title: Källén–Lehmann spectral representation
text: # Källén–Lehmann spectral representation
  
  The Källén–Lehmann spectral representation gives a general expression for the (time ordered) two-point function of an interacting quantum field theory as a sum of free propagators. It was discovered by Gunnar Källén and Harry Lehmann independently. This can be written as, using the mostly-minus metric signature,
  
      $\Delta (p)=\int _{0}^{\infty }d\mu ^{2}\rho (\mu ^{2}){\frac {1}{p^{2}-\mu ^{2}+i\epsilon }},$
  
  where $\rho (\mu ^{2})$ is the spectral density function that should be positive definite. In a gauge theory, this latter condition cannot be granted but nevertheless a spectral representation can be provided. This belongs to non-perturbative techniques of quantum field theory.
  
  ## Mathematical derivation
  
  The following derivation employs the mostly-minus metric signature.
  
  In order to derive a spectral representation for the propagator of a field $\Phi (x)$, one considers a complete set of states $\{|n\rangle \}$ so that, for the two-point function one can write
  
      $\langle 0|\Phi (x)\Phi ^{\dagger }(y)|0\rangle =\sum _{n}\langle 0|\Phi (x)|n\rangle \langle n|\Phi ^{\dagger }(y)|0\rangle .$
  
  We can now use Poincaré invariance of the vacuum to write down
  
      $\langle 0|\Phi (x)\Phi ^{\dagger }(y)|0\rangle =\sum _{n}e^{-ip_{n}\cdot (x-y)}|\langle 0|\Phi (0)|n\rangle |^{2}.$
  
  Next we introduce the spectral density function
  
      $\rho (p^{2})\theta (p_{0})(2\pi )^{-3}=\sum _{n}\delta ^{4}(p-p_{n})|\langle 0|\Phi (0)|n\rangle |^{2}$.
  
  Where we have used the fact that our two-point function, being a function of $p_{\mu }$, can only depend on $p^{2}$. Besides, all the intermediate states have $p^{2}\geq 0$ and $p_{0}>0$. It is immediate to realize that the spectral density function is real and positive. So, one can write
  
      $\langle 0|\Phi (x)\Phi ^{\dagger }(y)|0\rangle =\int {\frac {d^{4}p}{(2\pi )^{3}}}\int _{0}^{\infty }d\mu ^{2}e^{-ip\cdot (x-y)}\rho (\mu ^{2})\theta (p_{0})\delta (p^{2}-\mu ^{2})$
  
  and we freely interchange the integration, this should be done carefully from a mathematical standpoint but here we ignore this, and write this expression as
  
      $\langle 0|\Phi (x)\Phi ^{\dagger }(y)|0\rangle =\int _{0}^{\infty }d\mu ^{2}\rho (\mu ^{2})\Delta '(x-y;\mu ^{2})$
  
  where
  
      $\Delta '(x-y;\mu ^{2})=\int {\frac {d^{4}p}{(2\pi )^{3}}}e^{-ip\cdot (x-y)}\theta (p_{0})\delta (p^{2}-\mu ^{2})$.
  
  From the CPT theorem we also know that an identical expression holds for $\langle 0|\Phi ^{\dagger }(x)\Phi (y)|0\rangle$ and so we arrive at the expression for the time-ordered product of fields
  
      $\langle 0|T\Phi (x)\Phi ^{\dagger }(y)|0\rangle =\int _{0}^{\infty }d\mu ^{2}\rho (\mu ^{2})\Delta (x-y;\mu ^{2})$
  
  where now
  
      $\Delta (p;\mu ^{2})={\frac {1}{p^{2}-\mu ^{2}+i\epsilon }}$
  
  a free particle propagator. Now, as we have the exact propagator given by the time-ordered two-point function, we have obtained the spectral decomposition.
  
  