A study of $W^{+}W^{-}\gamma$ events at LEP

Abstract A study of W + W − events accompanied by hard photon radiation, E γ >2.5 GeV, produced in e + e − collisions at LEP is presented. Events consistent with being two on-shell W-bosons and an isolated photon are selected from 681 pb −1 of data recorded at 180  GeV s  GeV. From the sample of 187 selected W + W − γ candidates with photon energies greater than 2.5 GeV, the W + W − γ cross-section is determined at five values of s . The results are consistent with the Standard Model expectation. Averaging over all energies, the ratio of the observed cross-section to the Standard Model expectation is R( data/SM )=0.99±0.09±0.04, where the errors represent the statistical and systematic uncertainties respectively. These data provide constraints on the related O (α) systematic uncertainties on the measurement of the W-boson mass at LEP. Finally, the data are used to derive 95% confidence level upper limits on possible anomalous contributions to the W + W − γγ and W + W − Z 0 γ vertices: −0.020  GeV −2 a 0 Λ 2 GeV −2 , −0.053  GeV −2 a c Λ 2 GeV −2 , −0.16  GeV −2 a n Λ 2 GeV −2 , where Λ represents the energy scale for new physics and a 0 , a c and a n are dimensionless coupling constants.