The total and differential cross sections of the process e+e- -> n gamma with n >= 2 are measured using data collected by the L3 experiment at centre-of-mass energies of \sqrt{s}=183 and 189 GeV. The results are in agreement with the Standard Model expectations. Limits are set on deviations from QED, contact interaction cut-off parameters and masses of excited electrons.
Measured cross section.
Measured differential cross sections corrected for efficiency and additional photons as a function of cos(theta) where theta is the polar angle of the event defined as. cos(theta)=ABS((sin(theta1-theta2)/2)/(sin(theta1+theta2)/2)).
The total and the differential cross-sections for the reaction e + e − → γγ ( γ ) have been measured with the DELPHI detector at LEP at centre-of-mass energies from 130 to 183 GeV for an integrated luminosity of 78.19 pb −1 . The results agree with the QED predictions. The lower limits (obtained including previously published results at the Z 0 energies) on the QED cutoff parameters are Λ + >253 GeV and Λ − >225 GeV and the lower bound on the mass of an excited electron with an effective coupling constant λ γ =1 is 231 GeV/ c 2 . All the limits are at the 95% confidence level.
The cross section of the previously published data (sqrt(s)=91.25 GeV, see PL 327B, 386) is given at the mean of the CM energies weighted by the luminosityat each point.
Statistical errors only. Additional overall systematic uncertainty is givenabove.
Statistical errors only. Additional overall systematic uncertainty is givenabove.
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