Tuesday, April 10, 2018

quantum field theory - Constructing Ward identity associated with conserved currents


Consider constructing the Ward identity associated with Lorentz invariance. It is possible to find a 3rd rank tensor Bρμν antisymmetric in the first two indices, then the stress-energy tensor can be made symmetric. Once done, the conserved current coming from the classical analysis is of the form


jμνρ=TμνBxρTμρBxν


This ensures the symmetry of the conserved current which can be seen most easily be invoking the conservation law μjμνρ=0

and μTμνB=μ(TμνC+ρBρμν)=0.


Let X denote a set of n fields. The Ward identity associated with Lorentz invariance is then


μ(TμxρTμρxν)X=iδ(xxi)[xνiρixρiνiXiSνρiX].


This is then equal to


(TρνTνρ)X=iiδ(xxi)SνρiX,


which states that the stress tensor is symmetric within correlation functions, except at the position of the other fields of the correlator.


My question is: how is this last equation and statement derived?



I think the Ward identity associated with translation invariance is used after perhaps splitting (1) up like so:


nixνiniδ(xxi)ρiXnixρiniδ(xxi)νiXiniδ(xxi)SνρiX

and then replacing μTμρX=iδ(xxi)xρiX


for example. The result I am getting is that ((μTμν)xρ(μTμρ)xν+TρνTνρ)X=ixνiμTμρX+ixρiμTμνXiiδ(xxi)SνρiX

To obtain the required result, this means that e.gixνiμTμρX=(μTμρ)xνX,
but why is this the case? Regarding the statement at the end, do they mean that when the position in space x happens to coincide with one of the points where the field ΦiX takes on the value xi (so x=xi) then the r.h.s tends to infinity and the equation is then nonsensical?




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