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.^[1][2] This can be written as, using the mostly-minus metric signature,

${\displaystyle \Delta (p)=\int _{0}^{\infty }d\mu ^{2}\rho (\mu ^{2}){\frac {1}{p^{2}-\mu ^{2}+i\epsilon }},}$

where ${\displaystyle \rho (\mu ^{2})}$ 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.^[3] 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 ${\displaystyle \Phi (x)}$ , one consider a complete set of states ${\displaystyle \{|n\rangle \}}$  so that, for the two-point function one can write

${\displaystyle \langle 0|\Phi (x)\Phi ^{\dagger }(y)|0\rangle =\sum _{n}\langle 0|\Phi (x)|n\rangle \langle n|\Phi ^{\dagger }(y)|0\rangle .}$ 

We can now use Poincaré invariance of the vacuum to write down

${\displaystyle \langle 0|\Phi (x)\Phi ^{\dagger }(y)|0\rangle =\sum _{n}e^{-ip_{n}\cdot (x-y)}|\langle 0|\Phi (0)|n\rangle |^{2}.}$ 

Let us introduce the spectral density function

${\displaystyle \rho (p^{2})\theta (p_{0})(2\pi )^{-3}=\sum _{n}\delta ^{4}(p-p_{n})|\langle 0|\Phi (0)|n\rangle |^{2}}$ .

We have used the fact that our two-point function, being a function of ${\displaystyle p_{\mu }}$ , can only depend on ${\displaystyle p^{2}}$ . Besides, all the intermediate states have ${\displaystyle p^{2}\geq 0}$  and ${\displaystyle p_{0}>0}$ . It is immediate to realize that the spectral density function is real and positive. So, one can write

${\displaystyle \langle 0|\Phi (x)\Phi ^{\dagger }(y)|0\rangle =\int {\frac {d^{4}p}{(2\pi )^{3}}}\int _{0}^{\infty }d\mu ^{2}e^{-ip\cdot (x-y)}\rho (\mu ^{2})\theta (p_{0})\delta (p^{2}-\mu ^{2})}$ 

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

${\displaystyle \langle 0|\Phi (x)\Phi ^{\dagger }(y)|0\rangle =\int _{0}^{\infty }d\mu ^{2}\rho (\mu ^{2})\Delta '(x-y;\mu ^{2})}$ 

where

${\displaystyle \Delta '(x-y;\mu ^{2})=\int {\frac {d^{4}p}{(2\pi )^{3}}}e^{-ip\cdot (x-y)}\theta (p_{0})\delta (p^{2}-\mu ^{2})}$ .

From the CPT theorem we also know that an identical expression holds for ${\displaystyle \langle 0|\Phi ^{\dagger }(x)\Phi (y)|0\rangle }$  and so we arrive at the expression for the chronologically ordered product of fields

${\displaystyle \langle 0|T\Phi (x)\Phi ^{\dagger }(y)|0\rangle =\int _{0}^{\infty }d\mu ^{2}\rho (\mu ^{2})\Delta (x-y;\mu ^{2})}$ 

where now

${\displaystyle \Delta (p;\mu ^{2})={\frac {1}{p^{2}-\mu ^{2}+i\epsilon }}}$ 

a free particle propagator. Now, as we have the exact propagator given by the chronologically ordered two-point function, we have obtained the spectral decomposition.

References

1. Källén, Gunnar (1952). "On the Definition of the Renormalization Constants in Quantum Electrodynamics". Helvetica Physica Acta. 25: 417. doi:10.5169/seals-112316(pdf download available){{cite journal}}: CS1 maint: postscript (link)
2. Lehmann, Harry (1954). "Über Eigenschaften von Ausbreitungsfunktionen und Renormierungskonstanten quantisierter Felder". Nuovo Cimento (in German). 11 (4): 342–357. Bibcode:1954NCim...11..342L. doi:10.1007/bf02783624. ISSN 0029-6341. S2CID 120848922.
3. Strocchi, Franco (1993). Selected Topics on the General Properties of Quantum Field Theory. Singapore: World Scientific. ISBN 978-981-02-1143-1.

Bibliography

• Weinberg, S. (1995). The Quantum Theory of Fields: Volume I Foundations. Cambridge University Press. ISBN 978-0-521-55001-7.
• Peskin, Michael; Schoeder, Daniel (1995). An Introduction to Quantum Field Theory. Perseus Books Group. ISBN 978-0-201-50397-5.
• Zinn-Justin, Jean (1996). Quantum Field Theory and Critical Phenomena (3rd ed.). Clarendon Press. ISBN 978-0-19-851882-2.