Abstract: A search for charged-lepton-flavor violating μtau qt (q = u, c) interactions is presented, considering both top-quark production and decay. The data analyzed correspond to 140 fb(-1) of proton-proton collisions at a center-of-mass energy of root s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The analysis targets events containing two muons with the same electric charge, a hadronically decaying tau-lepton and at least one jet, with exactly one b-tagged jet, produced by a μtau qt interaction. Agreement with the Standard Model expectation within 1.6 standard deviations is observed, and limits are set at the 95% confidence level (CL) on the charged-lepton-flavor violation branching ratio of B(t -> μtau q) < 8.7 x 10(-7). An effective field theory interpretation is performed yielding 95% CL limits on Wilson coefficients, dependent on the flavor of the associated light quark and the Lorentz structure of the coupling. These range from vertical bar c(lequ)(3(2313))vertical bar/Lambda(2) < 0.10 TeV-2 for μtau ut to vertical bar c(lequ)(1(2323))vertical bar/Lambda(2) < 1.8 TeV-2 for μtau ct. An additional interpretation is performed for scalar leptoquark production inducing charged-lepton-flavor violation, with fixed intergenerational couplings. Upper limits on leptoquark coupling strengths are set at the 95% CL, ranging from lambda(LQ) = 1.3 to lambda(LQ) = 3.7 for leptoquark masses between 0.5 and 2.0 TeV.

Abstract: A search is presented for flavour-changing neutral-current interactions involving the top quark, the Higgs boson and an up-type quark (q = u, c) with the ATLAS detector at the Large Hadron Collider. The analysis considers leptonic decays of the top quark along with Higgs boson decays into two W bosons, two Z bosons or a tau(+)tau(-) pair. It focuses on final states containing either two leptons (electrons or muons) of the same charge or three leptons. The considered processes are t (t) over bar and Ht production. For the t (t) over bar production, one top quark decays via t -> Hq. The proton-proton collision data set analysed amounts to (140 fb(-1)) at (root s = 13 TeV). No significant excess beyond Standard Model expectations is observed and upper limits are set on the t -> Hq branching ratios at 95% confidence level, amounting to observed (expected) limits of B(t -> Hu) < 2.8 (3.0) x 10(-4) and B(t -> Hc) < 3.3 (3.8) x 10(-4). Combining this search with other searches for t Hq flavour-changing neutral-current interactions previously conducted by ATLAS, considering H -> b (b) over bar and H -> gamma gamma decays, as well as H -> tau(+)tau(-) decays with one or two hadronically decaying tau-leptons, yields observed (expected) upper limits on the branching ratios of B(t -> Hu) < 2.6 (1.8) x 10(-4) and B(t -> Hc) < 3.4 (2.3) x 10(-4).

Abstract: This paper presents a measurement of the production cross-section of a Z boson in association with b- or c-jets, in proton-proton collisions at root s = 13 TeV with the ATLAS experiment at the Large Hadron Collider using data corresponding to an integrated luminosity of 140 fb(-1). Inclusive and differential cross-sections are measured for events containing a Z boson decaying into electrons or muons and produced in association with at least one b-jet, at least one c-jet, or at least two b-jets with transverse momentum p(T) > 20 GeV and rapidity vertical bar y vertical bar < 2.5. Predictions from several Monte Carlo generators based on next-to-leading-order matrix elements interfaced with a parton-shower simulation, with different choices of flavour schemes for initial-state partons, are compared with the measured cross-sections. The results are also compared with novel predictions, based on infrared and collinear safe jet flavour dressing algorithms. Selected Z+ >= 1 c-jet observables, optimized for sensitivity to intrinsic-charm, are compared with benchmark models with different intrinsic-charm fractions.