# Questions tagged [correlation-functions]

A correlation function is a statistical correlation between random variables at two different points in space, time, or other parameter space, usually as a function of the variable distance between these points. In QFT, field autocorrelation functions are propagators, so use the "propagator" tag, instead.

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### Large $c$ limit and connected correlation functions in $2d$ QFT

I am having trouble reading this paper about $T \bar{T}$ deformations of $2d$-QFTs in the plane. All is fine until the beginning of section 3. There they start talking about the limit of a large ...
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### Correlation function at zero distance

I'm confused about the definition of the correlation function (at equal time). I know it is defined from the thermal average of the scalar product of two random variables (for example the spins of a ...
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### Does somebody know where to find original paper of Lehmann, Symanzik and Zimmerman translated in English?

does anybody know where to find original paper about LSZ reduction translated in english? unfortunantely, Ive found only original German article. H. Lehmann, K. Symanzik, and W. Zimmerman, "Zur ...
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### Spinwaves, Mermin-Wagner theorem, Two-point correlation function and Heisenberg model

I was looking at the Mermin-Wagner theorem (as following the previous question) and the Heisenberg model seems to be presented, and they split the Hamiltonian $H$ in the matrix or vector n-components ...
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### Expectation value of descendant fields

I'm trying to calculate the following quantity: $\left<(L_{-1}\phi)(w_1)(L_{-1}\phi)(w_2) \ldots (L_{-1}\phi)(w_N) \right>$ where $\phi(w_i)$ is a primary operator and $L_{-1}$ is the ...
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### Quantum field theory: corrections to excited state correlation functions

I want to know how to calculate the lowest-order-in-the-coupling-constant correction to $$M(x, y,k,p)=\langle k|\phi(x)\phi(y)|p\rangle$$ in $\phi^4$ scalar field theory in a relatively general ...
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