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This paper describes precision measurements of the transverse momentum $$p_\mathrm {T}^{\ell \ell }$$ p T ℓ ℓ ($$\ell =e,\mu $$ ℓ = e , μ ) and of the angular variable $$\phi ^{*}_{\eta }$$ ϕ η ∗ distributions of Drell–Yan lepton pairs in a mass range of 66–116 GeV. The analysis uses data from 36.1 fb$$^{-1}$$ - 1 of proton–proton collisions at a centre-of-mass energy of $$\sqrt{s}=13\,$$ s = 13 TeV collected by the ATLAS experiment at the LHC in 2015 and 2016. Measurements in electron-pair and muon-pair final states are performed in the same fiducial volumes, corrected for detector effects, and combined. Compared to previous measurements in proton–proton collisions at $$\sqrt{s}=7$$ s = 7 and $$8\,$$ 8 TeV, these new measurements probe perturbative QCD at a higher centre-of-mass energy with a different composition of initial states. They reach a precision of 0.2$$\%$$ % for the normalized spectra at low values of $$p_\mathrm {T}^{\ell \ell }$$ p T ℓ ℓ . The data are compared with different QCD predictions, where it is found that predictions based on resummation approaches can describe the full spectrum within uncertainties.

A search for direct pair production of scalar partners of the top quark (top squarks or scalar third-generation up-type leptoquarks) in the all-hadronic $$t{\bar{t}}$$tt¯ plus missing transverse momentum final state is presented. The analysis of 139 $$\hbox {fb}^{-1}$$fb-1 of $${\sqrt{s}=13}$$s=13 TeV proton–proton collision data collected using the ATLAS detector at the LHC yields no significant excess over the Standard Model background expectation. To interpret the results, a supersymmetric model is used where the top squark decays via $${\tilde{t}} \rightarrow t^{(*)} {\tilde{\chi }}^0_1$$t~→t(∗)χ~10, with $$t^{(*)}$$t(∗) denoting an on-shell (off-shell) top quark and $${\tilde{\chi }}^0_1$$χ~10 the lightest neutralino. Three specific event selections are optimised for the following scenarios. In the scenario where $$m_{{\tilde{t}}}> m_t+m_{{\tilde{\chi }}^0_1}$$mt~>mt+mχ~10, top squark masses are excluded in the range 400–1250 GeV for $${\tilde{\chi }}^0_1$$χ~10 masses below 200 GeV at 95% confidence level. In the situation where $$m_{{\tilde{t}}}\sim m_t+m_{{\tilde{\chi }}^0_1}$$mt~∼mt+mχ~10, top squark masses in the range 300–630 GeV are excluded, while in the case where $$m_{{\tilde{t}}}< m_W+m_b+m_{{\tilde{\chi }}^0_1}$$mt~<mW+mb+mχ~10 (with $$m_{{\tilde{t}}}-m_{{\tilde{\chi }}^0_1}\ge 5$$mt~-mχ~10≥5 GeV), considered for the first time in an ATLAS all-hadronic search, top squark masses in the range 300–660 GeV are excluded. Limits are also set for scalar third-generation up-type leptoquarks, excluding leptoquarks with masses below 1240 GeV when considering only leptoquark decays into a top quark and a neutrino.

Jet energy scale and resolution measurements with their associated uncertainties are reported for jets using 36–81 fb$$^{-1}$$ - 1 of proton–proton collision data with a centre-of-mass energy of $$\sqrt{s}=13$$ s = 13  $${\text {Te}}{\text {V}}$$ TeV collected by the ATLAS detector at the LHC. Jets are reconstructed using two different input types: topo-clusters formed from energy deposits in calorimeter cells, as well as an algorithmic combination of charged-particle tracks with those topo-clusters, referred to as the ATLAS particle-flow reconstruction method. The anti-$$k_t$$ k t jet algorithm with radius parameter $$R=0.4$$ R = 0.4 is the primary jet definition used for both jet types. This result presents new jet energy scale and resolution measurements in the high pile-up conditions of late LHC Run 2 as well as a full calibration of particle-flow jets in ATLAS. Jets are initially calibrated using a sequence of simulation-based corrections. Next, several in situ techniques are employed to correct for differences between data and simulation and to measure the resolution of jets. The systematic uncertainties in the jet energy scale for central jets ($$|\eta |<1.2$$ | η | < 1.2 ) vary from 1% for a wide range of high-$$p_{{\text {T}}}$$ p T jets ($$250<p_{{\text {T}}} <2000~{\text {Ge}}{\text {V}}$$ 250 < p T < 2000 GeV ), to 5% at very low $$p_{{\text {T}}}$$ p T ($$20~{\text {Ge}}{\text {V}}$$ 20 GeV ) and 3.5% at very high $$p_{{\text {T}}}$$ p T ($$>2.5~{\text {Te}}{\text {V}}$$ > 2.5 TeV ). The relative jet energy resolution is measured and ranges from ($$24 \pm 1.5$$ 24 ± 1.5 )% at 20 $${\text {Ge}}{\text {V}}$$ GeV to ($$6 \pm 0.5$$ 6 ± 0.5 )% at 300 $${\text {Ge}}{\text {V}}$$ GeV .

A search for dark matter is conducted in final states containing a photon and missing transverse momentum in proton-proton collisions at $$ \sqrt{s} $$ s = 13 TeV. The data, collected during 2015–2018 by the ATLAS experiment at the CERN LHC, correspond to an integrated luminosity of 139 fb−1. No deviations from the predictions of the Standard Model are observed and 95% confidence-level upper limits between 2.45 fb and 0.5 fb are set on the visible cross section for contributions from physics beyond the Standard Model, in different ranges of the missing transverse momentum. The results are interpreted as 95% confidence-level limits in models where weakly interacting dark-matter candidates are pair-produced via an s-channel axial-vector or vector mediator. Dark-matter candidates with masses up to 415 (580) GeV are excluded for axial-vector (vector) mediators, while the maximum excluded mass of the mediator is 1460 (1470) GeV. In addition, the results are expressed in terms of 95% confidence-level limits on the parameters of a model with an axion-like particle produced in association with a photon, and are used to constrain the coupling gaZγ of an axion-like particle to the electroweak gauge bosons.