Abstract: A search for decays of pair-produced neutral long-lived particles (LLPs) is presented using 139 fb(-1) of proton-proton collision data collected by the ATLAS detector at the LHC in 2015-2018 at a centre-of-mass energy of 13 TeV. Dedicated techniques were developed for the reconstruction of displaced jets produced by LLPs decaying hadronically in the ATLAS hadronic calorimeter. Two search regions are defined for different LLP kinematic regimes. The observed numbers of events are consistent with the expected background, and limits for several benchmark signals are determined. For a SM Higgs boson with a mass of 125 GeV, branching ratios above 10% are excluded at 95% confidence level for values of c times LLP mean proper lifetime in the range between 20 mm and 10 m depending on the model. Upper limits are also set on the cross-section times branching ratio for scalars with a mass of 60 GeV and for masses between 200 GeV and 1 Tev.

Abstract: A search for events with a dark photon produced in association with a dark Higgs boson via rare decays of the standard model Z boson is presented, using 139 fb(-1) of root p 1/4 13 TeV proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider. The dark boson decays into a pair of dark photons, and at least two of the three dark photons must each decay into a pair of electrons or muons, resulting in at least two same-flavor opposite-charge lepton pairs in the final state. The data are found to be consistent with the background prediction, and upper limits are set on the dark photon's coupling to the dark Higgs boson times the kinetic mixing between the standard model photon and the dark photon, alpha(D)epsilon(2), in the dark photon mass range of [5, 40] GeV except for the gamma mass window [8.8, 11.1] GeV. This search explores new parameter space not previously excluded by other experiments.

Abstract: A search for flavor-changing neutral-current couplings between a top quark, an up or charm quark, and a Z boson is presented, using proton-proton collision data at root s = 13 TeV collected by the ATLAS detector at the Large Hadron Collider. The analyzed data set corresponds to an integrated luminosity of 139 fb(-1). The search targets both single-top- quark events produced as gq -> tZ (with q = u, c) and top-quark-pair events, with one top quark decaying through the t -> Zq channel. The analysis considers events with three leptons (electrons or muons), a b-tagged jet, possible additional jets, and missing transverse momentum. The data are found to be consistent with the background-only hypothesis and 95% confidence-level limits on the t -> Zq branching ratios, assuming only tensor operators of the Standard Model effective field theory framework contribute to the tZq vertices. These are 6.2 x 10(-5) (13 x 10(-5)) for t -> Zu (t -> Zc) for a left-handed tZq coupling, and 6.6 x 10(-5) (12 x 10(-5)) in the case of a right-handed coupling. These results are interpreted as 95% CL upper limits on the strength of the corresponding couplings, yielding limits for |C-uW((13))*| and |C-uB((13))*| (|C-uW((31))| and |C-uB((31))|) of 0.15 (0.16), and limits for |C-uW((23))*| and |C-uB((23))*| (|C-uW((32))| and |C-uB((32))|) of 0.22 (0.21), assuming a new-physics energy scale Lambda(NP) of 1 TeV.