4 added more explanation to clarify the question

The "definition" of the normal ordering of a single operator in CFT is understandable looks a bit vague to mebut it is not clear as to how products of normal ordered operators or normal order of product of operators are defined? .

Some more elaboration on what about normal ordering I am concerned about.

The problem is that I can't these books give an honest definition of what it means to "normal order" operators in CFT. Like there is a very clean definition in rest of QFT whose relation to time-ordering is given by the Wick's Theorem. Here in CFT one is supposed to understand that while normal ordering a string of operators inserted at different points on the space-time one is subtracting away from the product every possible way in which one or more pairs of insertion points can coincide and produce a singularity

Like if A,B,C,D are 4 different Bosonic operators say inserted at 4 different space-time points. Then one would define normal ordering as,

$:ABCD: = ABCD - (AB):CD: - (AC):BD: - (AD):BC:-(BC):AD:-(BD):AC:$$-(CD):AB:-(AB)(CD)-(AC)(BD)-(AD)(BC)$$ where () denotes the correlation function of the operators. Now the point is whether one is supposed to take the above kind of equations as being just well-motivated definition or is there is anything more fundamental from which it is derivable? There is definitely an issue about defining difference of two divergent expressions here. 3 added explicit calculations ; added 24 characters in body # Normal Ordering withVertexOperators in Conformal Field Theory The "definition" of the normal ordering of a single operator in CFT is understandable to me but it is not clear as to how products of normal ordered operators or normal order of product of operators are defined? I found the definition in terms of exponentiated functional derivative pretty opaque. Also in this context it might help if someone can give a reference or if there is a short explanation to understand how the Operator Product Expansion is derived using products of normal ordered operators. I don't see the conceptual framework in which these ideas fit together. Some of the books I looked at gave a very disparate view as a collection of some complicated formulas. Let me give a precise example of the kind of calculation that I am stuck with, Refer to these lecture notes I can understand equation 4.26 of this but not the next 4 equations that seem to follow from it leading to 4.28. It would be helpful if someone can decrypt the calculation. In light of the kinds of references that came in as responses, I think it would help if I make the problematic calculation a little more explicit. This has to do with what are called "Vertex Operators" in CFT given as$:e^{ikX(z)}:$where$::$is the notation for normal ordering and$k$is some scalar and$X$is a conformally invariant free Bosonic field. Then I would like to understand the derivation of this equality, (all expressions are understood to be valid under the Feynman Path Integral)$:\partial X(z)\partial X(z)::e^{ikX(w)}: = -\frac{k^2\alpha ^2}{4}\frac{:e^{ikX(w)}:}{(z-w)^2}-ik\alpha\frac{:\partial X(z) e^{ikX(w)}:}{(z-w)}$where we have$X(z)X(w) = -\frac{\alpha}{2}ln \vert z - w \vert$and what would be the similar simplification of$:e^{ikX(z)}::e^{ikX(w)}: = ?\$

2 added 370 characters in body

The "definition" of the normal ordering of a single operator in CFT is understandable to me but it is not clear as to how products of normal ordered operators or normal order of product of operators are defined?

I found the definition in terms of exponentiated functional derivative pretty opaque.

Also in this context it might help if someone can give a reference or if there is a short explanation to understand how the Operator Product Expansion is derived using products of normal ordered operators.

I don't see the conceptual framework in which these ideas fit together. Some of the books I looked at gave a very disparate view as a collection of some complicated formulas.

Let me give a precise example of the kind of calculation that I am stuck with,

Refer to these lecture notes

I can understand equation 4.26 of this but not the next 4 equations that seem to follow from it leading to 4.28.

It would be helpful if someone can decrypt the calculation.

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