A Study of W+W-gamma Events at LEP

TL;DR

This study measures the WWγ production cross-section in e+e- collisions at LEP using the OPAL detector, testing the Standard Model’s treatment of real photon radiation in WW events. By defining a restricted signal region and employing sophisticated photon identification and kinematic fitting, the analysis achieves about 10% precision across five energy bins and finds results consistent with predictions from state-of-the-art generators (KandY, RacoonWW). The work further uses the WWγ data to bound O(α) electroweak corrections affecting the W mass and to set limits on anomalous quartic gauge couplings (WWγγ, WWZγ), demonstrating LEP sensitivity to new physics in the gauge sector. Overall, the results validate SM radiative corrections in WW production, constrain systematic uncertainties relevant for precision W-mass measurements, and provide competitive limits on quartic gauge interactions. These findings contribute to the robustness of LEP electroweak analyses and inform future collider studies of multi-boson processes.

Abstract

A study of W+W- events accomanied by hard photon radiation 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 < sqrt(s) < 209 GeV. For these data, 187 W+W- candidates are selected with photon energies greater than 2.5 GeV. The selected events are used to determine the W+ W- gamma cross section at five values of sqrt(s). The results are consistent with the Standard Model expectation. These data provide constraints on the related O(alpha) systematic uncertainties on the measurement of the W boson mass at LEP. Finally, the data are used to derive 95% C.L. upper limits on possible anomalous contributions to the W+ W- gamma gamma and W+ W- Z0 gamma vertices.

Figures (6)

• Figure 1: The five kinematic variables used to classify the photon in $\hbox{$\mathrm{W^+W^-}$$\gamma$}$ events as being from ISR, FSR or associated with the jet. Unless otherwise specified the distributions are shown for $\hbox{$\mathrm{q\overline{q}}$$\ell\overline{\nu}_{\ell}$}$ and $\hbox{$\mathrm{q\overline{q}}$$\mathrm{q\overline{q}}$}$ events combined. The variables are: a) the angle between the photon and the nearest jet, $\cos\theta_{\gamma-\mathrm{JET}}$; b) the angle between the photon and the charged lepton, $\cos\theta_{\gamma \ell}$ ($\mathrm{q\overline{q}}$$\ell\overline{\nu}_{\ell}$ events only); c) the reconstructed W-boson mass under the hypothesis that the photon is associated with jet; d) the reconstructed W-boson mass under the hypothesis that the photon is from ISR; e) the reconstructed W-boson mass under the hypothesis that the photon is from FSR ($\hbox{$\mathrm{q\overline{q}}$$\ell\overline{\nu}_{\ell}$}$ only). Plot f) shows the resulting relative likelihood distribution for the ISR hypothesis. In all cases the data are shown by points with error bars, the total SM expectation is shown by the histogram and the contributions from processes other than ISR are shown by the hatched histograms.
• Figure 2: For selected $\hbox{$\mathrm{W^+W^-}$}\gamma$ events $(180~\hbox{$\mathrm{GeV}$}<\hbox{$\sqrt{s}$}<209~\hbox{$\mathrm{GeV}$})$, a) shows the photon energy spectrum, b) the modulus of the cosine of the polar angle of the photon, and c) the cosine of the angle between the photon and the nearest charged fermion. The data are shown by the points with error bars and the SM expectations (KandY) are shown by the histograms. The doubly-hatched histograms indicate the contributions from non-$\mathrm{W^+W^-}$ background and background from photons associated with the parton-shower (either FSR or from hadron decay). The singly-hatched histograms show the contributions from FSR from leptons. The expected $\hbox{$E_\gamma$}$ and $|\hbox{$\cos\theta_\gamma$}|$ distributions for an anomalous QGC of $a_0/ \Lambda^2 = 0.040~\mathrm{GeV}^{-2}$ are also shown.
• Figure 3: Measured $\hbox{$\mathrm{W^+W^-}$}\gamma$ cross-section for the signal definition of Section \ref{['sec:sigdef']}. The points with error bars show the OPAL measurements. The curve shows the SM expectation obtained from the KandY program.
• Figure 4: Reconstructed invariant mass of $\mathrm{W}^\pm\gamma$ in selected $\mathrm{W^+W^-}$$\gamma$ events with $\hbox{$E_\gamma$}>2.5$ GeV (two entries per event). The data are shown by the points, the Standard Model expectation, determined from KandY, is shown by the histogram. The singly hatched histograms show the contribution from FSR from leptons and the doubly hatched histograms show the background.
• Figure 5: Standard Model production diagram for the $\hbox{$\mathrm{W^+W^-}$}\gamma$ final states involving the $\hbox{$\mathrm{W^+W^-}$}\gamma\gamma$ and $\hbox{$\mathrm{W^+W^-}$}\hbox{${\mathrm{Z}^0}$}\gamma$ quartic gauge couplings.