Study of hard double-parton scattering in four-jet events in pp collisions at √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 p _T ≥ 20 GeV and pseudorapidity |η| ≤ 4.4, and at least one having p _T ≥ 42.5 GeV, the contribution of hard double-parton scattering is estimated to be f _DPS = 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 cross-section, σ _eff , was determined to be σ _eff = 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 σ _eff , 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 √s = 7 TeV. The distributions of the observables sensitive to the contribution of hard double-parton scattering, corrected for detector effects, are also provided.