Tuesday, 1 September 2020

quantum field theory - Traceless of stress-energy tensor in d=2


This is a question regarding Francesco, section 4.3.3. In this section, he considers the two-point function Sμνρσ(x)=Tμν(x)Tρσ(0)

He then goes on to claim that symmetry of the stress-energy tensor implies Sμνρσ(x)=Sνμρσ(x)   (1)
Though he doesn't mention this, I presume this is true only when x0 since the EM tensor is symmetric in a correlation as long as the other fields in the correlator are not evaluated at the same point.





EDIT: Due to some comments, I'll explain why I think so. If a theory is Poincare invariant, it has conserved currents Tμν for translations and jμνρ=TμνxρTμρxν

for Lorentz transformations. For completeness, we also note that if the theory has scale invariance the dilation current is jμD=Tμνxν
In a classical theory, conservation of these currents implies symmetry and tracelessness of the stress-energy tensor. In a quantum theory, we have a Ward Identity, which for each of the currents reads μTμνX=ni=1δd(xxi)xνiXμjμνρX=ni=1δd(xxi)(xρixνixνixρiiSμνi)XμjμDX=ni=1δd(xxi)(xαixαi+Δi)X
where X=Φ1(x1)Φn(xn), Sμνi is the representation of the Lorentz algebra under which Φi(xi) transforms and Δi is the scaling dimension of Φi(xi). Now plugging in the exact forms of the currents jμνρ and jμD, we find μTμνX=ni=1δd(xxi)xνiX(TμνTνμ)X=ini=1δd(xxi)SμνiXTμμX=ni=1δd(xxi)ΔiX
Clearly, the EM tensor is not symmetric under correlation functions at the points x=xi.




Now, using these symmetry properties and certain other properties under parity, he argues that Sμμσσ(x)=Tμμ(x)Tσσ(0)=0

Following the above arguments, this should then only be true at x0. However, Francesco claims that this holds everywhere and therefore concludes that Tμμ(0)2=0. How does this makes sense?




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