# Are there any exact solutions in the Standard Model of particle physics?

I wonder if there are any exact solutions to any equations of the Standard Model (SM)?

Do we always have to use perturbation methods to solve anything within the SM? Are there any simple cases where exact solutions are available?

• Could you elaborate on your question? I'm not sure I understand it. Commented Nov 25, 2013 at 11:33
• What equations do you mean? Examples? Commented Nov 25, 2013 at 11:33
• I think that QFT in the lattice and twistors are non perturbational, but I'm not sure. Commented Nov 25, 2013 at 11:43
• Exact solutions in QFT (except free fields) are very rare. Peskin's QFT section 22.3 has a few examples. Commented Nov 25, 2013 at 15:50

## 2 Answers

I think that this question can be answered in three parts:

1. Are there any exact solutions to any equations of the SM?
2. Do we always have to use perturbation methods to solve anything within the SM?
3. Are there any exact solutions beyond the SM?

The answer to question 1 is: it depends on what equations you are referring to. For example, the propagators of fermions and photons are exact solutions to the equations of motion of the corresponding free Lagrangian (you cannot say that these are not the parts of SM). Surely, most (almost all) equations of the SM have no exact solutions, such as equations of motion with interactions, Dyson-Schwinger equations, Callan-Symanzik equations, Altarelli-Parisi equations, and so on.

The answer to question 2 is: no! We have many nonperturbative methods to deal with the SM, such as the lattice method, effective field theories, and recent advances on unitary methods. There are also some topological or geometrical aspects which should be treated as nonperturbative, such as anomalies, instantons, solitons, the Batalin-Vilkovsky formalism.

The answer to question 3 is: yes! (1) Many two-dimensional modes can have exact solutions, such as two-dimensional QED, the Thirring model, $d=2$ nonlinear sigma model, and so on. (2) Some supersymmetric gauge theories can have exact solutions, such as the famous Seiberg-Witten equations. (3) Large $N$ expansion allows exact solutuons to many models, such as the Gross-Neveu model, $1+1$-dimensional QCD.

If I have missed some examples or some misundertanding, please help to improve it.

Exact solutions, hmm... not so sure. Exact results, yes. Because of symmetry, there are a large number of things that are known to be zero exactly, and I believe in some cases we know the form that corrections take (they are "protected" by things like gauge invariance and chiral symmetry)

• Are we also missing non-renormalization of the anomaly here? e.g. $\pi\to\gamma\gamma$ is exact? and in fact non-zero? Commented Mar 20, 2014 at 9:57