criterion. Applications deal with the interrelation between functional flows and the…
When solving renormalisation group equations in a quantum field theory, one often specifies the boundary conditions at multiple renormalisation scales, such as the weak and grand-unified scales in a theory beyond the standard model. A point in the parameter space of such a model is usually specified by the values of couplings at these boundaries of the renormalisation group flow, but there is no theorem guaranteeing that such a point has a unique solution to the associated differential equations, and so there may exist multiple, phenomenologically distinct solutions, all corresponding to the s…
We study quantum gravity in more than four dimensions with renormalisation group methods. We find a non-trivial ultraviolet fixed point in the Einstein-Hilbert action. The fixed point connects with the perturbative infrared domain through finite renormalisation group trajectories. We show that our results for fixed points and related scaling exponents are stable. If this picture persists at higher order, quantum gravity in the metric field is asymptotically safe. We discuss signatures of the gravitational fixed point in models with low-scale gravity and compact extra dimensions.
We develop a system of equations for the propagators and three point functions of the $ϕ^3$ quantum field theory in six dimensions. Inspired from a refinement by Ward on the Schwinger--Dyson equations, the main characteristics of this system are to be formulated purely in terms of renormalized quantities and to give solutions satisfying renormalisation group equations. These properties were difficult to get together, due to the overlapping divergences in the propagator. The renormalisation group equations are an integral part of any efficient resolution scheme of this system and will be instru…
Euclidean quantum gravity is studied with renormalisation group methods. Analytical results for a non-trivial ultraviolet fixed point are found for arbitrary dimensions and gauge fixing parameter in the Einstein-Hilbert truncation. Implications for quantum gravity in four dimensions are discussed.
Physics-informed gauge theories
本文提出物理信息重整化群方法，构建规范不变的流方程，应用于杨-米尔斯理论和引力，恢复单圈普适性，并探讨了fRG流景观与潜在应用。
Renormalising the Field-Space Geometry
本文从几何视角研究标量有效场论的单圈修正，借助场重定义不变性与几何–运动学对偶，揭示 NLSM 和 ϕ^4 理论中场空间曲率跑动的普适结构。
We discuss the renormalisation of the initial value problem in quantum field theory using the two-particle irreducible (2PI) effective action formalism. The nonequilibrium dynamics is renormalised by counterterms determined in equilibrium. We emphasize the importance of the appropriate choice of initial conditions and go beyond the Gaussian initial density operator by defining self-consistent initial conditions. We study the corresponding time evolution and present a numerical example which supports the existence of a continuum limit for this type of initial conditions.
Radiative symmetry breaking (RSB) is a theoretically appealing framework for the generation of mass scales through quantum effects. It can be successfully implemented in models with extended scalar and gauge sectors. We provide a systematic analysis of RSB in such models: we review the common approximative methods of studying RSB, emphasising their limits of applicability and discuss the relevance of the relative magnitudes of tree-level and loop contributions as well as the dependence of the results on the renormalisation scale. The general considerations are exemplified within the context of…
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