Abstract: Several extensions of the Standard Model predict the production of dark matter particles at the LHC. An uncharted signature of dark matter particles produced in association with VV = (WW -/+)-W-+/- or ZZ pairs from a decay of a dark Higgs boson s is searched for using 139 fb(-1) of pp collisions recorded by the ATLAS detector at a center-of-mass energy of 13 TeV. The s -> V(q (q) over bar )V(q (q) over bar) decays are reconstructed with a novel technique aimed at resolving the dense topology from boosted VV pairs using jets in the calorimeter and tracking information. Dark Higgs scenarios with m(s) > 160 GeV are excluded.

Abstract: A search is presented for the pair production of heavy vector-like B quarks, primarily targeting B quark decays into a W boson and a top quark. The search is based on 36.1 fb(-1) of pp collisions at root s = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton-plus-jets final state, characterised by a high-transverse-momentum isolated electron or muon, large missing transverse momentum, and multiple jets, of which at least one is b-tagged. No significant deviation from the Standard Model expectation is observed. The 95% confidence level lower limit on the B mass is 1350 GeV assuming a 100% branching ratio to Wt. In the SU(2) singlet scenario, the lower mass limit is 1170 GeV. The 100% branching ratio limits are found to be also applicable to heavy vector-like X production, with charge +5/3, that decay into Wt. This search is also sensitive to a heavy vector-like B quark decaying into other final states (Zb and Hb) and thus mass limits on B production are set as a function of the decay branching ratios.

Abstract: This paper presents the combinations of single-top-quark production cross-section measurements by the ATLAS and CMS Collaborations, using data from LHC proton-proton collisions at = 7 and 8 TeV corresponding to integrated luminosities of 1.17 to 5.1 fb(-1) at = 7 TeV and 12.2 to 20.3 fb(-1) at = 8 TeV. These combinations are performed per centre-of-mass energy and for each production mode: t-channel, tW, and s-channel. The combined t-channel cross-sections are 67.5 +/- 5.7 pb and 87.7 +/- 5.8 pb at = 7 and 8 TeV respectively. The combined tW cross-sections are 16.3 +/- 4.1 pb and 23.1 +/- 3.6 pb at = 7 and 8 TeV respectively. For the s-channel cross-section, the combination yields 4.9 +/- 1.4 pb at = 8 TeV. The square of the magnitude of the CKM matrix element V-tb multiplied by a form factor f(LV) is determined for each production mode and centre-of-mass energy, using the ratio of the measured cross-section to its theoretical prediction. It is assumed that the top-quark-related CKM matrix elements obey the relation |V-td|, |V-ts| << |V-tb|. All the |f(LV)V(tb)|(2) determinations, extracted from individual ratios at = 7 and 8 TeV, are combined, resulting in |f(LV)V(tb)| = 1.02 +/- 0.04 (meas.) +/- 0.02 (theo.). All combined measurements are consistent with their corresponding Standard Model predictions.

Abstract: A search for exotic decays of the Higgs boson into a pair of spin-zero particles, H -> aa, where the a-boson decays into b-quarks promptly or with a mean proper lifetime c tau(a) up to 6 mm and has a mass in the range of 20-60GeV, is presented. The search is performed in events where the Higgs boson is produced in association with a W or Z boson, giving rise to a signature of one or two charged leptons (electrons or muons) and multiple jets from b-quark decays. The analysis is based on the dataset of proton-proton collisions at root s = 13TeV recorded in 2015 and 2016 by the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 36: 1 fb(-1). No significant excess of events above the Standard Model background prediction is observed, and 95% confidence-level upper limits are derived for the production cross-sections for pp -> WH, ZH and their combination, times the branching ratio of the decay chain H -> aa -> 4b. For a-bosons which decay promptly, the upper limit on the combination of cross-sections for WH and ZH times the branching ratio of H -> aa -> 4b ranges from 3.0 pb for m(a) = 20 GeV to 1.3 pb for m(a) = 60 GeV, assuming that the ratio of WH to ZH cros-ssections follows the Standard Model prediction. For a-bosons with longer proper lifetimes, the most stringent limits are 1.8 pb and 0.68 pb, respectively, at c tau(a) similar to 0.4 mm.

Abstract: A search for the associated production of the Higgs boson with a top quark pair ((tt) over barH) is reported. The search is performed in multilepton final states using a data set corresponding to an integrated luminosity of 36.1 fb(-1) of proton-proton collision data recorded by the ATLAS experiment at a center-of-mass energy root s = 13 TeV at the Large Hadron Collider. Higgs boson decays to WW*, tau tau, and ZZ* are targeted. Seven final states, categorized by the number and flavor of charged-lepton candidates, are examined for the presence of the Standard Model Higgs boson with a mass of 125 GeVand a pair of top quarks. An excess of events over the expected background from Standard Model processes is found with an observed significance of 4.1 standard deviations, compared to an expectation of 2.8 standard deviations. The best fit for the (tt) over barH production cross section is sot (tt) over barH) = 790(-210)(+230) fb, in agreement with the Standard Model prediction of 507(-50)(+35) fb. The combination of this result with other t <overline> tH searches from the ATLAS experiment using the Higgs boson decay modes to b (b) over bar, gamma gamma and ZZ* -> 4l, has an observed significance of 4.2 standard deviations, compared to an expectation of 3.8 standard deviations. This provides evidence for the (tt) over barH production mode.