Measurements of four-lepton production in pp collisions at √s = 8 TeV with the ATLAS detector

CERN-LHC. The four-lepton (4ℓ, ℓ=e,μ) production cross section is measured in the mass range from 80 to 1000 GeV using 20.3 fb−1
of data in pp collisions at √s=8 TeV collected with the ATLAS detector at the LHC. The 4ℓ events are produced in the decays of resonant Z
and Higgs bosons and the non-resonant ZZ continuum originating from
qq, gg, and qg initial states. A total of 476 signal candidate events are observed with a background expectation of 26.2±3.6 events, enabling the measurement of the integrated cross section and the differential cross section as a function of the invariant mass and transverse momentum of the four-lepton system. In the mass range above 180 GeV, assuming the theoretical constraint on the qq production cross section calculated with perturbative NNLO QCD and NLO electroweak corrections, the signal strength of the gluon-fusion component relative to its leading-order prediction is determined.

The experimental fiducial region for four-lepton analysis is defined as:
- 4e, 4mu or 2e2mu.
- The leading dilepton pair is selected as the one with invariant mass closest to the Z-boson mass.
- The sub-leading dilepton pair is selected among the remaining possible pairs with largest invariant mass.
- ptLepton > 6 (7) GeV for muons (electrons).
- |etaLepton| < 2.7 (2.5) for muons (electrons).
- ptDilepton > 2 GeV.
- ptLepton > 20, 15, 8 (10) GeV for leading, sub-leading and third muons (electrons).
- min(DeltaR(l,l)) > 0.1 (0.2) for same- (different-) flavour leptons.
- Leading dilepton invariant mass between 50 and 120 GeV.
- Sub-leading dilepton invariant mass between 12 and 120 GeV.
- All same-flavor opposite-sign lepton pairs should have invariant mass larger than 5 GeV.
- Four-lepton invariant mass between 80 and 1000 GeV.

UPDATE 25/04/2016: Covariance matrices were previously given without systematic uncertainties. This mistake has been corrected.