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

Inclusive four-jet events produced in proton--proton collisions at a center-of-mass energy of $\sqrt{s} = 7~\mathrm{TeV}$ have been analysed for the presence of double parton scattering using data corresponding to an integrated luminosity of $(37.3\pm1.3)~\mathrm{pb}^{-1}$, collected with the ATLAS detector at the LHC. The fraction of events arising from double parton scattering has been extracted using an artificial neural network, under the assumption that the four-jet topology originating from double parton scattering can be represented by a random combination of dijet events. The fraction was estimated to be $f_{\mathrm{DPS}} = 0.084 ^{+0.009}_{-0.012} (\mathrm{stat.}) ^{+0.062}_{-0.031} (\mathrm{syst.})$ in four-jet events, where each event contains at least four reconstructed jets with transverse momentum, $p_{\mathrm{T}} \geq 20~\mathrm{GeV}$, pseudo-rapidity, $\eta \leq 4.4$, and the highest-$p_{\text{T}}$ jet has $p_{\text{T}} \geq 42.5~\mathrm{GeV}$. Combined with the measurements of the dijet and four-jet cross-sections in the appropriate phase-space regions, the effective overlap area between the interacting protons, $\sigma_{\mathrm{eff}}$, was found to be $\sigma_{\mathrm{eff}} = 16.1 ^{+2.0}_{-1.5} (\mathrm{stat.}) ^{+6.1}_{-6.8} (\mathrm{syst.}) \mathrm{mb}$. This value is consistent within the quoted uncertainties with previous measurements of $\sigma_{\mathrm{eff}}$ at center-of-mass energies above 1 $\mathrm{TeV}$, using various final states, and it is roughly a quarter of the inelastic cross-section. A sample enriched with double parton scattering events was extracted and some characteristics of these events were studied.