Abstract
A study of W+W− events accompanied by hard photon radiation, Eγ>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<209 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−Z0γ vertices: −0.020 GeV−2<a0Λ2<0.020 GeV−2,−0.053 GeV−2<acΛ2<0.037 GeV−2,−0.16 GeV−2<anΛ2<0.15 GeV−2, where Λ represents the energy scale for new physics and a0, ac and an are dimensionless coupling constants.
Highlights
The W+W− pair production cross-section has been precisely measured at LEP over a range of centre-of-mass energies [1,2,3]
This paper presents a study of the process e+e− → W+W−γ and probes the modelling of real photonic corrections to the W+W− pair creation process
The difference between the cross-section for the full set of 4f γ diagrams relative to cross-section for the CC03 diagrams alone is less than 0.5 %
Summary
The W+W− pair production cross-section has been precisely measured at LEP over a range of centre-of-mass energies [1,2,3]. The inclusion of real and virtual photonic corrections in the YFSWW [4] and RacoonWW [5] programs has reduced the theoretical uncertainty on the CC03 e+e− → W+W− cross-section to below 0.5 % [6] Uncertainties in these O(α) corrections may lead to small, but non-negligible, systematic uncertainties in the determination of the W-boson mass, MW, at LEP [7]. The measurements of the W+W−γ cross-section are compared with the predictions of the KORALW [8], KandY [9] (the concurrent Monte Carlo KORALW1.51 and YFSWW3) and RacoonWW [5] programs These comparisons are used to obtain the first data-driven estimate of the systematic uncertainty on MW due to the Monte Carlo description of real photon radiation in W+W− events. These limits are more than a factor three tighter than previous OPAL results from e+e− → W+W−γ [10] and are consistent with other measurements [11]
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