On the geometry of operator mixing in massless QCD-like theories

We revisit the operator mixing in massless QCD-like theories. In particular, we address the problem of determining under which conditions a renormalization scheme exists where the renormalized mixing matrix in the coordinate representation, $Z(x, \mu)$, is diagonalizable to all perturbative orders. As a key step, we provide a differential-geometric interpretation of renormalization that allows us to apply the Poincar\'e-Dulac theorem to the problem above: We interpret a change of renormalization scheme as a (formal) holomorphic gauge transformation, $-\frac{\gamma(g)}{\beta(g)}$ as a (formal) meromorphic connection with a Fuchsian singularity at $g=0$, and $Z(x,\mu)$ as a Wilson line, with $\gamma(g)=\gamma_0 g^2 + \cdots$ the matrix of the anomalous dimensions and $\beta(g)=-\beta_0 g^3 +\cdots$ the beta function. As a consequence of the Poincar\'e-Dulac theorem, if the eigenvalues $\lambda_1, \lambda_2, \cdots$ of the matrix $\frac{\gamma_0}{\beta_0}$, in nonincreasing order $\lambda_1 \geq \lambda_2 \geq \cdots$, satisfy the nonresonant condition $\lambda_i -\lambda_j -2k \neq 0$ for $i\leq j$ and $k$ a positive integer, then a renormalization scheme exists where $-\frac{\gamma(g)}{\beta(g)} = \frac{\gamma_0}{\beta_0} \frac{1}{g}$ is one-loop exact to all perturbative orders. If in addition $\frac{\gamma_0}{\beta_0}$ is diagonalizable, $Z(x, \mu)$ is diagonalizable as well, and the mixing reduces essentially to the multiplicatively renormalizable case. We also classify the remaining cases of operator mixing by the Poincar\'e-Dulac theorem.