Study of hard double-parton scattering in four-jet events in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment

Inclusive four-jet events produced in proton--proton collisions at a centre-of-mass energy of s√=7 TeV are analysed for the presence of hard double-parton scattering using data corresponding to an integrated luminosity of 37.3 pb−1, collected with the ATLAS detector at the LHC. The contribution of hard double-parton scattering to the production of four-jet events is extracted using an artificial neural network, assuming that hard double-parton scattering can be approximated by an uncorrelated overlaying of dijet events. For events containing at least four jets with transverse momentum pT≥20 GeV and pseudorapidity η≤4.4, and at least one having pT≥42.5 GeV, the contribution of hard double-parton scattering is estimated to be fDPS=0.092+0.005−0.011(stat.)+0.033−0.037(syst.). After combining this measurement with those of the inclusive dijet and four-jet cross-sections in the appropriate phase space regions, the effective overlap area between the interacting protons, σeff, was determined to be σeff=14.9+1.2−1.0(stat.)+5.1−3.8(syst.) mb. This result is consistent within the quoted uncertainties with previous measurements of σeff, performed at centre-of-mass energies between 63 GeV and 8 TeV using various final states, and it corresponds to 21+7−6% of the total inelastic cross-section measured at sqrt(s) = 7 TeV. The distributions of the observables sensitive to the contribution of hard double-parton scattering, corrected for detector effects, are also provided.