A search for a heavy neutral Higgs boson, $A$, decaying into a $Z$ boson and another heavy Higgs boson, $H$, is performed using a data sample corresponding to an integrated luminosity of 139 fb$^{-1}$ from proton-proton collisions at $\sqrt{s}$ = 13 TeV recorded by the ATLAS detector at the LHC. The search considers the $Z$ boson decaying into electrons or muons and the $H$ boson into a pair of $b$-quarks or $W$ bosons. The mass range considered is 230-800 GeV for the $A$ boson and 130-700 GeV for the $H$ boson. The data are in good agreement with the background predicted by the Standard Model, and therefore 95% confidence-level upper limits for $\sigma \times B(A\rightarrow ZH) \times B(H\rightarrow bb$ or $H\rightarrow WW)$ are set. The upper limits are in the range 0.0062-0.380 pb for the $H\rightarrow bb$ channel and in the range 0.023-8.9 pb for the $H\rightarrow WW$ channel. An interpretation of the results in the context of two-Higgs-Doublet models is also given.

A search is presented for the pair production of heavy vector-like $B$ quarks, primarily targeting $B$ quark decays into a $W$ boson and a top quark. The search is based on $36.1$ $fb^{-1}$ of $pp$ collisions at $\sqrt{s}$ = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton-plus-jets final state, characterised by a high-transverse-momentum isolated electron or muon, large missing transverse momentum, and multiple jets, of which at least one is $b$-tagged. No significant deviation from the Standard Model expectation is observed. The 95% confidence level lower limit on the $B$ mass is 1350 GeV assuming a 100% branching ratio to $Wt$. In the SU(2) singlet scenario, the lower mass limit is 1170 GeV. This search is also sensitive to a heavy vector-like $B$ quark decaying into other final states ($Zb$ and $Hb$) and thus mass limits on $B$ production are set as a function of the decay branching ratios. The 100% branching ratio limits are found to be also applicable to heavy vector-like $X$ production, with charge $+$5/3, that decay into $Wt$.

The results of a search for electroweakino pair production $pp \rightarrow \tilde\chi^\pm_1 \tilde\chi^0_2$ in which the chargino ($\tilde\chi^\pm_1$) decays into a $W$ boson and the lightest neutralino ($\tilde\chi^0_1$), while the heavier neutralino ($\tilde\chi^0_2$) decays into the Standard Model 125 GeV Higgs boson and a second $\tilde\chi^0_1$ are presented. The signal selection requires a pair of $b$-tagged jets consistent with those from a Higgs boson decay, and either an electron or a muon from the $W$ boson decay, together with missing transverse momentum from the corresponding neutrino and the stable neutralinos. The analysis is based on data corresponding to 139 $\mathrm{fb}^{-1}$ of $\sqrt{s}=13$ TeV $pp$ collisions provided by the Large Hadron Collider and recorded by the ATLAS detector. No statistically significant evidence of an excess of events above the Standard Model expectation is found. Limits are set on the direct production of the electroweakinos in simplified models, assuming pure wino cross-sections. Masses of $\tilde{\chi}^{\pm}_{1}/\tilde{\chi}^{0}_{2}$ up to 740 GeV are excluded at 95% confidence level for a massless $\tilde{\chi}^{0}_{1}$.

Several extensions of the Standard Model predict the production of dark matter particles at the LHC. An uncharted signature of dark matter particles produced in association with $VV=W^\pm W^\mp$ or $ZZ$ pairs from a decay of a dark Higgs boson $s$ is searched for using 139 fb$^{-1}$ of $pp$ collisions recorded by the ATLAS detector at a center-of-mass energy of 13 TeV. The $s\to V(q\bar q)V(q\bar q)$ decays are reconstructed with a novel technique aimed at resolving the dense topology from boosted $VV$ pairs using jets in the calorimeter and tracking information. Dark Higgs scenarios with $m_s > 160$ GeV are excluded.

A search for a heavy neutral Higgs boson, $A$, decaying into a $Z$ boson and another heavy Higgs boson, $H$, is performed using a data sample corresponding to an integrated luminosity of 36.1 fb$^{-1}$ from proton-proton collisions at $\sqrt{s} = 13$ TeV recorded in 2015 and 2016 by the ATLAS detector at the Large Hadron Collider. The search considers the $Z$ boson decaying to electrons or muons and the $H$ boson into a pair of $b$-quarks. No evidence for the production of an $A$ boson is found. Considering each production process separately, the 95% confidence-level upper limits on the $pp\rightarrow A\rightarrow ZH$ production cross-section times the branching ratio $H\rightarrow bb$ are in the range of 14-830 fb for the gluon-gluon fusion process and 26-570 fb for the $b$-associated process for the mass ranges 130-700 GeV of the $H$ boson and process for the mass ranges 130-700 GeV of the $H$ boson and 230-800 GeV of the $A$ boson. The results are interpreted in the context of the two-Higgs-doublet model.

A search for heavy resonances decaying to a $W$ or $Z$ boson and a Higgs boson in the $q\bar{q}^{(\prime)}b\bar{b}$ final state is described. The search uses 36.1 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} =$ 13 TeV collected by the ATLAS detector at the CERN Large Hadron Collider in 2015 and 2016. The data are in agreement with the Standard Model expectations, with the largest excess found at a resonance mass of 3.0 TeV with a local (global) significance of 3.3 (2.1) $\sigma$. The results are presented in terms of constraints on a simplified model with a heavy vector triplet. Upper limits are set on the production cross-section times branching ratio for resonances decaying to a $W$ ($Z$) boson and a Higgs boson, itself decaying to $b\bar{b}$, in the mass range between 1.1 and 3.8 TeV; the limits range between 83 and 1.6 fb (77 and 1.1 fb) at 95% confidence level.

A search is presented for the pair production of heavy vector-like $T$ and $B$ quarks, primarily targeting the $T$ quark decays to a $W$ boson and a $b$-quark. The search is based on $36.1$ fb$^{-1}$ of $pp$ collisions at $\sqrt{s}$ = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton-plus-jets final state, including at least one $b$-tagged jet and a large-radius jet identified as originating from the hadronic decay of a high-momentum $W$ boson. No significant deviation from the Standard Model expectation is observed in the reconstructed $T$ mass distribution. The observed 95% confidence level lower limit on the $T$ mass are 1350 GeV assuming 100% branching ratio to $Wb$. In the SU(2) singlet scenario, the lower mass limit is 1170 GeV. This search is also sensitive to a heavy vector-like $B$ quark decaying to $Wt$ and other final states. The results are thus reinterpreted to provide a 95% confidence level lower limit on the $B$ quark mass at 1250 GeV assuming 100% branching ratio to $Wt$; in the SU(2) singlet scenario, the limit is 1080 GeV. Mass limits on both $T$ and $B$ production are also set as a function of the decay branching ratios. The 100% branching ratio limits are found to be applicable to heavy vector-like $Y$ and $X$ production that decay to $Wb$ and $Wt$, respectively.

A search is conducted for new resonances decaying into a $W$ or $Z$ boson and a 125 GeV Higgs boson in the $\nu\bar{\nu}b\bar{b}$, $\ell^{\pm}{\nu}b\bar{b}$, and $\ell^+\ell^-b\bar{b}$ final states, where $\ell ^{\pm}= e^{\pm}$ or $\mu^{\pm}$, in $pp$ collisions at $\sqrt s = 13$ TeV. The data used correspond to a total integrated luminosity of 36.1 fb$^{-1}$ collected with the ATLAS detector at the Large Hadron Collider during the 2015 and 2016 data-taking periods. The search is conducted by examining the reconstructed invariant or transverse mass distributions of $Wh$ and $Zh$ candidates for evidence of a localised excess in the mass range of 220 GeV up to 5 TeV. No significant excess is observed and the results are interpreted in terms of constraints on the production cross-section times branching fraction of heavy $W^\prime$ and $Z^\prime$ resonances in heavy-vector-triplet models and the CP-odd scalar boson $A$ in two-Higgs-doublet models. Upper limits are placed at the 95 % confidence level and range between $9.0\times 10^{-4}$ pb and $8.1\times 10^{-1}$ pb depending on the model and mass of the resonance.

The production of $Z$ bosons in association with a high-energy photon ($Z\gamma$ production) is studied in the neutrino decay channel of the $Z$ boson using $pp$ collisions at $\sqrt{s}$ = 13 TeV. The analysis uses a data sample with an integrated luminosity of 36.1 fb$^{-1}$ collected by the ATLAS detector at the LHC in 2015 and 2016. Candidate $Z\gamma$ events with invisible decays of the $Z$ boson are selected by requiring significant transverse momentum ($p_{T}$) of the dineutrino system in conjunction with a single isolated photon with large transverse energy ($E_{T}$). The rate of $Z\gamma$ production is measured as a function of photon $E_{T}$, dineutrino system $p_{T}$ and jet multiplicity. Evidence of anomalous triple gauge-boson couplings is sought in $Z\gamma$ production with photon $E_{T}$ greater than 600 GeV. No excess is observed relative to the Standard Model expectation, and upper limits are set on the strength of $ZZ\gamma$ and $Z\gamma\gamma$ couplings.

A search for flavour-changing neutral-current decays of a top quark into an up-type quark (either up or charm) and a light scalar particle $X$ decaying into a bottom anti-bottom quark pair is presented. The search focuses on top-quark pair production where one top quark decays to $qX$, with $X \rightarrow b\bar{b}$, and the other top quark decays according to the Standard Model, with the $W$ boson decaying leptonically. The final state is thus characterised by an isolated electron or muon and at least four jets. Events are categorised according to the multiplicity of jets and jets tagged as originating from $b$-quarks, and a neural network is used to discriminate between signal and background processes. The data analysed correspond to 139 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC. The 95% confidence-level upper limits between 0.019% and 0.062% are derived for the branching fraction $\mathcal{B}$($t \rightarrow uX$) and between 0.018% and 0.078% for the branching fraction $\mathcal{B}$($t \rightarrow cX$), for masses of the scalar particle $X$ between 20 and 160 GeV.