Abstract
Inclusive four-jet events produced in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 7$ 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 $p_{\mathrm{T}} \geq 20$ GeV and pseudorapidity $\eta \leq 4.4$, and at least one having $p_{\mathrm{T}} \geq 42.5$ GeV, the contribution of hard double-parton scattering is estimated to be $f_{\mathrm{DPS}} = 0.092 ^{+0.005}_{-0.011} (\mathrm{stat.}) ^{+0.033}_{-0.037} (\mathrm{syst.})$. 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, $\sigma_{\mathrm{eff}}$, was determined to be $\sigma_{\mathrm{eff}} = 14.9 ^{+1.2}_{-1.0} (\mathrm{stat.}) ^{+5.1}_{-3.8} (\mathrm{syst.})$ mb. This result is consistent within the quoted uncertainties with previous measurements of $\sigma_{\mathrm{eff}}$, 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 $\sqrt{s} = 7$ TeV. The distributions of the observables sensitive to the contribution of hard double-parton scattering, corrected for detector effects, are also provided.
Highlights
Incl√usive four-jet events produced in proton-proton collisions at a centre-ofmass 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
A measurement of the rate of hard double-parton scattering in four-jet events was performed using a sample of data collected with the ATLAS experiment at the LHC in 2010, with an average of approximately 0.4 proton-proton interactions per bunch crossing, corresponding to an integrated luminosity of 37.3 ± 1.3 pb−1
Summary
The differential cross-sections in eq (1.1) are rewritten for the four-jet and dijet final states and integrated over the phase space defined by the selection requirements of the dijet phase space regions. This yields the following expression for the DPS cross-section in the four-jet final state: σ4DjPS σ2Ajσ2Bj σeff. Where σ2Aj and σ2Bj are the cross-sections for dijet events in the phase space regions labelled A and B respectively. Where σ4j is the inclusive cross-section for four-jet events in the phase-space region A ⊕ B, including all four-jet final states, namely both the SPS and DPS topologies, and where fDPS represents the fraction of DPS events in these four-jet final states. To extract σeff , it is necessary to measure three cross-sections, σ2Aj, σ2Bj and σ4j, and estimate fDPS. The differential distributions of these observables are presented here
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