The four-dimensional $SU(N)$ Yang-Mills Lagrangian is given by $$\mathcal{L}=\frac{1}{2e^2}\mathrm{Tr}F_{\mu\nu}F^{\mu\nu}$$

and gives rise to the Euclidean equations of motion $\mathcal{D}_\mu F^{\mu\nu}=0$ with covariant derivative $\mathcal{D}_\mu$. Finite action solutions $A_\mu$ satisfy the condition that,

$$A \to ig^{-1}\partial_\mu g$$

as we approach $\partial \mathbb{R}^4 \cong \mathbb{S}^3_{\infty}$, with $g$ an element of $SU(N)$. These provide a map from $\mathbb{S}^3_\infty$ to $SU(N)$, and are classified by homotopy theory. In Tong's lecture notes on solitons, he states without proof that the second Chern class, or Pontryagin number $k \in \mathbb{Z}$ is given by

$$k = \frac{1}{24\pi^2}\int_{S^3_\infty}\mathrm{d}S^3_\mu \, \, \mathrm{Tr} \,(\partial_\nu g)g^{-1}(\partial_\rho g)g^{-1} (\partial_\sigma g)g^{-1}\epsilon^{\mu \nu \rho \sigma}$$

As I understand them, Chern classes are characteristic classes of bundles on manifolds; in this case what bundle and manifold is $k$ associated with? How does one obtain $k$ in this case? Tong states "The integer... counts how many times the group wraps itself around spatial $\mathbb{S}^3_\infty$."

In addition, Tong states without rigorous proof the metric of the moduli space (the space of all solutions to the equations of motion which are self-dual):

$$g_{\alpha \beta} = \frac{1}{2e^2} \int \mathrm{d}^4 x \mathrm{Tr} \, \, (\delta_\alpha A_\mu)(\delta_\beta A_\mu)$$

with $\delta_\alpha A_\mu = \partial A_\mu / \partial X^{\alpha} + \mathcal{D}_\mu \Omega_\alpha$ where $\Omega_\alpha$ is an infinitesimal transformation, and $X^{\alpha}$ are the collective coordinates. How does one compute such a metric of a moduli space? Why should it be given by the sum of all zero modes?

I would prefer an answer which utilizes arguments from differential geometry, and topology.


1 Answer 1


First let me refer you to Eric Weinberg's book where the instanton moduli space is described in more detail.

Principal bundles over 4-dimensional Riemannian manifolds are classified by the second Chern class = Instanton number and the t' Hooft discrete Abelian magnetic fluxes. Please see the following Lecture notes by Måns Henningson.

t' Hooft fluxes are present only when the gauge group has a nontrivial center, thus in the case of $SU(N)$, the classification is according to the instanton numbers.

A compactified Minkowski space, can be thought of as a four dimensional ball $B^4$, with the boundary points ( at infinity) $S^3$ identified. Thus by the Stokes theorem, the instanton number is given by:

$$ k = \int_{B^4}*F \wedge F =\int_{B^4}dCS(A) = \int_{S^3_{\infty}} CS(A) = \int_{S^3_{\infty}} WZW(g) $$

Where $CS(A)$ and $WZW(g)$ are the Chern-Simons and the Wess-Zumino-Novikov-Witten functionals respectively and the last step is derived from the substitution of the pure gauge condition at infinity.

The local geometry of the moduli space can be understood as follows: The instanton solutions are of the form:$ A_{\mu} = A_{\mu}(X^\alpha)$, where $X^\alpha$ are the coordinates of the moduli space. These solutions are local minima of the action, for all constant values of the moduli but the action is not extremal when these coordinates are made to depend on time. This time dependence is introduced to study the dynamics of the moduli near the classical solution.

The difference between the action with time varying moduli and time invariant ones is due to the time dependence of the moduli coordinates, assuming the time derivatives are small(i.e., by substituting $X^{\alpha}= X_0^{\alpha} + t \dot{X}_0^{\alpha}$, the leading terms have the least number of time derivatives thus the varied action must have the form:

$$ I = \frac{1}{2g^2} \int d^4x F_{\mu\nu}(A) F^{\mu\nu}(A) = \frac{8\pi^2}{g^2} k + \int dt B_{\alpha}(X)\dot{X}^{\alpha} +g_{\alpha \beta}(X)\dot{X}^{\alpha}\dot{X}^{\beta} + ...$$

Where the last step is obtained after the integration of the "known" solution over the spatial coordinates.

This action has just the form of a particle moving on a Riemannian manifold having a metric $g$ in a magnetic field $B$. The exterior derivative of the magnetic field can be interpreted as the symplectic structure of the moduli space. The form of the metric taken by David Tong will give exactly the same metric, because, the leading terms in the moduli time derivatives of the Yang Mills action will include a time derivative, thus we are left with the variation of the gauge fields themselves.

This is only the local structure of the moduli space. This analysis will not tell us, for example, if the symplectic structure is exact or closed. Of course, the global structure of the moduli space requires deeper analysis.

One global property that can be "relatively" easily computed is the moduli space dimension, even if a simple closed form of the solution is not available: The dimension is the count perturbation $A_{\mu}+a_{\mu}$ of a self dual solution $A_{\mu}$ which is also self dual modulo gauge transformations. Inserting the gauge potential in the self duality equation, The following condition is obtained (Weinberg equation 10.112):

$$ \eta^{\mu \nu} D_{A \mu} a_{\nu} = 0$$

where $D_{A}$ is the covariant derivative of the classical solution, and $ \eta^{\mu \nu} $ is a self dual matrix defined in Weinberg (equation 10.74). To remove the pure gauge direction, there exist another orthogonality condition to all pure gauge directions:

$$D_{A\mu} a^{\mu} = 0$$.

These two conditions can be combined together into a single Dirac equation:

$$\not{D}_{A} \Psi = 0$$.

where, the relation of the gauge field perturbation and the spinor $\Psi$ are given by: $a_{\mu} = \sigma_{\mu}^{\alpha \dot{\alpha}} \Psi_{\alpha \dot{\alpha}}$

This construction converts the problem of counting the number of moduli to counting the number of a Dirac equation zero modes. For the Dirac equation, the number of zero modes is given by the Atiyah-Singer index theorem.

  • $\begingroup$ I've only briefly encountered the Atiyah-Singer index theorem in the context of the Dirac operator and quantum field theory anomalies. Can you provide a set of lecture notes, or another text which focuses on the applications of the index theorem in physics? $\endgroup$
    – JamalS
    Commented Mar 24, 2014 at 16:13
  • 1
    $\begingroup$ @jamalS There are analogous computations for the monopole moduli space dimension in the following review arxiv.org/abs/hep-th/0609055v2 by Weinberg and Yi, and for nonAbelian vortices in arxiv.org/abs/hep-th/0306150v1 by Hannay and Tong. There is a more mathematical review of the instanton and Seiberg-Witten moduli spaces dimension computation by David Bleecker milne.ruc.dk/~Booss/A-S-Index-Book/BlckBss_2012_04_25.pdf. $\endgroup$ Commented Mar 25, 2014 at 12:45
  • $\begingroup$ @jamalS cont. There are more applications besides anomalies such as spinc quantization, charge quantization and fractionalization, Brane charges, supersymmetric quantum mechanics, Berezin-Toeplitz quantization and may be more. I don’t know of a single review comprising all these applications. $\endgroup$ Commented Mar 25, 2014 at 12:46
  • $\begingroup$ "Principal bundles over 4-dimensional Riemannian manifolds are classified by the second Chern class" This statement looks like it needs for further qualifications. Are you thinking about principal $G$-bundles for some fixed $G$? Which $G$? $SU(2)$, perhaps? Currently, I don't quite see how to make sense out of this statement because it seems to fail for $U(1)$-bundles, for instance. $\endgroup$
    – Danu
    Commented Aug 19, 2017 at 11:44
  • $\begingroup$ @Danu Sorry for not being explicit, the mentioned classification result is true for simple compact Lie groups. (The question was about SU(N) but the result is a little more general). A detailed explanation was given in Henningson lecture that I attached but the link is now broken and I couldn't find it elsewhere. However, this result is quite known in the physics literature. Please see Witten in: arxiv.org/abs/hep-th/0006010v1 page 4 around equation (2.3). $\endgroup$ Commented Aug 20, 2017 at 9:24

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