Measurement of the W+W−γ cross-section and first direct limits on anomalous electroweak quartic gauge couplings

Abstract A study of W + W − events accompanied by hard photon radiation produced in e + e − collisions at LEP is presented. Events consistent with two on-shell W-bosons and an isolated photon are selected from 183 pb −1 of data recorded at s =189 GeV. From these data, 17 W + W − γ candidates are selected with photon energy greater than 10 GeV, consistent with the Standard Model expectation. These events are used to measure the e + e − →W + W − γ cross-section within a set of geometric and kinematic cuts, σ WW γ =136±37±8 fb, where the first error is statistical and the second systematic. The photon energy spectrum is used to set the first direct, albeit weak, limits on possible anomalous contributions to the W + W − γγ and W + W − γ Z 0 vertices: −0.070 GeV −2 0 /Λ 2 GeV −2 , −0.13 GeV −2 c /Λ 2 GeV −2 , −0.61 GeV −2 n /Λ 2 GeV −2 , where Λ represents the energy scale for new physics.