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 − 0.011 + 0.005 (stat.) − 0.037 + 0.033 (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 − 1.0 + 1.2 (stat.) − 3.8 + 5.1 (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 − 6 + 7  % 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.