The reconstruction of the signal from hadrons and jets emerging from the proton–proton collisions at the Large Hadron Collider (LHC) and entering the ATLAS calorimeters is based on a three-dimensional topological clustering of individual calorimeter cell signals. The cluster formation follows cell signal-significance patterns generated by electromagnetic and hadronic showers. In this, the clustering algorithm implicitly performs a topological noise suppression by removing cells with insignificant signals which are not in close proximity to cells with significant signals. The resulting topological cell clusters have shape and location information, which is exploited to apply a local energy calibration and corrections depending on the nature of the cluster. Topological cell clustering is established as a well-performing calorimeter signal definition for jet and missing transverse momentum reconstruction in ATLAS.

Combined measurements of Higgs boson production cross sections and branching fractions are presented. The combination is based on the analyses of the Higgs boson decay modes $H\to \gamma \gamma$, $Z{Z}^{*}$, $W{W}^{*}$, $\tau \tau$, $b\overline{b}$, $\mu \mu$, searches for decays into invisible final states, and on measurements of off-shell Higgs boson production. Up to $79.8{\mathrm{fb}}^{-1}$ of proton–proton collision data collected at $\sqrt{s}=13\mathrm{TeV}$ with the ATLAS detector are used. Results are presented for the gluon–gluon fusion and vector-boson fusion processes, and for associated production with vector bosons or top-quarks. The global signal strength is determined to be $\mu =1.11_{-0.08}^{+0.09}$. The combined measurement yields an observed (expected) significance for the vector-boson fusion production process of $6.5\sigma$ ($5.3\sigma$). Measurements in kinematic regions defined within the simplified template cross section framework are also shown. The results are interpreted in terms of modifiers applied to the Standard Model couplings of the Higgs boson to other particles, and are used to set exclusion limits on parameters in two-Higgs-doublet models and in the simplified minimal supersymmetric Standard Model. No significant deviations from Standard Model predictions are observed.

Jet energy scale measurements and their systematic uncertainties are reported for jets measured with the ATLAS detector using proton-proton collision data with a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$, corresponding to an integrated luminosity of $3.2{\mathrm{fb}}^{-1}$ collected during 2015 at the LHC. Jets are reconstructed from energy deposits forming topological clusters of calorimeter cells, using the anti-$k_t$ algorithm with radius parameter $R=0.4$. Jets are calibrated with a series of simulation-based corrections and in situ techniques. In situ techniques exploit the transverse momentum balance between a jet and a reference object such as a photon, $Z$ boson, or multijet system for jets with $20<p_{\mathrm{T}}<2000\mathrm{GeV}$ and pseudorapidities of $|\eta |<4.5$, using both data and simulation. An uncertainty in the jet energy scale of less than 1% is found in the central calorimeter region ($|\eta |<1.2$) for jets with $100<p_{\mathrm{T}}<500\mathrm{GeV}$. An uncertainty of about 4.5% is found for low-$p_{\mathrm{T}}$ jets with $p_{\mathrm{T}}=20\mathrm{GeV}$ in the central region, dominated by uncertainties in the corrections for multiple proton-proton interactions. The calibration of forward jets ($|\eta |>0.8$) is derived from dijet $p_{\mathrm{T}}$ balance measurements. For jets of $p_{\mathrm{T}}=80\mathrm{GeV}$, the additional uncertainty for the forward jet calibration reaches its largest value of about 2% in the range $|\eta |>3.5$ and in a narrow slice of $2.2<|\eta |<2.4$.

The algorithms used by the ATLAS Collaboration during Run 2 of the Large Hadron Collider to identify jets containing b-hadrons are presented. The performance of the algorithms is evaluated in the simulation and the efficiency with which these algorithms identify jets containing b-hadrons is measured in collision data. The measurement uses a likelihood-based method in a sample highly enriched in $$t{\bar{t}}$$tt¯ events. The topology of the $$t \rightarrow W b$$t→Wb decays is exploited to simultaneously measure both the jet flavour composition of the sample and the efficiency in a transverse momentum range from 20 to 600 GeV. The efficiency measurement is subsequently compared with that predicted by the simulation. The data used in this measurement, corresponding to a total integrated luminosity of 80.5 $$\hbox {fb}^{-1}$$fb-1 , were collected in proton–proton collisions during the years 2015–2017 at a centre-of-mass energy $$\sqrt{s}=$$s= 13 TeV. By simultaneously extracting both the efficiency and jet flavour composition, this measurement significantly improves the precision compared to previous results, with uncertainties ranging from 1 to 8% depending on the jet transverse momentum.