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A search for new particles decaying into a pair of top quarks is performed using proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at a center-ofmass energy of √ s = 13 TeV corresponding to an integrated luminosity of 36.1 fb −1 . Events consistent with top-quark pair production and the fully hadronic decay mode of the top quarks are selected by requiring multiple high transverse momentum jets including those containing b-hadrons. Two analysis techniques, exploiting dedicated top-quark pair reconstruction in different kinematic regimes, are used to optimize the search sensitivity to new hypothetical particles over a wide mass range. The invariant mass distribution of the two reconstructed top-quark candidates is examined for resonant production of new particles with various spins and decay widths. No significant deviation from the Standard Model prediction is observed and limits are set on the production cross-section times branching fraction for new hypothetical Z bosons, dark-matter mediators, Kaluza-Klein gravitons and Kaluza-Klein gluons. By comparing with the predicted production cross sections, the Z boson in the topcolor-assisted-technicolor model is excluded for masses up to 3.1-3.6 TeV, the dark-matter mediators in a simplified framework are excluded in the mass ranges from 0.8 to 0.9 TeV and from 2.0 to 2.2 TeV, and the Kaluza-Klein gluon is excluded for masses up to 3.4 TeV, depending on the decay widths of the particles.

The production of a prompt photon in association with a Z boson is studied in proton-proton collisions at a centre-of-mass energy √ s = 13 TeV. The analysis uses a data sample with an integrated luminosity of 139 fb −1 collected by the ATLAS detector at the LHC from 2015 to 2018. The production cross-section for the process pp → + − γ + X (= e, µ) is measured within a fiducial phase-space region defined by kinematic requirements on the photon and the leptons, and by isolation requirements on the photon. An experimental precision of 2.9% is achieved for the fiducial cross-section. Differential crosssections are measured as a function of each of six kinematic variables characterising the + − γ system. The data are compared with theoretical predictions based on next-toleading-order and next-to-next-to-leading-order perturbative QCD calculations. The impact of next-to-leading-order electroweak corrections is also considered.

A search for the decays of the Higgs and Z bosons to a ϕ meson and a photon is performed with a pp collision data sample corresponding to an integrated luminosity of 2.7  fb^{-1} collected at sqrt[s]=13  TeV with the ATLAS detector at the LHC. No significant excess of events is observed above the background, and 95% confidence level upper limits on the branching fractions of the Higgs and Z boson decays to ϕγ of 1.4×10^{-3} and 8.3×10^{-6}, respectively, are obtained.

The ATLAS CollaborationResults of a search for gluino pair production with subsequent R-parity-violating decays to quarks are presented. This search uses 36.1 fb −1 of data collected by the ATLAS detector in proton-proton collisions with a center-of-mass energy of √ s = 13 TeV at the LHC. The analysis is performed using requirements on the number of jets and the number of jets tagged as containing a b-hadron as well as a topological observable formed by the scalar sum of masses of large-radius jets in the event. No significant excess above the expected Standard Model background is observed. Limits are set on the production of gluinos in models with the R-parity-violating decays of either the gluino itself (direct decay) or the neutralino produced in the R-parity-conserving gluino decay (cascade decay). In the gluino cascade decay model, gluino masses below 1850 GeV are excluded for 1000 GeV neutralino mass. For the gluino direct decay model, the 95% confidence level upper limit on the cross section times branching ratio varies between 0.80 fb at mg = 900 GeV and 0.011 fb at mg = 1800 GeV. 1 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point in the centre of the detector and the z-axis along the beam direction. The x-axis points toward the centre of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the beam pipe. The pseudorapidity η is defined in terms of the polar angle θ by η ≡ − ln[tan(θ/2)]. Analysis strategyThe analysis uses a kinematic observable, the total jet mass, M Σ J [64-66], as the primary discriminating variable to separate signal and background. The observable M Σ J is defined as the sum of the masses of the four leading large-R jets.The use of M Σ J in this analysis provides an opportunity to employ the fully data-driven jet mass template method to estimate the background contribution in signal regions. The jet mass template method is discussed in Ref. [66], and its first experimental implementation is described in Ref. [21]. In this method, single-jet mass templates are extracted from signal-depleted control regions. These jet mass templates are created in bins that are defined by a number of observables, which include jet p T and |η|, and the b-matching status. They provide a probability density function that describes the relative probability for a jet with a given p T and η to have a certain mass. This method assumes that jet mass templates only depend on these observables and are the same in the control regions and signal regions. A sample where the background M Σ J distribution needs to be estimated, such as a validation region or a signal region, is referred to as the kinematic sample. The only information used is the jet p T and η, as well as its b-matching status, which are inputs to the templates. For each jet in the kinematic sample, its corresponding jet mass template is used to generate a random jet mass. An M Σ J distribution can be constructed from ...