Saturday, 18 July 2020

conformal field theory - Question about correlation functions of 2d CFTs


I have a question regarding equation (2.22) in Ginsparg's lecture notes on CFTs. Equation (2.22) is T(z)ϕ1(w1,ˉw1)=ni=1(hi(zwi)2+1zwiwi)ϕ1(w1,ˉw1)

Here, T(z) is the stress tensor of the CFT and ϕi is a primary operator of weight (hi,0) which transforms under conformal transformations as δϵϕi=(hiϵ+ϵ)ϕi
He derives (2.22) from (2.21) which reads dz2πiϵ(z)T(z)ϕ1(w1,ˉw1)=ni=1ϕ1(w1,ˉw1)δϵϕi(wi,ˉwi)
by setting ϵ(x)=1xz.


My question is - Is (2.22) correct?


Here are my reasons to believe that it is not -




  1. I believe he derives (2.22) from (2.21) by setting ϵ(x)=1xz in (2.21). (2.22) is then derived if the following holds dx2πiT(x)xzϕ1(w1,ˉw1)=T(z)ϕ1(w1,ˉw1)

    This would be true if the integrand on the LHS had only a pole at xz. However, it has also has poles at each x=wi, but those contributions aren't considered.





  2. I can try and derive (2.22) in a different way - namely via contractions. I start with the LHS of (2.22) and contract T(z) with each ϕi. Each contraction is replaced with the operator product T(z)ϕi(wiˉwi)=hiϕi(wiˉwi)(zwi)2+ϕi(wiˉwi)zwi+:T(z)ϕi(wiˉwi):

    Again, if I only consider the singular terms, I reproduce the RHS of (2.22). But what about :T(z)ϕi(wiˉwi):?? In a general CFT, conformal normal ordering : : is not equivalent to creation-annihilation normal ordering  . The latter would vanish in a correlation function, but not the former. So, I believe in general there would be extra terms on the right of (2.22).





What am I misunderstanding?





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