Search for additional heavy neutral Higgs and gauge bosons in the ditau final state produced in 36 fb −1 of pp collisions at √ s = 13 TeV with the ATLAS detectorThe ATLAS collaboration E-mail: atlas.publications@cern.ch Abstract: A search for heavy neutral Higgs bosons and Z bosons is performed using a data sample corresponding to an integrated luminosity of 36.1 fb −1 from proton-proton collisions at √ s = 13 TeV recorded by the ATLAS detector at the LHC during 2015 and 2016. The heavy resonance is assumed to decay to τ + τ − with at least one tau lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2-2.25 TeV for Higgs bosons and 0.2-4.0 TeV for Z bosons. The data are in good agreement with the background predicted by the Standard Model. The results are interpreted in benchmark scenarios. In the context of the hMSSM scenario, the data exclude tan β > 1.0 for m A = 0.25 TeV and tan β > 42 for m A = 1.5 TeV at the 95% confidence level. For the Sequential Standard Model, Z SSM with m Z < 2.42 TeV is excluded at 95% confidence level, while Z NU with m Z < 2.25 TeV is excluded for the non-universal G(221) model that exhibits enhanced couplings to third-generation fermions. 6 Background estimation 10 6.1 Jet background estimate in the τ had τ had channel 10 6.1. The ATLAS collaboration 37-1 - JHEP01(2018)0551 IntroductionThe discovery of a scalar particle [1, 2] at the Large Hadron Collider (LHC) [3] has provided important insight into the mechanism of electroweak symmetry breaking. Experimental studies of the new particle [4][5][6][7][8] demonstrate consistency with the Standard Model (SM) Higgs boson [9][10][11][12][13][14]. However, it remains possible that the discovered particle is part of an extended scalar sector, a scenario that is predicted by a number of theoretical arguments [15,16]. The Minimal Supersymmetric Standard Model (MSSM) [15,17,18] is the simplest extension of the SM that includes supersymmetry. The MSSM requires two Higgs doublets of opposite hypercharge. Assuming that CP symmetry is conserved, this results in one CPodd (A) and two CP-even (h, H) neutral Higgs bosons and two charged Higgs bosons (H ± ). At tree level, the properties of the Higgs sector in the MSSM depend on only two non-SM parameters, which can be chosen to be the mass of the CP-odd Higgs boson, m A , and the ratio of the vacuum expectation values of the two Higgs doublets, tan β. Beyond tree level, a number of additional parameters affect the Higgs sector, the choice of which defines various MSSM benchmark scenarios. In the m mod+ h scenario [19], the top-squark mixing parameter is chosen such that the mass of the lightest CP-even Higgs boson, m h , is close to the measured mass of the Higgs boson that was discovered at the LHC. A different approach is employed in the hMSSM scenario [20,21] in which the measured value of m h can be used, with certain assumptions, to predict the remaining masses and couplings of the MSSM Higgs bosons without explicit reference to the sof...

Search for pair production of Higgs bosons in the bbbb final state using proton-proton collisions at √ s = 13 TeV with the ATLAS detectorThe ATLAS Collaboration A search for Higgs boson pair production in the bbbb final state is carried out with up to 36.1 fb −1 of LHC proton-proton collision data collected at √ s = 13 TeV with the ATLAS detector in 2015 and 2016. Three benchmark signals are studied: a spin-2 graviton decaying into a Higgs boson pair, a scalar resonance decaying into a Higgs boson pair, and Standard Model non-resonant Higgs boson pair production. Two analyses are carried out, each implementing a particular technique for the event reconstruction that targets Higgs bosons reconstructed as pairs of jets or single boosted jets. The resonance mass range covered is 260-3000 GeV. The analyses are statistically combined and upper limits on the production cross section of Higgs boson pairs times branching ratio to bbbb are set in each model. No significant excess is observed; the largest deviation of data over prediction is found at a mass of 280 GeV, corresponding to 2.3 standard deviations globally. The observed 95% confidence level upper limit on the non-resonant production is 13 times the Standard Model prediction.The discovery of the Standard Model (SM) Higgs boson (H) [1, 2] at the Large Hadron Collider (LHC) motivates searches for new physics using the Higgs boson as a probe. In particular, many models predict cross sections for Higgs boson pair production that are significantly greater than the SM prediction. Resonant Higgs boson pair production is predicted by models such as the bulk Randall-Sundrum model [3,4], which features spin-2 Kaluza-Klein gravitons, G KK , that subsequently decay to pairs of Higgs bosons. Extensions of the Higgs sector, such as two-Higgs-doublet models [5,6], propose the existence of a heavy spin-0 scalar that can decay into H pairs. Enhanced non-resonant Higgs boson pair production is predicted by other models, for example those featuring light coloured scalars [7] or direct ttHH vertices [8,9].Previous searches for Higgs boson pair production have all yielded null results. In the bbbb channel, ATLAS searched for both non-resonant and resonant production in the mass range 400-3000 GeV using 3.2 fb −1 of 13 TeV data [10] collected during 2015. CMS searched for the production of resonances with masses 750-3000 GeV using 13 TeV data [11] and with masses 270-1100 GeV with 8 TeV data [12]. Using 8 TeV data, ATLAS has examined the bbbb [13], bbγγ [14], bbτ + τ − and W + W − γγ channels, all of which were combined in Ref. [15]. CMS has performed searches using 13 TeV data for the bbτ + τ − [16] and bb ν ν [17] final states, and used 8 TeV data to search for bbγγ [18] in addition to a search in multilepton and multilepton+photons final states [19].The analyses presented in this paper exploit the decay mode with the largest branching ratio, H → bb, to search for Higgs boson pair production in both resonant and non-resonant production. Two analyses, which are complementary in the...

A measurement of the four-lepton invariant mass spectrum is made with the ATLAS detector, using an integrated luminosity of 36.1 fb −1 of proton-proton collisions at √ s = 13 TeV delivered by the Large Hadron Collider. The differential cross-section is measured for events containing two same-flavour opposite-sign lepton pairs. It exhibits a rich structure, with different mass regions dominated in the Standard Model by single Z boson production, Higgs boson production, and Z boson pair production, and non-negligible interference effects at high invariant masses. The measurement is compared with state-of-the-art Standard Model calculations, which are found to be consistent with the data. These calculations are used to interpret the data in terms of gg → Z Z → 4 and Z → 4 subprocesses, and to place constraints on a possible contribution from physics beyond the Standard Model.

A determination of the top-quark mass is presented using 20.2 fb −1 of 8 TeV proton-proton collision data produced by the Large Hadron Collider and collected by the ATLAS experiment. The normalised differential cross section of top-quark pair production in association with an energetic jet is measured in the lepton+jets final state and unfolded to parton and particle levels. The unfolded distribution at parton level can be described using next-to-leading-order QCD predictions in terms of either the top-quark pole mass or the running mass as defined in the (modified) minimal subtraction scheme. A comparison between the experimental distribution and the theoretical prediction allows the top-quark mass to be extracted in the two schemes. The value obtained for the pole-mass scheme is: m pole t = 171.1 ± 0.4 (stat) ± 0.9 (syst) +0.7 −0.3 (theo) GeV. The extracted value in the running-mass scheme is: m t (m t) = 162.9 ± 0.5 (stat) ± 1.0 (syst) +2.1 −1.2 (theo) GeV. The results for the top-quark mass using the two schemes are consistent, when translated from one scheme to the other.

The ATLAS CollaborationA search for vectorlike quarks is presented, which targets their decay into a Z boson and a third-generation Standard Model quark. In the case of a vectorlike quark T (B) with charge + 2 /3e (− 1 /3e), the decay searched for is T → Zt (B → Z b). Data for this analysis were taken during 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 fb −1 of pp collisions at √ s = 13 TeV. The final state used is characterized by the presence of b-tagged jets, as well as a Z boson with high transverse momentum, which is reconstructed from a pair of opposite-sign same-flavor leptons. Pair and single production of vectorlike quarks are both taken into account and are each searched for using optimized dileptonic exclusive and trileptonic inclusive event selections. In these selections, the high scalar sum of jet transverse momenta, the presence of high-transversemomentum large-radius jets, as well as-in the case of the single-production selections-the presence of forward jets are used. No significant excess over the background-only hypothesis is found and exclusion limits at 95% confidence level allow masses of vectorlike quarks of m T > 1030 GeV (m T > 1210 GeV) and m B > 1010 GeV (m B > 1140 GeV) in the singlet (doublet) model. In the case of 100% branching ratio for T → Zt (B → Z b), the limits are m T > 1340 GeV (m B > 1220 GeV). Limits at 95% confidence level are also set on the coupling to Standard Model quarks for given vectorlike quark masses.