Abstract
A test of the QED process e + e −→ γγ( γ) is reported. The data analysed were collected with the DELPHI detector in 1998 and 1999 at the highest energies achieved at LEP, reaching 202 GeV in the centre-of-mass. The total integrated luminosity amounts to 375.7 pb −1. The differential and total cross-sections for the process e + e −→ γγ were measured, and found to be in agreement with the QED prediction. 95% confidence level (C.L.) lower limits on the QED cut-off parameters of Λ +>330 GeV and Λ −>320 GeV were derived. A 95% C.L. lower bound on the mass of an excited electron of 311 GeV/ c 2 (for λ γ =1) was obtained. s-channel virtual graviton exchange was searched for, resulting in 95% C.L. lower limits on the string mass scale, M S : M S >713 GeV/ c 2 ( λ=1) and M S >691 GeV/ c 2 ( λ=−1).
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