Measurement of the WW Production Cross Section in Proton-Proton Collisions with the ATLAS Detector

. Measurements of the W + W  production using the leptonic decay channels are analysed using data collected by the ATLAS detector at the CERN Large Hadron Collider. The precise measurement of W + W-production is important for comparing to the Standard Model expectation, and also provides a detailed under-standing of this process as a background in searches for new physics


Introduction
Measurements of vector boson pair production at particle colliders provide important tests of the electroweak sector of the Standard Model (SM).Vector boson pair production at the Large Hadron Collider [1] (LHC) also represents an important source of background to Higgs boson production and to searches for physics beyond the SM.Diboson processes also provide new constraints on Triple Gauge Couplings.The latest ATLAS results on WW production are presented using data collected at centre-of-mass energy of 7 TeV.

ATLAS detector
The ATLAS detector [2] consists of an inner tracking system (inner detector, or ID) surrounded by a superconducting solenoid providing a 2 T magnetic field, electromagnetic and hadronic calorimeters, and a muon spectrometer (MS) incorporating three large superconducting toroid magnets arranged with an eight-fold azimuthal coil symmetry around the calorimeters.The ID consists of silicon pixel and microstrip detectors, surrounded by a transition radiation tracker.The electromagnetic calorimeter is a lead/liquid-argon (LAr) detector.Hadron calorimetry is based on two di↵erent detector technologies, with scintillator tiles or LAr as active media, and with either steel, copper, or tungsten as the absorber material.The MS comprises three layers of chambers for the trigger and for track measurements.

SM WW production
We present a measurement of W + W inclusive and di↵erential production cross sections in pp collisions and limits on anomalous WWZ and WW triple gauge couplings (TGCs) in purely leptonic decay channels WW !`⌫`0⌫ 0 a e-mail: jiri.hejbal@cern.chwith `,`0 = e, µ.Processes with ⌧ leptons decaying into electrons or muons with additional neutrinos are also included.Three final states are considered based on the lepton flavor, namely e + e E miss T , µ + µ E miss T and e ± µ ⌥ E miss T .Leading-order (LO) Feynman diagrams for WW production at the LHC include s-channel production with either a Z boson or a virtual photon as the mediating particle or uand t-channel quark exchange.The sand t-channel diagrams are shown in Fig. 1.
Gluon-gluon fusion processes contribute a background of about 3% to the total cross section [3].The processes involving SM Higgs Boson production (⇡ 3% for Higgs mass of 126 GeV), vector boson fusion (VBF) and double parton scattering (DPS) are not considered in estimating the contribution of background processes.cut for the ee, µµ and eµ channels, respectively [3].
ton invariant mass m ``0 and a modified missing transverse energy, E miss T, Rel , definied as: where is the di↵erence in the azimuthal angle between the Ẽmiss T and the nearest lepton or jet.The E miss T, Rel variable is designed to reject events where the apparent E miss T arises from a mismeasurement of lepton momentum or jet energy.The selection criteria applied to m ``0 and E miss T, Rel are: T, Rel > 45, 45, 25 GeV for the ee, µµ and eµ channels, respectively.With the application of the m ``0 and E miss T, Rel selection criteria, the remaining background events come mainly from t t and single top-quark processes.To reject this background contribution, events are vetoed if there is at least one jet candidate with p T > 25 GeV and |⌘| < 4.5.To further reduce the Drell-Yan contribution, the transverse momentum of the dilepton system, p T (``0), is required to be greater than 30 GeV for all three channels.
For illustration, Figure 2 shows comparison between data and predictions of the MC generators at detector level for the E miss T, Rel distribution before the E miss T, Rel cut is applied to the ee, µµ and eµ channels, respectively.

Cross sections
The WW cross section is measured in the fiducial phase space and extrapolated to the total phase space.The fiducial phase space is defined at the particle level by selection criteria similar to those used at detector level and includes the jet veto as well.The fiducial cross section for the pp !WW + X !`⌫`0⌫ 0 + X process is calculated using: where N data and N bkg are numbers of observed data events and estimated background events, respectively.C WW is defined as the ratio of the number of events satisfying all selection criteria at detector level to the number of events produced in the fiducial phase space and is estimated from simulations.L is the integrated luminosity of the data sample.
The total cross section WW for the inclusive pp !WW + X process is calculated for each channel using: where A WW represents the kinematic and geometric acceptance from the total phase space to the fiducial phase space, and BR is the branching ratio for both W bosons decaying into e⌫ or µ⌫ (including decays through ⌧ leptons with additional neutrinos).
To obtain the normalized di↵erential WW cross section in the fiducial phase space (1/ fid WW ⇥ d fid WW /dp T ), the reconstructed leading lepton p T distribution is corrected for detector e↵ects after the subtraction of background contamination and unfolded to the particle level.The resulting normalized fiducial cross sections extracted in bins of the leading lepton p T together with the SM predictions is shown in Fig. 3.

Signal and background yields
The number of events selected in data and the estimated background contributions for the three individual chan-  1.
The expected numbers of WW signal events for the individual and the combined channels are also shown.In total 1325 ``0 + E miss T candidates are observed in data with 824 ± 4 (stat) ± 69 (syst) signal events expected from the WW process and 369 ± 31 (stat) ± 53 (syst) background events expected from non-WW processes.

Background
The processes producing the ``0 + E miss T signature with no reconstructed jets in the final state are Z+jet (Drell-Yan background) when apparent E miss T is generated from the mismeasurement of the p T of the two leptons from Z boson decay; top-quark production when b-jets are not reconstructed or identified (top-quark background); W+jet production when a jet is misidentified as a lepton (W+jets background); W+ production with photon identified as an electron; W + ⇤ when one lepton is not detected; WZ !lll⌫ where one lepton not detected; ZZ !ll⌫⌫ where invariant mass is not near the Z-mass.The contribution from QCD multijet production when two jets are reconstructed as leptons is found to be negligible.

Extracted TGC Limits
The reconstructed leading lepton p T distribution is also used to set limits on anomalous WWZ and WW TGCs.The Lorentz invariant Lagrangian describing the WWZ and WW interactions has 14 independent coupling parameters.Assuming electromagnetic gauge invariance and C and P conservation, the number of independent parameters reduces to five: g Z 1 ,  Z ,  , Z and .Each of the couplings is usually modified by a factor depending on scale ⇤ with the general form ↵( ŝ) = ↵ 0 /(1 + ŝ/⇤ 2 ) 2 , where ↵ 0 is the coupling value at low energy and p ŝ is the invariant mass of the WW pair.
Table 2 shows expected and observed 95% confidence level limits on anomalous WWZ and WW couplings in  In this scenario we assume that the WWZ and WW couplings are equal (  Z =  , Z = , and g Z 1 = 1).

Summary and results
The WW inclusive production cross section in protonproton collisions at p s = 7 TeV is measured using 4.6 fb 1 of data collected with the ATLAS detector at the LHC.In total 1325 candidates are selected with an estimated background of 369 ± 61 events channels into ee, µµ and eµ final states.The combined production cross section (pp !WW + X) 51.9 ± 2.0 (stat) ± 3.9 (syst) ± 2.0 (lumi) pb, compatible with the SM NLO prediction of 44.7 +2. 1  1.9 pb.

Figure 1 .
Figure 1.SM LO Feynman diagrams for WW production through the q q initial state at the LHC for (a) the s-channel and (b) the t-channel [3].

Figure 2 .
Figure 2. Comparison between data and predictions of various generators at detector level for the E miss T, Rel distribution before the E miss T, Rel

Figure 3 .
Figure3.The normalized di↵erential WW cross section for data corrected for detector e↵ects as a function of the leading lepton p T compared to the SM prediction[3].

Table 1 .
[3]mary of observed and expected numbers of signal and background events in three individual channels and their combination[3].