Measurements of the ZZ production cross sections in the 2 l 2 nu channel in proton-proton collisions at sqrt(s) = 7 and 8 TeV and combined constraints on triple gauge couplings

The CMS collaboration
Eur.Phys.J.C 75 (2015) 511, 2015.

Abstract (data abstract)
CERN-LHC. Measurements of the ZZ production cross section in proton-proton collisions at center-of-mass energies of 7 and 8 TeV are presented. Candidate events for the leptonic decay mode ZZ->2l2nu, where l denotes an electron or a muon, are reconstructed and selected from data corresponding to an integrated luminosity of 5.1 (19.6)/fb at 7 (8) TeV collected with the CMS experiment. The selected data are analyzed to search for anomalous triple gauge couplings involving the ZZ final state. These limits are then combined with the previously published CMS results for ZZ in 4l final states. We define a reduced missing transverse energy (reduced-MET) variable: - The transverse momenta (pT) of all the individual particle candidates in the event and of the clustered jets are decomposed along an orthogonal set of axes in the transverse plane of the detector. One of the axes is defined by the pT of the charged dilepton system (pT(ll)), the other perpendicular to it. - We define the recoil of the l+l' system in two different ways: (1) the clustered recoil (Rc) is the vectorial sum of the momenta of the jets reconstructed in the event (2) the unclustered recoil (Ru) is the vectorial sum of the transverse momenta of all particle candidates in the event, with the exception of the two leptons. - On each axis (i = parallel/orthogonal to the dilepton system pT), the reduced MET projection is defined as: reduced-MET_i = - pT(ll)_i - Rc/u_i, where Rc/u_i represents the choice of Rc or Ru that minimizes the absolute value of that reduced-MET component, and pT(ll)_i is a projection of the transverse momentum of the Z boson. - The absolute reduced-MET variable is the sum in quadrature of the two components. Selection is defined as: - 2l (l=e+,e-,mu+,mu-). No opposite charge sign is required. - pT(mu) > 20 GeV; |eta(mu)| < 2.4 - pT(e) > 20 GeV; |eta(e)| < 2.5 - Jet: particle-flow (PF) candidates, using the anti-kT algorithm with a distance parameter of 0.5 (pT > 20 GeV). - |Dilepton invariant mass - 91| < 7.5 GeV - pT(ll) > 45 GeV - Events containing jets with pT > 20 GeV and identified as b-quark jets are rejected - Events containing jets with pT > 30 GeV are rejected - Reduced-MET > 65 GeV - 0.4 > MET/pT(ll) > 1.8 - DeltaPhi(MET,jet) > 0.5 rad - DeltaPhi(MET,lepton) > 0.2 rad - Events containing additional leptons with pT > 10 GeV (electron) or 3 GeV (muon) are rejected.

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