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A<sc>bstract</sc> This paper describes a search for dark photons (γ_d) in proton-proton collisions at$$ \sqrt{s} $$ $\sqrt{s}$= 13 TeV at the Large Hadron Collider (LHC). The dark photons are searched for in the decay of Higgs bosons (H→γγ_d) produced through theZHproduction mode. The transverse mass of the system, made of the photon and the missing transverse momentum from the non-interactingγ_d, presents a distinctive signature as it peaks near the Higgs boson mass. The results presented use the total Run-2 integrated luminosity of 139 fb⁻¹recorded by the ATLAS detector at the LHC. The dominant reducible background processes are estimated using data-driven techniques. A Boosted Decision Tree technique is adopted to enhance the sensitivity of the search. As no excess is observed with respect to the Standard Model prediction, an observed (expected) upper limit on the branching ratio BR(H→γγ_d) of 2.28% ($$ {2.82}_{-0.84}^{+1.33}\% $$ ${2.82}_{-0.84}^{+1.33}\%$) is set at 95% CL for masslessγ_d. For massive dark photons up to 40 GeV, the observed (expected) upper limits on BR(H→γγ_d) at 95% confidence level is found within the [2.19,2.52]% ([2.71,3.11]%) range. 

Abstract This paper presents the observation of four-top-quark ($$t\bar{t}t\bar{t}$$ $t\overline{t}t\overline{t}$) production in proton-proton collisions at the LHC. The analysis is performed using an integrated luminosity of 140 $$\hbox {fb}^{-1}$$ ${\text{fb}}^{-1}$at a centre-of-mass energy of 13 TeV collected using the ATLAS detector. Events containing two leptons with the same electric charge or at least three leptons (electrons or muons) are selected. Event kinematics are used to separate signal from background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The observed (expected) significance of the measured$$t\bar{t}t\bar{t}$$ $t\overline{t}t\overline{t}$signal with respect to the standard model (SM) background-only hypothesis is 6.1 (4.3) standard deviations. The$$t\bar{t}t\bar{t}$$ $t\overline{t}t\overline{t}$production cross section is measured to be$$22.5^{+6.6}_{-5.5}$$ $22.5_{-5.5}^{+6.6}$ fb, consistent with the SM prediction of$$12.0 \pm 2.4$$ $12.0\pm 2.4$fb within 1.8 standard deviations. Data are also used to set limits on the three-top-quark production cross section, being an irreducible background not measured previously, and to constrain the top-Higgs Yukawa coupling and effective field theory operator coefficients that affect$$t\bar{t}t\bar{t}$$ $t\overline{t}t\overline{t}$production.