AbstractWe 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, $$Z(x, \mu )$$
Z
(
x
,
μ
)
, is diagonalizable to all perturbative orders. As a key step, we provide a differential-geometric interpretation of renormalization that allows us to apply the Poincaré-Dulac theorem to the problem above: We interpret a change of renormalization scheme as a (formal) holomorphic gauge transformation, $$-\frac{\gamma (g)}{\beta (g)}$$
-
γ
(
g
)
β
(
g
)
as a (formal) meromorphic connection with a Fuchsian singularity at $$g=0$$
g
=
0
, and $$Z(x,\mu )$$
Z
(
x
,
μ
)
as a Wilson line, with $$\gamma (g)=\gamma _0 g^2 + \cdots $$
γ
(
g
)
=
γ
0
g
2
+
⋯
the matrix of the anomalous dimensions and $$\beta (g)=-\beta _0 g^3 +\cdots $$
β
(
g
)
=
-
β
0
g
3
+
⋯
the beta function. As a consequence of the Poincaré-Dulac theorem, if the eigenvalues $$\lambda _1, \lambda _2, \ldots $$
λ
1
,
λ
2
,
…
of the matrix $$\frac{\gamma _0}{\beta _0}$$
γ
0
β
0
, in nonincreasing order $$\lambda _1 \ge \lambda _2 \ge \cdots $$
λ
1
≥
λ
2
≥
⋯
, satisfy the nonresonant condition $$\lambda _i -\lambda _j -2k \ne 0$$
λ
i
-
λ
j
-
2
k
≠
0
for $$i\le j$$
i
≤
j
and k a positive integer, then a renormalization scheme exists where $$-\frac{\gamma (g)}{\beta (g)} = \frac{\gamma _0}{\beta _0} \frac{1}{g}$$
-
γ
(
g
)
β
(
g
)
=
γ
0
β
0
1
g
is one-loop exact to all perturbative orders. If in addition $$\frac{\gamma _0}{\beta _0}$$
γ
0
β
0
is diagonalizable, $$Z(x, \mu )$$
Z
(
x
,
μ
)
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 Poincaré–Dulac theorem.