Klein transformation

In quantum field theory, the Klein transformation ^[1] is a redefinition of the fields to amend the spin-statistics theorem.

Bose–Einstein

Suppose φ and χ are fields such that, if x and y are spacelike-separated points and i and j represent the spinor/tensor indices,

${\displaystyle [\varphi _{i}(x),\varphi _{j}(y)]=[\chi _{i}(x),\chi _{j}(y)]=\{\varphi _{i}(x),\chi _{j}(y)\}=0.}$

Also suppose χ is invariant under the Z₂ parity (nothing to do with spatial reflections!) mapping χ to −χ but leaving φ invariant. Obviously, free field theories always satisfy this property. Then, the Z₂ parity of the number of χ particles is well defined and is conserved in time. Let's denote this parity by the operator K_χ which maps χ-even states to itself and χ-odd states into their negative. Then, K_χ is involutive, Hermitian and unitary.

Needless to say, the fields φ and χ above don't have the proper statistics relations for either a boson or a fermion. i.e. they are bosonic with respect to themselves but fermionic with respect to each other. But if you look at the statistical properties alone, we find it has exactly the same statistics as the Bose–Einstein statistics. Here's why:

Define two new fields φ' and χ' as follows:

${\displaystyle \varphi '=iK_{\chi }\varphi \,}$

and

${\displaystyle \chi '=K_{\chi }\chi .\,}$

This redefinition is invertible (because K_χ is). Now, the spacelike commutation relations become

${\displaystyle [\varphi '_{i}(x),\varphi '_{j}(y)]=[\chi '_{i}(x),\chi '_{j}(y)]=[\varphi '_{i}(x),\chi '_{j}(y)]=0.\,}$

Fermi–Dirac

Now, let's work with the example where

${\displaystyle \{\phi ^{i}(x),\phi ^{j}(y)\}=\{\chi ^{i}(x),\chi ^{j}(y)\}=[\phi ^{i}(x),\chi ^{j}(y)]=0}$

(spacelike-separated as usual).

Assume once again we have a Z₂ conserved parity operator K_χ acting upon χ alone.

Let

${\displaystyle \phi '=iK_{\chi }\phi \,}$

and

${\displaystyle \chi '=K_{\chi }\chi .\,}$

Then

${\displaystyle \{\phi '^{i}(x),\phi '^{j}(y)\}=\{\chi '^{i}(x),\chi '^{j}(y)\}=\{\phi '^{i}(x),\chi '^{j}(y)\}=0.}$

More than two fields

If there are more than two fields, then one can keep applying the Klein transformation to each pair of fields with the "wrong" commutation/anticommutation relations until the desired result is obtained.

This explains the equivalence between parastatistics and the more familiar Bose–Einstein/Fermi–Dirac statistics.

References

1. R. F. Streater and A. S. Wightman, PCT, Spin and Statistics and All That, §4.4, Princeton University Press, Landmarks in Mathematics and Physics, 2000 (1st edn., New York, Benjamin 1964).

See also

• Jordan–Schwinger transformation
• Jordan–Wigner transformation
• Bogoliubov–Valatin transformation
• Holstein–Primakoff transformation