Double parton scattering in four-jet events in $pp$ collisions at $\sqrt{s}$=7 TeV with the ATLAS experiment at the LHC

The dijet double-differential cross section is measured as a function of the dijet invariant mass, using data taken during 2010 and during 2011 with the ATLAS experiment at the LHC, with a center-of-mass energy, $\sqrt{s}=7$ TeV. The measurements are sensitive to invariant masses between 70 GeV and 4.27 TeV with center-of-mass jet rapidities up to 3.5. A novel technique to correct jets for pile-up (additional proton-proton collisions) in the 2011 data is developed and subsequently used in the measurement. The data are found to be consistent over 12 orders of magnitude with fixed-order NLO pQCD predictions provided by NLOJET++. The results constitute a stringent test of pQCD, in an energy regime previously unexplored. The dijet analysis is a confidence building step for the extraction of the signal of hard double parton scattering in four-jet events, and subsequent extraction of the effective overlap area between the interacting protons, expressed in terms of the variable, $\sigma_{\mathrm{eff}}$. The measurement of double parton scattering is performed using the 2010 ATLAS data. The rate of double parton scattering events is estimated using a neural network. A clear signal is observed, under the assumption that the double parton scattering signal can be represented by a random combination of exclusive dijet production. The fraction of double parton scattering candidate events is determined to be $f_\mathrm{DPS} = 0.081~\pm~0.004~\mathrm{(stat.)}~^{+\;0.025}_{-0.014}~\mathrm{(syst.)}$ in the analyzed phase-space of four-jet topologies. Combined with the measurement of the dijet and four-jet cross sections in the appropriate phase-space regions, the effective cross section is found to be $\sigma_{\mathrm{eff}} = 16.0~^{+\;0.5}_{-0.8}~\mathrm{(stat.)}~^{+\;1.9}_{-3.5}~\mathrm{(syst.)}$ mb. This result is consistent within the quoted uncertainties with previous measurements of $\sigma_{\mathrm{eff}}$ at center-of-mass energies between 63 GeV and 7 TeV, using several final states.