Abstract: The production of W-+/- Z events in proton-proton collisions at a centre-of-mass energy of 13 TeV is measured with the ATLAS detector at the LHC. The collected data correspond to an integrated luminosity of 3.2 fb(-1). The W-+/- Z candidates are reconstructed using leptonic decays of the gauge bosons into electrons or muons. The measured inclusive cross section in the detector fiducial region for leptonic decay modes is sigma(fid.)(W +/- Z -> L'vll) = 63.2 +/- 3.2 (stat.) +/- 2.6 (sys.) +/- 1.5 (lumi.) fb. In comparison, the next-to-leading-order Standard Model prediction is 53.4(-2.8)(+3.6) fb. The extrapolation of the measurement from the fiducial to the total phase space yields sigma(tot.)(W +/- Z) = 50.6 +/- 2.6 (stat.) +/- 2.0 (sys.) +/- 0.9 (th.) +/- 1.2 (lumi.) pb, in agreement with a recent next-to-next-to-leading-order calculation of 48.2(-1.0)(+1.1) pb. The cross section as a function of jet multiplicity is also measured, together with the charge-dependent W+ Z and W- Z cross sections and their ratio.

Abstract: The W boson angular distribution in events with high transverse momentum jets is measured using data collected by the ATLAS experiment from proton-proton collisions at a centre-of-mass energy root s = 8 TeV at the Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb(-1). The focus is on the contributions to W + jets processes from real W emission, which is achieved by studying events where a muon is observed close to a high transverse momentum jet. At small angular separations, these contributions are expected to be large. Various theoretical models of this process are compared to the data in terms of the absolute cross-section and the angular distributions of the muon from the leptonic W decay.

Abstract: Measurements of dijet Pt correlations in Pb+Pb and pp collisions at a nucleon-nucleon centre-of-mass energy of root s(NN) = 2.76 TeV are presented. The measurements are performed with the ATLAS detector at the Large Hadron Collider using Pb+Pb and pp data samples corresponding to integrated luminosities of 0.14 nb(-1)and 4.0 pb(-1) respectively. Jets are reconstructed using the anti-k(t) algorithm with radius parameter values R = 0.3 and R = 0.4. A background subtraction procedure is applied to correct the jets for the large underlying event present in Pb+Pb collisions. The leading and sub-leading jet transverse momenta are denoted p(T1) and p(T2). An unfolding procedure is applied to the two-dimensional (p(T1) , p(T2)) distributions to account for experimental effects in the measurement of both jets. Distributions of (1/N)dN/dx(J), where X-J = p(T2)/p(T1) , are presented as a function of p(T1) and collision centrality. The distributions are found to be similar in peripheral Pb+Pb collisions and pp collisions, but highly modified in central Pb+Pb collisions. Similar features are present in both the R = 0.3 and R = 0.4 results, indicating that the effects of the underlying event are properly accounted for in the measurement. The results are qualitatively consistent with expectations from partonic energy loss models.

Abstract: A search for the Standard Model Higgs boson has been performed in the H -> WW -> lvjj channel using 4.7 fb(-1) of pp collision data recorded at a centre-of-mass energy of root s = 7 TeV with the ATLAS detector at the Large Hadron Collider. Higgs boson candidates produced in association with zero, one or two jets are included in the analysis to maximize the acceptance for both gluon fusion and weak boson fusion Higgs boson production processes. No significant excess of events is observed over the expected background and limits on the Higgs boson production cross section are derived for a Higgs boson mass in the range 300 GeV < m(H) < 600 GeV. The best sensitivity is reached for m(H) = 400 GeV, where the observed (expected) 95% confidence level upper bound on the cross section for H -> WW produced in association with zero or one jet is 2.2 pb (1.9 pb), corresponding to 1.9 (1.6) times the Standard Model prediction. In the Higgs boson plus two jets channel, which is more sensitive to the weak boson fusion process, the observed (expected) 95% confidence level upper bound on the cross section for H -> WW production with m(H) = 400 GeV is 0.7 pb (0.6 pb), corresponding to 7.9 (6.5) times the Standard Model prediction.

Abstract: A search for the decay of the Higgs boson to a Z boson and a light, pseudoscalar particle, a, decaying respectively to two leptons and to two photons is reported. The search uses the full LHC Run 2 proton-proton collision data at root s = 13 TeV, corresponding to 139 fb-1 collected by the ATLAS detector. This is one of the first searches for this specific decay mode of the Higgs boson, and it probes unexplored parameter space in models with axion-like particles (ALPs) and extended scalar sectors. The mass of the a particle is assumed to be in the range 0.1-33 GeV. The data are analysed in two categories: a merged category where the photons from the a decay are reconstructed in the ATLAS calorimeter as a single cluster, and a resolved category in which two separate photons are detected. The main background processes are from Standard Model Z boson production in association with photons or jets. The data are in agreement with the background predictions, and upper limits on the branching ratio of the Higgs boson decay to Za times the branching ratio a -> yy are derived at the 95% confidence level and they range from 0.08% to 2% depending on the mass of the a particle. The results are also interpreted in the context of ALP models.