# Physics Notes by Jakob Schwichtenberg

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quantum_field_theory:methods:non_perturbative_qft

# Non Perturbative Quantum Field Theory

Instead of using the usual perturbation expansion one can try to find solutions to the classical field equations and use these as starting points for calculations.

However, in contrast to electromagnetism, the classical field equations of general Yang-Mills theory are highly complicated. While the Maxwell equations are linear and can be easily solved (the solutions describe electromagnetic waves), the analogous Yang-Mills equations are non-linear and complicated to solve.

This complicated structure gives rise to many interesting phenomena that can not be described by a perturbative approach. The most important example is the structure of the QCD vacuum.

“Perturbative physics is determined by the Lie algebra of the gauge group, but nonperturbative physics is determined by the Lie group, not just the algebra.” https://arxiv.org/pdf/1012.5999.pdf

Non-perturbative methods are also approximation methods, but a different kind of approximation. Instead small perturbations, described by an expansion in the gauge coupling, one considers semi-classical approximations. Non-perturbative methods are important to describe scenarios where the usual perturbation theory is not applicable. A good example are tunneling processes. For example, QCD vacuum posses an infinite number of ground states with equal energy. With perturbation theory we can only describe small perturbations around such a minimum. However in a quantum theory, there can be tunneling processes between minimums of a potential. Such processes are called instantons. A perturbative approach would never notice anything about the other minimas and is therefore not able to describe the true ground state, which is a superpositon of all “classical” ground states, with tunneling processes connecting them. At first it is confusing why such processes are described by a semi-classical approximation, because there is no tunneling in classical physics. However, the semi-classical approach is merely a trick to identify the dominant contributions to the path integral. We focus on these dominant contributions, because we can not compute the path integral exactly. We call the saddle points of the action classical paths and these dominante the sum over all possible paths. Another trick to make this idea work, is to redefine the time as $i t$, i.e. make the time complex. This is necessary, because there is no tunneling in classical physics. By making the time imaginary, in some sense, we flip the potential upside down, and thus classical paths become possible. Another way to see why imaginary times are useful to describe tunneling processes, is to remember the usual quantum mechanical situation of tunneling through a potential barrier. Outside of the barrier, the wave function oscillated $\mathrm{e} {i\omega t}$. However, in the potential the wave function is damped exponentially:$\mathrm{e} {- \omega t}$. The connection between a “usual” wave function, and a tunneling wave function is therefore $t \to i t$. These points are described in the following sections in more detail.

## Why are non-perturbative methods important?

Calculation methods beyond the usual perturbation expansion are especially important in QCD, because the coupling becomes large at large distances and therefore the expansion is useless.

Since QCD is strongly coupled and therefore non-perturbative at low energies, we expect the vacuum to be populated by strong fields. (This is in contrast to standard QED, for example, where the vacuum contains mostly zero-point fluctuations, i.e. weakly interacting electron-positron pairs and photons which can be handled perturbatively.) […] Strong fields can contain a very large number of quanta, and those quanta can become coherent and render the corresponding action large compared to ℏ. In other words, such fields behave (semi-) classically since quantum fluctuations are of O (ℏ) and thus contribute only relatively small corrections. The above reasoning suggests that insight into the vacuum fields of QCD may be gained from a semiclassical perspective.

## Why do we use a semi-classical approximation?

Classical solutions dominate the path integral. (Source: page 416 in Topological Solitons by Manton, Sutcliff)

The corresponding quantum field theory (including the quarks) determines the structure of the QCD vacuum, i.e. the unique state of lowest energy on which the Fock space is built. Since QCD is strongly coupled and therefore non-perturbative at low energies, we expect the vacuum to be populated by strong fields. (This is in contrast to standard QED, for example, where the vacuum contains mostly zero-point fluctuations, i.e. weakly interacting electron-positron pairs and photons which can be handled perturbatively.) […] Strong fields can contain a very large number of quanta, and those quanta can become coherent and render the corresponding action large compared to ℏ. In other words, such fields behave (semi-) classically since quantum fluctuations are of O (ℏ) and thus contribute only relatively small corrections. The above reasoning suggests that insight into the vacuum fields of QCD may be gained from a semiclassical perspective.

Usually in QFT we use perturbation theory. However, there are phenomena that can not be described by this method. A good example is the double well potential $V(q)=\frac{\lambda}{4!}(q^2-a^2)^2$:

There are two classical ground states. However, in the usual perturbation approach, we ignore this fact and expand $V$ around one of these minimas. The potential then looks exactly as the potential of the harmonic oscillator about that minimum plus anharmonic terms (cubic, quartic). The perturbation approach then yields possible wave functions and energies. However, we never see any evidence of the second minimum. Of course, it makes no difference which minimum we choose. Equally, we could expand around the other minimum. The energy levels are exactly the same in all orders of perturbation theory. However through perturbative effects there is only one true ground state and not two degenerate ground states which is suggested by perturbation theory. This shows that perturbation theory fails to describe all possible phenomena. (Source: http://www.weizmann.ac.il/particle/perez/Courses/QMII16/TA4.pdf)

The approximation method that is used in such situations is called semi-classical approximation or the method of steepest descend. See page 2 in http://www.weizmann.ac.il/particle/perez/Courses/QMII16/TA4.pdf for an illustration of the idea behind this method.

As far as I know there is currently no better approach to deal with such effects. For example, QCD can not be solved exactly and therefore, we need to use approximation methods. Perturbation theory does not work and thus we need another approximation method. At the moment, non-perturbative effects are described using the semi-classical approximation. However this method also has limitations.

[Instanton physics] is both exciting and frustrating. It is exciting because, at last, a substantial body of evidence shows that the original vision of the role of instantons in QCD is borne out in nature. However, as will be clear below, it is also frustrating because of intrinsic limits to the precision with which one can apply ultimately semiclassical concepts.

Instantons, the QCD Vacuum, and Hadronic Physics by J. W. Negele

## Why imaginary time?

Tunneling processes can not be described with perturbation theory. Instead one employs a semi-classical approach. This works, because we can describe a tunneling trajectory as a classical trajectory in imaginary time. This can be seen by considering the following example:

Consider a particle with energy $E$ sitting in the minimum of a double well potential, for example, at B in the following picture:

The classical energy momentum relation is: $$E=\frac{m}{2}\dot{x}^{2}+V \quad \Rightarrow \quad \dot{x}=\sqrt{\frac{2(E-V)}{m}}$$

We can the use separation of variables for $\dot{x} = \frac{dx}{d\tau}$:

$$\sqrt{\frac{m}{2}}\frac{dx}{\sqrt{(E-V)\left( x\right) }}=d\tau,$$ and then integrate the equation to find the trajectory

\begin{align} \int_{B}^{F}\sqrt{\frac{m}{2}}\frac{dx}{\sqrt{(E-V)\left( x\right) }}&= \int_{\tau_1}^{\tau_2}d\tau \notag \\ \int_{B}^{F}\sqrt{\frac{m}{2}}\frac{dx}{\sqrt{(E-V)\left( x\right) }}&= \tau_1 - \tau_2 . \end{align}

Classically there is no solution if $V>E$, because then the time becomes complex through the negative root above. However, we know that in Quantum Mechanics tunneling through such a potential barrier is possible. Therefore, we see here that formally such a trajectory through a forbidden region corresponds to a classical trajectory in imaginary time.

Another point of view is that if we work with $t \rightarrow i \tau$, we flip the potential upside down: $V \rightarrow -V$.

Since no classical trajectory can be associated to barrier penetration, one may wonder how it is possible to evaluate such effects in the semi-classical limit. Actually, it has been noticed that, formally, barrier penetration has a semi-classical interpretation in terms of classical particles moving in imaginary time. […] To calculate instanton contributions at leading order, one must master two problems that are increasingly difficult: find the saddle points by solving classical equations, expand the integrand around the saddle point and evaluate the path integral at leading order by integrating over Gaussian fluctuations.

The seemingly artificial analytical continuation to imaginary times has allowed us to identify those paths whose neighborhoods give the dominant contributions to the path integral for a tunneling process in the semiclassical limit, and to evaluate this path integral to O (~) in the saddle-point approximation. In more physical terms the situation can be described as follows: for tunneling problems there exist no minimal-action trajectories (i.e. classical solutions) with the appropriate boundary conditions in real time. Therefore all trajectories between those boundary conditions (over which we sum in the real-time path integral) interfere highly destructively. Still, their net effect can be approximately gathered in a finite number of regions in function space, namely those in the neighborhood of the saddle points in imaginary time. In other words, while the tunneling amplitudes would have to be recovered at real times from a complex mixture of non-stationary paths (a forbidding task in practice), they are concentrated around the classical paths in imaginary time, and are therefore accessible to the saddlepoint approximation. The destructive interference at real times leaves a conspicuous trace, however, namely the exponential suppression of (2.40) due to the Gamov factor exp (−SE/h), which is typical for tunneling amplitudes.

To bring the tunneling interpretation into clearer view, it is helpful to pass to the imaginary time picture, i.e. to discuss $<n'|e^{-HT}|n>$ instead of $<n'|e^{-HTi}|n>$, The reason for this is that we know from experience with ordinary quantum mechanics that imaginary time solutions of the classical eguations of motion can be used to obtain a WKB (or small H) treatment of barrier penetration problems. In real time a classically forbidden process defines no stationary path which dominates the functional integral and there is no simple way to study tunneling. […] Since the action is positive definite, the dominant history is the one of minimum action consistent with the boundary conditions. Such a path satisfies all the Euclidean Yang-Mills equations - except the Gauss' law constraint. Upon varying the end points of the Ai path history, one will finally pick out the path which satisfies the constraint as well. This path is guaranteed to have the absolute minimum action consistent with the constraint that it describe a transition n ~ n' and satisfies the full set of Euclidean Yang-Hills equations. A rather large class of Euclidean Yang-Hills solutions are known by now, 19 but we need only discuss the original one of Belavin et al. out of which, in a sense, all the others are constructed.

Toward a Theory of the Strong Interactions by Curtis G. Callan et. al.

## What is the role of the path integral formalism for non-perturbative methods?

As mentioned above, we use a semi-classical approach to investigate effects that are not captured by the usual perturbative approach. This semi-classical approximation can be realized nicely by using Feynman's path integral formalism, which, in addition, can also be used in quantum field theory.

Most of the time it is impossible to carry out the summation over all paths. Therefore, in practice one looks for those paths that dominate the summation, i.e. yield the largest contributions. The paths that yield are the extremal points of the action and these paths are exactly what we usually call the classical paths. This can be understand nicely, for example, by considering “cornu spirals”.

Thus, we can approximate the highly complicated sum over all paths by just keeping these dominant contributions and small fluctuations about them. However, as already mentioned above, there are no classical paths that describe, for example, the tunneling through a potential barrier.

The clever trick one then employs is to continue the time variable to imaginary values. This enables us to find those path that dominate the sum over all paths and thus find a sensible approximation.

Another way of looking at this trick is that the potential gets flipped. We start with the action

$$S[x]^{t_2}_{t_1} = \int^{t_2}_{t_1} \left[ \frac{1}{2m}\left(\frac{dx}{dt}\right)^2-V(x) \right] dt$$

and then change the integration variable by defining $t= -i\tau$:

$$S[x]^{-i\tau_2}_{-i\tau_1} = \int^{-i\tau_2}_{-i\tau_1} \left[ \frac{1}{2m}\left(\frac{dx}{-id\tau}\right)^2-V(x) \right] (-i d\tau) .$$

We then define the Euclidean action $S_E$ as

$$S_E [x]^{\tau_2}_{\tau_1} \equiv i S[x]^{t_2}_{t_1}$$

and thus have

$$S_E [x]^{\tau_2}_{\tau_1} = \int^{\tau_2}_{\tau_1} \left[ \frac{1}{2m}\left(\frac{dx}{d\tau}\right)^2+V(x) \right] d\tau$$

where $\tau$ is called the Euclidean time. This is exactly the action for a “mirror image” of the problem that we started with, i.e. the same problem with a flipped potential $-V(x)$. The potential barrier has become a well and therefore, now there exist classical paths!

As mentioned above, we investigate this situation to find the classical paths that dominate the sum over all paths. In the approximate solution, we must at the end switch back to the correct time by using $\tau \to i t$.

(Source: page 315 in An Introduction to Gauge Theories and Modern Particle Physics, Vol 2 by Elliot Leader,Enrico Predazzi)

The most useful approach to the quantization of gauge theories appears to be Feynman's path integral method. From a geometric point of view, the path integral has the advantage of being able to take into account the global topology of the gauge potentials, while the canonical perturbation theory approach to quantization is sensitive only to the local topology.

## Important Concepts

### The QCD Vacuum

There is a non-zero energy density of the QCD vacuum due to non-perturbative effects (source: Eq. 1.26 |here)

$$\epsilon_{vac} \simeq -\frac{b}{128\pi^2}\langle 0|(gG_{\mu\nu}^a)^2|0\rangle \simeq 0.5 \mathrm{\ GeV/fm^3}$$

This contribution is negative, which means that non-perturbative effects lower the vacuum density we get if we only consider perturbative effects. The absolute value of $\epsilon_{vac}$ sets a scale for the energy density that is necessary to rearrange the vacuum structure.

It is not known completely what is really going in the QCD vacuum, because the final solution of the QCD equations has not been found yet. The values for the vacuum energy density are inferred, for example, by fitting QCD sum rules to experimental data.

If we use the standard perturbative form for the fields and vacuum state then $\langle \bar \Psi \Psi \rangle = 0$ and $\langle G_{\mu \nu}^a G_{\mu \nu}^a \rangle = 0$ etc. So we might think these terms do not even exist. However, we know from chiral symmetry breaking, i.e. from the fact that $m_\rho\neq m_{a_1}$ and that the pion exists, that expressions like $\langle \bar \Psi \Psi \rangle \equiv \langle \text{true. vac. } | \bar \Psi \Psi |\text{true. vac. } \rangle \neq 0$. We cannot calculate these matrix elements. They reflect the deepest non-perturbative aspects of the theory.

But we can calculate the $k^2$ dependence of the coefficients of these operators in the operator product expansion. Thus we can produce a QCD calculation of $\Pi(k^2)$ [call it $\Pi^{QCD}(k^2)$] which includes the perturbative term plus corrections to it that involve powers of $(k^2)^{-d/2}$ multiplied by unknown numbers, the unknown values of the vacuum matrix elements. […] Schematically then $$\Pi^{QCD}(k^2) = \Pi^{pert.}(k^2)+a \frac{\langle G_{\mu \nu}^a G_{\mu \nu}^a \rangle}{k^4} + b \frac{ \langle \bar \Psi \Psi \rangle }{k^4} + \ldots$$ where $a$ and $b$ are known. page 303 in An Introduction to Gauge Theories and Modern Particle Physics, Vol 2 by Elliot Leader,Enrico Predazzi

### Instantons

see: Instantons

In perturbation theory one deals with zero-point quantum-mechanical fluctuations of the YM fields near one of the minima, say, at NCS = 0 . The non-linearity of the YM theory is taken into account as a perturbation, and results in series in g 2 where g is the gauge coupling. In that approach one is apparently missing a possibility for the system to tunnel to another minimum, say, at NCS = 1 . The tunneling is a typical non-perturbative effect in the coupling constant. Instanton is a large fluctuation of the gluon field in imaginary (or Euclidean) time corresponding to quantum tunneling from one minimum of the potential energy to the neighbour one. Physically, one can think of instantons in two ways: on the one hand it is a tunneling process occuring in time, on the other hand it is a localized pseudoparticle in the Euclidean space. […] tunneling of electrons from one atom to another in a metal is also a non-perturbative effect.

### Sphalerons

Sphalerons are static, unstable, finite-energy solutions of the electroweak $SU(2)_L \times U(1)$ Yang-Mills equations.

Because of the scaling property of the instanton solutions and the running of the coupling $α_{QCD}$ in QCD, it is difficult to determine the barrier potential in QCD. On the other hand, the electroweak theory has a natural scale, namely the Higgs vacuum expectation value v, or equivalently, the W-boson mass $m_W = gv/2$, where g is the SU(2) gauge coupling. The existence of the closely related sphaleron in the electroweak theory [8] was first studied by Manton [9] and Klinkhamer and Manton [10]. Although the sphaleron is not a topological soliton, it does have a ChernSimons (CS) number (half-integer) and is important to the dynamics in the electroweak theory. The sphaleron energy $E_{sph}$ measures the height of the potential barrier to the baryon- and lepton-number (B + L)-violating processes (which conserve the (B − L) number).

The energy of the sphaleron solution is (source)

$$E_S \approx \frac{v}{g} \approx 10 \text{ TeV} ,$$ where $v$ is the electroweak Higgs expectation value and $g$ is the $SU(2)$ coupling constant.

[A sphaleron] is a slightly elongated blob of field energy with size of order $1/M_W ∼ 10^{- 2}$ fm and energy density of order $(1/α ) M_W^4$ [and] corresponds to an unstable configuration of fields, which, after a small perturbation, decays to the vacuum by emission of many particles (number of order $1/α ∼ 100$).

In contrast, static, stable, finite energy solutions are called “topological solitons”.

See Solitons

### Monopoles

A great introduction can be found at page 14 in https://hepthboun.files.wordpress.com/2015/10/zainab-thesis.pdf

See Some elementary gauge theory concepts by Hong-Mo Chan, Sheung Tsun Tsou

As analogy Monopole is also a minima of the equation of motion of Yang-Mills theory coupled to an adjoint valued scalar field in 4-dimensional space-time, we can also consider instanton solutions as monopole solutions, in the static limit where all time derivatives are zero.

## Problems

For a while there were high hopes, especially at Princeton, that this work might lead to a calculational method that would allow for reliable calculations in strongly interacting QCD. In the end these hopes were not to be realised. It seems that semi-classical calculations still rely too much on forces being weak and, like the standard perturbation expansion, fail at the point that the forces in QCD become strong.

Not Even Wrong by P. Woit

## Static Solitons exist only in 4-spatial dimension

There are no static solitons in pure Yang-Mills theory except in 4 spatial dimensions.[…] ONe might wonder why a phenomenon that exists only in Euclidean 4-space should concern us. The answer lies in the fact that a quantum field theory in Minkowski 4-space can be described in terms of the classical action in Euclidean 4-space, as we will show in Chapter 7.

page 87 in Quarks, Leptons & Gauge Fields by Kerson Huang

## Experimental Status

One would have thought that when we come to four-dimensional spontaneously broken non-Abelian gauge theories (such as the standard model) - where topologically non-trivial gauge transformations are possible - the existence of topological solitons with interesting topological conservations laws would be a frequent occurrence. Unfortunately, as we have seen, the SSB of the standard gauge group does not give rise to 't Hooft-Polyakov magnetic monopoles and searches motivated by enlargements of the standard model have proved to be futile. This is where instantons enter the picture and raise the hope that the topologically non-trivial phenomena that they define in Yang-Mills theories are capable of giving rise to testable non-perturbative effects in the standard model. After all, instanton solutions are rigorous classical solutions of the Yang-Mills equations in Euclidean four-space and the translation from Euclidean to Minkowski space and quantization is presumed to be tractable! […] Thus far, we can only claim modes phenomenological support for the usefulness of topological conservation laws in particle physics: while the topological non-trivial instanton solutions of QCD have led to a solution of the “U(1) problem”, the same color instantons and their associated $\theta$ vacuum have created the “strong CP” problem in QCD. Moreover, neither the Dirac nor the 't Hooft-Polyakov magnetic monopole has been found to confirm the existence of topologically non-trivial structures in $D=4$ spacetime dimensional particle physics; nor has any evidence been forthcoming as yet for the spectacular sphaleron-induced effects in QFD. Despite these disappointments, the study of topological aspect of field theories - bot classical and quantum - has given us greater insight into the conceptual foundations of the standard model and it's likely future merging with a theory of quantum gravity. page 623 in Conceptual Foundations of Modern Particle Physics by Robert E Marshak

## Quotes

Consider the theory of instantons and remember that standard QCD theory says that the vacuum which we inhabit is an instanton sea with about one instanton per femtometer. This means that the physical reality which we inhabit, if you remove everything and just consider the plain vacuum, is already densely filled with, if you wish, physical incarnation of identity types.