The electroweak production of $ Z\left(\nu \overline{\nu}\right)\gamma $ in association with two jets is studied in a regime with a photon of high transverse momentum above 150 GeV using ...proton–proton collisions at a centre-of-mass energy of 13 TeV at the Large Hadron Collider. The analysis uses a data sample with an integrated luminosity of 139 fb$^{−1}$ collected by the ATLAS detector during the 2015–2018 LHC data-taking period. This process is an important probe of the electroweak symmetry breaking mechanism in the Standard Model and is sensitive to quartic gauge boson couplings via vector-boson scattering. The fiducial $ Z\left(\nu \overline{\nu}\right)\gamma jj $ cross section for electroweak production is measured to be $ {0.77}_{-0.30}^{+0.34} $ fb and is consistent with the Standard Model prediction. Evidence of electroweak $ Z\left(\nu \overline{\nu}\right)\gamma jj $ production is found with an observed significance of 3.2σ for the background-only hypothesis, compared with an expected significance of 3.7σ. The combination of this result with the previously published ATLAS observation of electroweak $ Z\left(\nu \overline{\nu}\right)\gamma jj $ production yields an observed (expected) signal significance of 6.3σ (6.6σ). Limits on anomalous quartic gauge boson couplings are obtained in the framework of effective field theory with dimension-8 operators.graphic not available: see fulltext
A measurement of the top-quark mass (m$_{t}$) in the $ t\overline{t} $ → lepton + jets channel is presented, with an experimental technique which exploits semileptonic decays of b-hadrons produced in ...the top-quark decay chain. The distribution of the invariant mass m$_{ℓμ}$ of the lepton, ℓ (with ℓ = e, μ), from the W-boson decay and the muon, μ, originating from the b-hadron decay is reconstructed, and a binned-template profile likelihood fit is performed to extract m$_{t}$. The measurement is based on data corresponding to an integrated luminosity of 36.1 fb$^{−1}$ of $ \sqrt{s} $ = 13 TeV pp collisions provided by the Large Hadron Collider and recorded by the ATLAS detector. The measured value of the top-quark mass is m$_{t}$ = 174.41 ± 0.39 (stat.) ± 0.66 (syst.) ± 0.25 (recoil) GeV, where the third uncertainty arises from changing the Pythia8 parton shower gluon-recoil scheme, used in top-quark decays, to a recently developed setup.graphic not available: see fulltext
A search for a new heavy scalar or pseudo-scalar Higgs boson (H/A) produced in association with a pair of top quarks, with the Higgs boson decaying into a pair of top quarks (H/A → $ t\overline{t} $) ...is reported. The search targets a final state with exactly two leptons with same-sign electric charges or at least three leptons. The analysed dataset corresponds to an integrated luminosity of 139 fb$^{−1}$ of proton–proton collisions collected at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Two multivariate classifiers are used to separate the signal from the background. No significant excess of events over the Standard Model expectation is observed. The results are interpreted in the context of a type-II two-Higgs-doublet model. The observed (expected) upper limits at 95% confidence level on the $ t\overline{t}H/A $ production cross-section times the branching ratio of H/A → $ t\overline{t} $ range between 14 (10) fb and 6 (5) fb for a heavy Higgs boson with mass between 400 GeV and 1000 GeV, respectively. Assuming that only one particle, either the scalar H or the pseudo-scalar A, contributes to the $ t\overline{t}t\overline{t} $ final state, values of tan β below 1.2 or 0.5 are excluded for a mass of 400 GeV or 1000 GeV, respectively. These exclusion ranges increase to tan β below 1.6 or 0.6 when both particles are considered.graphic not available: see fulltext
The inclusive top-quark pair ($ t\overline{t} $) production cross-section $ {\sigma}_{t\overline{t}} $ is measured in proton–proton collisions at a centre-of-mass energy $ \sqrt{s} $ = 5.02 TeV, ...using 257 pb$^{−1}$ of data collected in 2017 by the ATLAS experiment at the LHC. The $ t\overline{t} $ cross-section is measured in both the dilepton and single-lepton final states of the $ t\overline{t} $ system and then combined. The combination of the two measurements yields$ {\sigma}_{t\overline{t}}=67.5\pm 0.9\left(\textrm{stat}.\right)\pm 2.3\left(\textrm{syst}.\right)\pm 1.1\left(\textrm{lumi}.\right)\pm 0.2\left(\textrm{beam}\right)\textrm{pb}, $where the four uncertainties reflect the limited size of the data sample, experimental and theoretical systematic effects, and imperfect knowledge of both the integrated luminosity and the LHC beam energy, giving a total uncertainty of 3.9%. The result is in agreement with theoretical quantum chromodynamic calculations at next-to-next-to-leading order in the strong coupling constant, including the resummation of next-to-next-to-leading logarithmic soft-gluon terms, and constrains the parton distribution functions of the proton at large Bjorken-x.graphic not available: see fulltext
This Letter reports the observation of $WWW$ production and a measurement of its cross section using 139 fb$^{-1}$ of proton-proton collision data recorded at a center-of-mass energy of 13 TeV by the ...ATLAS detector at the Large Hadron Collider. Events with two same-sign leptons (electrons or muons) and at least two jets, as well as events with three charged leptons, are selected. A multivariate technique is then used to discriminate between signal and background events. Events from $WWW$ production are observed with a significance of 8.0 standard deviations, where the expectation is 5.4 standard deviations. The inclusive $WWW$ production cross section is measured to be $820 \pm 100\,\text{(stat)} \pm 80\,\text{(syst)}$ fb, approximately 2.6 standard deviations from the predicted cross section of $511 \pm 18$ fb calculated at next-to-leading-order QCD and leading-order electroweak accuracy.
A full-size prototype of a Micromegas precision tracking chamber for the upgrade of the ATLAS detector at the LHC Collider has been built between October 2015 and April 2016. This paper describes in ...detail the procedures followed in the construction of the components of the chamber in various INFN laboratories and the final assembly in the Laboratori Nazionali di Frascati (LNF). In addition, the results of the chamber exposure to a particle beam at SPS/H8 at CERN in June 2016 are presented. The performances obtained in the construction and the results of the test beam are compared with the requirements set in order to sustain the high radiation levels expected during the data-taking of the LHC in the next years.
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