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The N=4 string is the same as the N=2 string

W. Siegel
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Abstract

We redo the quantization of the N=4 string, taking into account the reducibility of the constraints. The result is equivalent to the N=2 string, with critical dimension D=4 and signature (++--). The N=4 formulation has several advantages: the sigma-model field equations are implied classically, rather than by quantum/beta-function calculations; self-duality/chirality is one of the super-Virasoro constraints; SO(2,2) covariance is manifest. This reveals that the theory includes fermions, and is apparently spacetime supersymmetric.

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W. Siegel. 1992-05-28. The N=4 string is the same as the N=2 string. https://doi.org/10.1103/physrevlett.69.1493

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