Abstract: Inclusive four-jet events produced in proton-proton collisions at a centre-of mass energy of root 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 PT >= 20 GeV and pseudorapidity vertical bar eta vertical bar <= 4.4, and at least one having pT >= 42.5 GeV, the contribution of hard double-parton scattering is estimated to be fDps = 0.092(-0.011)(+0.0005) (stat.) (+0.03337)(-0.011) (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, sigma(eff,) was determined to be sigma(eff) = 14.9(-1.0)(+1.2) (stat.) (+5.1)(-3.8) (syst.) mb. This result is consistent within the quoted uncertainties with previous measurements of sigma(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 root s = 7 TeV. The distributions of the observables sensitive to the contribution of hard double-parton scattering, corrected for detector effects, are also provided.

Abstract: The differential cross section for the process Z/gamma* -> ll (l = e, mu) as a function of dilepton invariant mass is measured in pp collisions at root s = 7 TeV at the LHC using the ATLAS detector. The measurement is performed in the e and μchannels for invariant masses between 26 GeV and 66 GeV using an integrated luminosity of 1.6 fb(-1) collected in 2011 and these measurements are combined. The analysis is extended to invariant masses as low as 12 GeV in the muon channel using 35 pb(-1) of data collected in 2010. The cross sections are determined within fiducial acceptance regions and corrections to extrapolate the measurements to the full kinematic range are provided. Next-to-next-to-leading-order QCD predictions provide a significantly better description of the results than next-to-leading-order QCD calculations, unless the latter are matched to a parton shower calculation.

Abstract: A search with the ATLAS detector is presented for the Standard Model Higgs boson produced by vector-boson fusion and decaying to a pair of bottom quarks, using 20.2 fb(-1) of LHC proton-proton collision data at root s – 8 TeV. The signal is searched for as a resonance in the invariant mass distribution of a pair of jets containing b-hadrons in vector-boson-fusion candidate events. The yield is measured to be -0.8 +/- 2.3 times the Standard Model cross-section for a Higgs boson mass of 125 GeV. The upper limit on the cross-section times the branching ratio is found to be 4.4 times the Standard Model cross-section at the 95% confidence level, consistent with the expected limit value of 5.4 (5.7) in the background-only (Standard Model production) hypothesis.

Abstract: A search for new-physics resonances decaying into a lepton and a jet performed by the ATLAS experiment is presented. Scalar leptoquarks pair-produced inppcollisions at root s = 13 TeV at the Large Hadron Collider are considered using an integrated luminosity of 139 fb(-1), corresponding to the full Run 2 dataset. They are searched for in events with two electrons or two muons and two or more jets, including jets identified as arising from the fragmentation ofc- orb-quarks. The observed yield in each channel is consistent with the Standard Model background expectation. Leptoquarks with masses below 1.8 TeV and 1.7 TeV are excluded in the electron and muon channels, respectively, assuming a branching ratio into a charged lepton and a quark of 100%, with minimal dependence on the quark flavour. Upper limits on the aforementioned branching ratio are also given as a function of the leptoquark mass.

Abstract: The production of W boson pairs in association with one jet in pp collisions at root s = 8 TeV is studied using data corresponding to an integrated luminosity of 20.3 fb(-1) collected by the ATLAS detector during 2012 at the CERN Large Hadron Collider. The cross section is measured in a fiducial phase-space region defined by the presence of exactly one electron and one muon, missing transverse momentum and exactly one jet with a transverse momentum above 25 GeV and a pseudorapidity of vertical bar eta vertical bar < 4.5. The leptons are required to have opposite electric charge and to pass transverse momentum and pseudorapidity requirements. The fiducial cross section is found to be sigma(fid,1-jet)(WW) = 136 +/- 6 (stat) +/- 14 (syst) +/- 3 (lumi) fb. In combination with a previous measurement restricted to leptonic final states with no associated jets, the fiducial cross section of WW production with zero or one jet is measured to be sigma(fid,<1-jet)(WW) = 511 +/- 9 (stat) +/- 26 (syst) +/- 10 (lumi) fb. The ratio of fiducial cross sections in final states with one and zero jets is determined to be 0.36 +/- 0.05. Finally, a total cross section extrapolated from the fiducial measurement of WWproduction with zero or one associated jet is reported. The measurements are compared to theoretical predictions and found in good agreement.