The onset of high-energy operation of the Large Hadron Collider (LHC) at CERN in 2010 heralded a new and transformative era in physics research. A powerful and versatile hadron collider paired with sophisticated particle detectors and worldwide grid computing, all performing beyond expectations, allowed the LHC experiments to chart new frontiers and advance particle physics. While the discovery of the Higgs boson by the ATLAS and CMS Collaborations in 2012 stands out as the LHC’s magnum opus, the wealth of results includes the observation of countless new processes and states, precision measurements matched with continuously refined theoretical predictions, and a broad and deep exploration of the new physics landscape. The non-observation of physics beyond the Standard Model so far, despite strong theoretical motivation for new TeV-scale particles, is another paramount outcome of the LHC. How can one fathom the puzzling tailoring or fine-tuning that seems to haunt the scalar sector? This mystery has fuelled many theoretical developments. It has prompted phenomenologists and model builders to propose new ways to preserve naturalness, and sparked renewed interest in more speculative anthropic concepts, stimulated by the string landscape and eternal inflation.

The discovery of the Higgs boson, a manifestation of the condensed Brout-Englert-Higgs (BEH) field, has enabled ATLAS and CMS physicists for the first time to directly study electroweak symmetry breaking and the process of mass generation. The first run of the LHC (Run 1), comprising the data-taking periods between 2010 and 2012 with proton collisions at 7 and 8 TeV centre-of-mass energies, marked the discovery of the Higgs boson via its decays to boson pairs, the measurement of its mass, a critical parameter in the energy evolution of the scalar field potential, and the confirmation of its basic quantum numbers and coupling properties. The second run of the LHC (Run 2), delivering seven times more data during the years 2015–2018 at 13 TeV centre-of-mass energy, allowed ATLAS and CMS to establish Higgs boson interactions with fermions and observe the expected non-universal, mass-dependent interaction strengths (Figure 1). This confirmed the BEH mechanism of spontaneous symmetry breaking of the electroweak vacuum through a scalar doublet field with an energy potential whose ground state is non-zero. Masses arise from particles interacting with this ever-present background field that permeates the universe. However, it remains unknown why some particles interact more strongly with the field, acquiring greater mass, than others. This is the flavour problem, and perhaps the most striking feature is not the order-one coupling strength of the top quark, but rather the exceptionally small coupling of the electron.

Despite its success, the newly discovered scalar sector raises profound questions about its naturalness (the Higgs boson is the only known elementary particle with a bare mass term — how come it is so light?) and the stability of the electroweak vacuum at high energy, in addition to the enigmatic flavour pattern. It may also relate to the matter-antimatter asymmetry in the universe, potentially through additional scalar fields, and to cosmological dark matter if it is composed of massive elementary particles. The remarkable similarity between the BEH mechanism and the Bardeen–Cooper–Schrieffer theory of superconductivity, where the Bose condensation of a Cooper pair of two electrons plays the role of the BEH field, raises the question whether the Higgs boson might not be elementary after all.

ATLAS data-taking during Run 2 benefited from several detector upgrades implemented during the LHC’s long shutdown 1 (2013–2015), most prominently the addition of the insertable B-layer (IBL), a pixel layer with radiation hard sensors mounted at a radius of only 33 mm from the beam (see Figure 2). The IBL significantly improved the vertex resolution of the ATLAS Inner Detector and thus the heavy-flavour jet tagging performance, among other benefits. The ATLAS experiment achieved record data-taking and data quality efficiency during Run 2 (Figure 3). Of the 156 fb^–1 integrated proton–proton luminosity delivered by the LHC, 140 fb^–1 (90%) is used for physics analysis. Additional low-luminosity runs were conducted to enable precision measurements of W-boson properties. The Run-2 dataset further comprises 2.2 nb^–1 of lead–lead collisions taken in the years 2015 and 2018 at 5.02 TeV nucleon–nucleon centre-of-mass energy, 179 nb^–1 of proton–lead collisions taken in 2016 at 5.02 and 8.16 TeV, a small dataset of xenon–xenon collisions taken in 2017, and reference proton–proton collision datasets at corresponding energies. The luminosity measurement, calibrated using LHC beam-separation scans, reached the tremendous precision of 0.8% for the full ATLAS Run-2 proton–proton dataset, never before attained at a hadron collider. Online event selection employing low-threshold single and di-object triggers as mainstays, and a new first-level topological trigger for object combination, ensured high efficiency across the ATLAS physics programme.

The worldwide LHC computing grid performed superbly. Heterogeneous, pledged and opportunistic computing resources were efficiently integrated, allowing ATLAS to promptly reconstruct the recorded data and produce substantial amounts of simulated data. The compute performance of the Geant-4 based full detector simulation was continuously improved, and a new fast calorimeter simulation, enhanced with generative machine-learning, was brought to completion during Run 2. New developments in the reconstruction and identification of charged leptons, flavoured and unflavoured jets, missing transverse momentum, and highly boosted objects benefited from ever more sophisticated machine learning algorithms, where in particular recent advances using graph neural networks stand out in performance (Figure 4). The excellent performance of the ATLAS detector and reconstruction software, together with its meticulous calibration, and the accuracy of the physics modelling and detector simulation, has made the Run-2 dataset the largest, most comprehensive, and highest quality collection of high-energy physics data at the time.

Run 2 has seen a surge in the breadth and depth of ATLAS’ physics results. The Collaboration has released 415 papers with the full Run-2 dataset to date (and counting), and a similar number of results using partial Run-2 datasets. Numerous rare processes have been observed for the first time. Each iteration of an analysis improved the results beyond the statistical yield, owing to better performance and analysis methods, notably by widely embracing machine learning algorithms.

ATLAS Run-2 results have set new benchmarks in multi-boson production. Forty years after the discovery of the W boson at CERN’s proton–antiproton collider, ATLAS observed for the first time the simultaneous production of three W bosons (Figure 5) as well as that of a W-boson pair through the interaction of two photons (Figure 6). Numerous vector-boson scattering processes were newly observed and measured. Angular analyses were performed to detect, again for the first time, the joint longitudinal polarisation of weak bosons in such events. These studies pave the way to testing another key property of the BEH mechanism: the regulation of longitudinal vector boson scattering at high momentum transfer.

The production of almost 300 million top-quarks during Run 2, recorded with high trigger efficiency, has prompted a plethora of top-quark production and property measurements. It allowed ATLAS to measure rare high-mass processes such as the associated production of a top-quark pair with a Z boson, directly probing the top–Z coupling, and to observe four-top-quark production for the first time (Figure 7), a process four thousand times rarer than Higgs boson production. The first observation of quantum entanglement of top-quark pairs by ATLAS marked the beginning of high-energy quantum information research at the LHC. Top quarks also serve as a clean source of W bosons. When produced in pairs, one W boson can be used for identification while the other is studied in detail. This approach has enabled highly sensitive tests of lepton universality in the weak interaction and resolved a long-standing discrepancy from LEP.

Higgs physics underwent a transformation during Run 2. All major Higgs boson production modes at the LHC were observed and measured. Previously thought to be impossible or highly challenging, the decay of the Higgs boson into a bottom quark pair — which accounts for more than half of all Higgs boson decays — was observed for the first time by ATLAS in summer 2018 using a partial Run-2 dataset (Figure 8, left). The associated production of the Higgs boson with top-quark pairs, a rare process representing less than one percent of all Higgs boson production modes, was unambiguously observed by ATLAS in the same year through its decay into two photons, confirming the large predicted top–Higgs coupling (Figure 8, right). Evidence for exceedingly rare Higgs-boson decay channels was also seen. The small Higgs boson decay width, predicted to be 4.1 MeV in the Standard Model, has been constrained to better than 60% precision — well beyond expectations — by measuring off-shell production at high mass and comparing the measured rate to that observed on the mass shell.

The full suite of Run-2 analyses and their combination allowed ATLAS to draw a detailed map of the Higgs boson and its interactions with unprecedented depth and precision (Figure 9), a decade after its discovery. The possibility to directly constrain the quartic field coefficient in the BEH energy potential through the measurement of Higgs-pair production is a fascinating opportunity that has already been explored by ATLAS during Run 2, despite its minuscule cross section, three orders of magnitude smaller than that of single Higgs boson production. Spectacular improvements with each iteration of the analyses have sharpened the prospects for observing and measuring this channel in the upcoming High-Luminosity LHC era.

While the LHC was primarily built for the direct production and study of new particles, processes, and phenomena, progress in the field also relies on precision measurements that refine our understanding of Standard Model parameters and probe new physics through quantum loops. ATLAS has played a key role in this endeavour, delivering the first measurements of the W boson mass and width at the LHC, both found in agreement with the Standard Model predictions.

ATLAS further released the most precise measurement of the Higgs boson mass to date (Figure 10), a precise preliminary measurement of the weak mixing angle, the most precise measurement yet of the top-quark mass in combination with CMS, the world’s most precise measurement of the strong coupling constant (these latter two results still based on the Run-1 dataset), the most precise result to date of the total proton–proton cross section by measuring forward elastic scattering, comprehensive constraints on parton distribution functions, as well as numerous measurements of total, fiducial, and differential cross-sections (Figure 11), angular properties and correlations. Sensitive measurements of rare B-meson decays and CP violation, along with the most precise measurement to date of the B-meson lifetime, complement these results. Effective field theoretical extensions of the Standard Model Lagrangian provide a general framework for quantifying the reach of direct and indirect searches for new physics in terms of higher dimensional field operators, allowing ATLAS to consistently combine measurements of different processes and thus enhance their impact.

Collisions among ionised lead nuclei, each consisting of 208 nucleons (82 protons and 126 neutrons), generate temperatures several hundred thousand times hotter than the core of the Sun — hot enough to melt nucleons within the nuclei, forming a plasma of deconfined quarks and gluons, a near-perfect fluid with almost no viscosity. Due to its ephemerality (lifetime of about 10⁻²³ s) and microscopic size (10⁻¹⁴ m), the quark-gluon plasma cannot be observed directly, but only through the particles it emits.

ATLAS studies of heavy-ion collisions collected during Run 2 have deepened our understanding of this new state of matter by employing soft probes via measurements of multi-particle correlations and flow harmonics, alongside hard probes to investigate energy-loss (“quenching”) effects in a variety of final states. Further insights have been gained by measuring the production of quarkonia and electroweak bosons as a function of the collision centrality. Jet quenching, a process where partons lose energy through collisions and gluon radiation in the quark-gluon plasma, was first observed at the LHC by ATLAS in 2010. Since then, the study of this striking phenomenon has evolved significantly with measurements of transverse momentum asymmetries in dijet, photon-jet, and Z-jet events (Figure 12), and by investigating varying jet energies and shapes.

Ultraperipheral collisions have enabled ATLAS to directly observe, for the first time, light-by-light scattering (Figure 13) and tau-lepton pair production. These processes are made possible by the extreme electric fields — up to seven orders of magnitude beyond the Schwinger limit of 10¹⁸ Volt per metre and among the highest observed in nature — generated by the quasi-real photons emitted by the colliding lead ions. Similarly, the enormous magnetic fields, reaching up to 10¹⁶ Tesla, in these processes allowed ATLAS to perform highly sensitive searches for hypothetical magnetic monopoles.

The vast field of new physics searches featured prominently in the ATLAS physics programme during Run 2. The higher proton–proton collision energy and increased data sample provided a substantial gain in sensitivity. Searches for supersymmetry have excluded gluino masses up to 2.4 TeV, top-squark masses up to 1.2 TeV, and chargino and neutralino masses up to 1 TeV, depending on the scenario (Figure 14). These results set strong constraints on natural supersymmetry. Sensitivity gaps could be addressed through specialised searches.

Supersymmetry and other new physics models posit an extended scalar sector, explored through a variety of channels closing in on additional Higgs bosons. Heavy, strongly produced resonances such as excited quarks could be excluded up to almost 7 TeV (Figure 15, left), and heavy Z-like bosons up to about 5 TeV. Analyses looking for heavy particles decaying into boosted boson pairs exploited jet substructure techniques to increase their sensitivity far beyond the TeV mass scale. An apparent diphoton excess near 750 GeV in the 2015 data was not confirmed with larger datasets. The direct production of dark matter particles in proton–proton collisions was explored in a broad set of recoil modes involving jets (Figure 15, right), photons, weak bosons, Higgs boson, and top quarks, as well as in searches for supersymmetry.

Dark matter was also searched for via invisible decays of the Higgs boson produced through weak boson fusion or in association with other particles. These processes provide strong constraints on dark matter originating from weakly interacting massive particles, complementary to direct recoil-based searches underground and indirect searches for annihilation signals in space (Figure 16). Long-lived heavy particles may commonly occur in decay chains involving new mass-degenerate particles, weak couplings, or quantum loops with high virtuality (Figure 17). Searches for such phenomena have seen a rise of interest in ATLAS and produced numerous innovative analyses, which also benefited from advances in reconstruction techniques such as large impact parameter tracking. While the Standard Model is subjected to increasingly stringent tests and the ATLAS searches become ever deeper and more inventive, none have yet revealed compelling evidence of new physics.

ATLAS published a collection of six articles in Physics Reports to highlight the key findings and transformative implications of the landmark results obtained from its Run-2 proton–proton dataset. The articles cover Higgs boson physics, electroweak, QCD and flavour physics, top-quark physics, as well as searches for new phenomena covering an extended Higgs sector, supersymmetry, and other scenarios. A summary of heavy-ion results will be released at a later stage. As Run-3 data-taking progresses and the Collaboration prepares for major detector upgrades ahead of the High-Luminosity LHC — the next major milestone for the global community — the Run-2 results stand as the state-of-the-art in particle physics at the high-energy frontier.

A version of this text was released as a foreword to the six ATLAS summary articles on Run-2 physics results published early 2025 in Physics Reports.