Measurement of the $W^{+}{W}^{-}$ Cross Section in $\sqrt{s}=7\mathrm{TeV}$ $pp$ Collisions with ATLAS

This Letter presents a measurement of the $W^{+}{W}^{-}$ production cross section in $\sqrt{s}=7\mathrm{TeV}$ $pp$ collisions by the ATLAS experiment, using $34{\mathrm{pb}}^{-1}$ of integrated luminosity produced by the Large Hadron Collider at CERN. Selecting events with two isolated leptons, each either an electron or a muon, 8 candidate events are observed with an expected background of $1.7\pm 0.6$ events. The measured cross section is ${41}_{-16}^{+20}(\mathrm{stat})\pm 5(\mathrm{syst})\pm 1(\mathrm{lumi})\mathrm{pb}$, which is consistent with the standard model prediction of $44\pm 3\mathrm{pb}$ calculated at next-to-leading order in QCD.

Figure 1

$E_{T,\mathrm{rel}}^{\mathrm{miss}}$ distributions for the selected $ee$ and $\mu \mu$ (left) and $e\mu$ (center) events and the multiplicity distribution for jets with $p_{\mathrm{T}}>20\mathrm{GeV}$ and $|\eta |<3.0$ for all three dilepton channels combined (right). The distributions show events with all selection criteria applied except for $E_{T,\mathrm{rel}}^{\mathrm{miss}}$ in the $E_{T,\mathrm{rel}}^{\mathrm{miss}}$ distribution and the jet-veto in the jet multiplicity distribution. Simulation is used for the QCD jet and $W+\mathrm{\text{jets}}$ background contributions in these plots as opposed to the data-driven method used for $W+\mathrm{\text{jets}}$ in the signal region described in the text. The QCD jet contribution is negligible in the signal region.

Figure 2

Distributions of the leading lepton $p_T$ (left), transverse momentum of the dilepton system (center), and azimuthal angle between the leptons (right) for the sum of the selected $ee$, $\mu \mu$ and $e\mu$ samples compared to the expectation. The gray band indicates the combined statistical and systematic uncertainty on the sum of the signal and background expectations.