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 %

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Summary

Introduction

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]

The OPAL Detector
Data Sample
Monte Carlo
Photon Identification
Photon Acceptance
Kinematic Requirements
Systematic Uncertainties
QED and Electroweak Corrections in KandY
Anomalous Quartic Gauge Boson Couplings
Theoretical Framework
Experimental Limits
Conclusions
Findings
10 Appendix
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