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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. 

A<sc>bstract</sc> A search for long-lived particles decaying into hadrons is presented. The analysis uses 139 fb⁻¹ofppcollision data collected at$$ \sqrt{s} $$ $\sqrt{s}$= 13 TeV by the ATLAS detector at the LHC using events that contain multiple energetic jets and a displaced vertex. The search employs dedicated reconstruction techniques that significantly increase the sensitivity to long-lived particles decaying in the ATLAS inner detector. Background estimates for Standard Model processes and instrumental effects are extracted from data. The observed event yields are compatible with those expected from background processes. The results are used to set limits at 95% confidence level on model-independent cross sections for processes beyond the Standard Model, and on scenarios with pair-production of supersymmetric particles with long-lived electroweakinos that decay via a smallR-parity-violating coupling. The pair-production of electroweakinos with masses below 1.5 TeV is excluded for mean proper lifetimes in the range from 0.03 ns to 1 ns. When produced in the decay of$$ m\left(\overset{\sim }{g}\right) $$ $m\left(\tilde{g}\right)$= 2.4 TeV gluinos, electroweakinos with$$ m\left({\overset{\sim }{\chi}}_1^0\right) $$ $m\left({\tilde{\chi }}_1^0\right)$= 1.5 TeV are excluded with lifetimes in the range of 0.02 ns to 4 ns.