Measurement of the WW cross section in sqrt(s) = 7 TeV pp collisions with ATLAS

TL;DR

Measuring the $W^+W^-$ production cross section in $\sqrt{s}=7$ TeV $pp$ collisions using ATLAS with $34~\mathrm{pb}^{-1}$ of data. WW events are reconstructed in the leptonic $\ell\ell\nu\nu$ channel, with two isolated leptons and missing transverse energy; primary backgrounds include $W$+jets, Drell–Yan, top, and other dibosons. Eight events are observed, with an expected background of $1.7\pm0.6$ events, yielding a measured cross section of $\sigma_{W^+W^-} = 41^{+20}_{-16}$ (stat.) $\pm 5$ (syst.) $\pm 1$ (lumi.)~pb, compatible with the SM prediction of $44 \pm 3$ pb at NLO in QCD. The observation corresponds to a $3.0$-sigma significance, and the result confirms $W^+W^-$ production at the LHC, providing a baseline for Higgs searches and studies of triple gauge couplings.

Abstract

This Letter presents a measurement of the $\WW$ production cross section in $\sqrt{s} = 7 \TeV\ pp$ collisions by the ATLAS experiment, using 34 $\pbi$ 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 $1.7\pm0.6$ events. The measured cross section is $41^{+20}_{-16}(\mathrm{stat.})\pm 5(\mathrm{syst.})\pm 1(\mathrm{lumi.})$ pb, which is consistent with the standard model prediction of $44 \pm 3$ pb calculated at next-to-leading order in QCD.

Figures (2)

• Figure 1: $E_{\mathrm{T,rel}}^{\mathrm{\mathrm{miss}}}$ distributions for the selected $ee$ and $\mu\mu$ (left) and $e\mu$ (center) events and the multiplicity distribution for jets with $p_{\mathrm{T}}\xspace>20$ GeV and $|\eta|<3.0$ for all three dilepton channels combined (right). The distributions show events with all selection criteria applied except for $E_{\mathrm{T,rel}}^{\mathrm{\mathrm{miss}}}$ in the $E_{\mathrm{T,rel}}^{\mathrm{\mathrm{miss}}}$ distribution and the jet veto in the jet multiplicity distribution. Simulation is used for the QCD jet and $W$+jets background contributions in these plots as opposed to the data-driven method used for $W$+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 $p_{\mathrm{T}}\xspace$ (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.