I edited this question to remomve the reference request issue and think it should be reopend now.
I'll copy my reasoning here: while it's certainly better than it was as a resource recommendation, the question is still unfocused. It says
I am looking for an explanation about the idea of "operator mixing" and its associated concept about when anomalous dimension has to be thought of as a matrix.
but it does not specify what about operator mixing needs to be explained. What about operator mixing and anomalous dimensions does the OP want to know? The definitions? What they are used for? Some particular aspect of how they are used? Etc. We're waiting for clarification from the OP about what exactly he/she wants to ask.
At a first look, one could think that the question still asks too many things in a too incoherent manner, but Mitchell Porter was able to correctly understand the question.
To me, asking about two nearly related enough concepts such as operator mixing and anomalous dimension, giving an example of what one wants to better understand from knowing these concepts (the equation in the paper), plus investigating if these two concepts are known under other names in the literature (which could be confusing) too, is legitimate and makes a lot of sense to me too.