Measurements of $W^\pm Z$ production cross sections in $pp$ collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector and limits on anomalous gauge boson self-couplings

TL;DR

<3-5 sentence high-level summary> This ATLAS study measures $W^{\pm}Z$ production in $pp$ collisions at $\sqrt{s}=8$ TeV using leptonic decays, delivering fiducial and total cross sections, plus differential spectra and cross-channel ratios. It combines four leptonic channels, unfolds detector effects, and assesses systematic uncertainties to compare with SM predictions (NLO QCD and VBS NLO) and to extract limits on anomalous triple and quartic gauge couplings. The analysis also probes vector boson scattering in $WZjj$ final states, setting upper limits on the VBS cross section and constraining new physics in a model-independent EFT framework. Overall, the results are consistent with the SM within uncertainties and provide some of the most stringent constraints to date on aTGC and aQGC parameters in the $WWZ$ sector.

Abstract

This paper presents measurements of $W^\pm Z$ production in $pp$ collisions at a center-of-mass energy of 8 TeV. The gauge bosons are reconstructed using their leptonic decay modes into electrons and muons. The data were collected in 2012 by the ATLAS experiment at the Large Hadron Collider, and correspond to an integrated luminosity of 20.3 fb$^{-1}$. The measured inclusive cross section in the detector fiducial region is $σ_{W^\pm Z \rightarrow \ell^{'} ν \ell \ell} = 35.1 \pm$ 0.9 (stat.) $\pm 0.8$ (sys.) $\pm 0.8$ (lumi.) fb, for one leptonic decay channel. In comparison, the next-to-leading-order Standard Model expectation is 30.0 $\pm$ 2.1 fb. Cross sections for $W^+Z$ and $W^-Z$ production and their ratio are presented as well as differential cross sections for several kinematic observables. Limits on anomalous triple gauge boson couplings are derived from the transverse mass spectrum of the $W^\pm Z$ system. From the analysis of events with a $W$ and a $Z$ boson associated with two or more forward jets an upper limit at 95% confidence level on the $W^\pm Z$ scattering cross section of 0.63 fb, for each leptonic decay channel, is established, while the Standard Model prediction at next-to-leading order is 0.13 fb. Limits on anomalous quartic gauge boson couplings are also extracted.

Figures (16)

• Figure 1: Distributions, summed over all channels, of the following kinematic variables: (a) the transverse momentum of the reconstructed $Z$ boson $p_\textrm{T}^Z$, (b) the mass of the $Z$$m_Z$, (c) the transverse mass of the reconstructed $W$ boson $m_\textrm{T}^W$ and (d) the transverse-mass like variable for the $WZ$ system $m_\textrm{T}^{WZ}$. The points correspond to the data and the histograms to the expectations of the different SM processes. All Monte Carlo expectations are scaled to the integrated luminosity of the data using the predicted MC cross sections of each sample. The sum of background events containing misidentified leptons is labeled "Misid. leptons". The Powheg+Pythia MC prediction is used for the $W^{\pm}Z$ signal contribution. It is scaled by a global factor of $1.17$ to match the measured inclusive $W^{\pm}Z$ cross section. The open red histogram shows the total prediction and the shaded orange band its estimated total uncertainty. The last bin contains the overflow.
• Figure 2: Ratio of the measured $W^{\pm}Z$ integrated cross sections in the fiducial phase space to the NLO SM prediction from Powheg+Pythia using the CT10 PDF set and renormalisation and factorisation scales $\mu_R = \mu_F = m_{WZ}/2$, in each of the four channels and for their combination. The inner and outer error bars on the data points represent the statistical and total uncertainties, respectively. The shaded orange band represents the uncertainty associated with the SM prediction.
• Figure 3: Measured ratios $\sigma^{\mathrm{fid.}}_{W^{+}Z} / \sigma^{\mathrm{fid.}}_{W^{-}Z}$ of $W^{+}Z$ and $W^{-}Z$ integrated cross sections in the fiducial phase space in each of the four channels and for their combination. The error bars on the data points represent the total uncertainties, dominated by statistical uncertainties. The NLO SM prediction from Powheg+Pythia using the CT10 PDF set and renormalisation and factorisation scales $\mu_R = \mu_F = m_{WZ}/2$ is represented by the red line and the shaded orange band for the associated uncertainty. The Powheg+Pythia prediction using the ATLAS-epWZ12 PDF set is also displayed as the dashed line.
• Figure 4: The measured $W^{\pm}Z$ differential cross section in the fiducial phase space as a function of (a) $p_\textrm{T}^Z$ and (b) $p_\textrm{T}^W$. The inner and outer error bars on the data points represent the statistical and total uncertainties, respectively. The measurements are compared to the prediction from Powheg+Pythia (red line, see text for details). The orange band represents its total theoretical uncertainty and the hatched red area the part of the theoretical uncertainty arising from the PDF and parton shower uncertainties. The predictions from the MC@NLO and Sherpa MC generators are also indicated by dashed and dotted-dashed lines, respectively. The Sherpa prediction is rescaled to the integrated cross section predicted by Powheg+Pythia. The right $y$-axis refers to the last cross-section point, separated from the others by a vertical dashed line, as this last bin is integrated up to the maximum value reached in the phase space.
• Figure 5: The measured $W^{\pm}Z$ differential cross section in the fiducial phase space as a function of $m_\textrm{T}^{WZ}$. The inner and outer error bars on the data points represent the statistical and total uncertainties, respectively. The measurements are compared to the prediction from Powheg+Pythia (red line, see text for details). The orange band represents its total theoretical uncertainty and the hatched red area the part of the theoretical uncertainty arising from the PDF and shower uncertainties. The predictions from the MC@NLO and Sherpa MC generators are also indicated by dashed and dotted-dashed lines, respectively. The Sherpa prediction is rescaled to the integrated cross section predicted by Powheg+Pythia. The right $y$-axis refers to the last cross-section point, separated from the others by a vertical dashed line, as this last bin is integrated up to the maximum value reached in the phase space.