This letter documents a search for flavour-changing neutral currents (FCNCs), which are strongly suppressed in the Standard Model, in events with a photon and a top quark with the ATLAS detector. The analysis uses data collected in pp collisions at √s = 13 TeV during Run 2 of the LHC, corresponding to an integrated luminosity of 139 fb⁻¹. Both FCNC top-quark production and decay are considered. The final state consists of a charged lepton, missing transverse momentum, a b-tagged jet, one high-momentum photon and possibly additional jets. A multiclass deep neural network is used to classify events either as signal in one of the two categories, FCNC production or decay, or as background. No significant excess of events over the background prediction is observed and 95% CL upper limits are placed on the strength of left- and right-handed FCNC interactions. The 95% CL bounds on the branching fractions for the FCNC top-quark decays, estimated (expected) from both top-quark production and decay, are B(t → uγ ) < 0.85 (0.88) × 10⁻⁵ and B(t → cγ ) < 4.2 (3.40) × 10⁻⁵ for a left-handed tqγ coupling, and B(t → uγ ) < 1.2 (1.20)×10⁻⁵ and B(t → cγ ) < 4.5 (3.70)×10⁻⁵ for a right-handed coupling.

This paper presents a search for hypothetical massive, charged, long-lived particles with the ATLAS detector at the LHC using an integrated luminosity of 139 fb⁻¹ of proton–proton collisions at √s = 13 TeV. These particles are expected to move significantly slower than the speed of light and should be identifiable by their high transverse momenta and anomalously large specific ionisation losses, dE/dx. Trajectories reconstructed solely by the inner tracking system and a dE/dx measurement in the pixel detector layers provide sensitivity to particles with lifetimes down to O(1) ns with a mass, measured using the Bethe–Bloch relation, ranging from 100 GeV to 3 TeV. Interpretations for pair-production of R-hadrons, charginos and staus in scenarios of supersymmetry compatible with these particles being long-lived are presented, with mass limits extending considerably beyond those from previous searches in broad ranges of lifetime.

A search for heavy Higgs bosons produced in association with a vector boson and decaying into a pair of vector bosons is performed in final states with two leptons (electrons or muons) of the same electric charge, missing transverse momentum and jets. A data sample of proton–proton collisions at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider between 2015 and 2018 is used. The data correspond to a total integrated luminosity of 139 fb⁻¹. The observed data are in agreement with Standard Model background expectations. The results are interpreted using higher-dimensional operators in an effective field theory. Upper limits on the production cross-section are calculated at 95% confidence level as a function of the heavy Higgs boson’s mass and coupling strengths to vector bosons. Limits are set in the Higgs boson mass range from 300 to 1500 GeV, and depend on the assumed couplings. The highest excluded mass for a heavy Higgs boson with the coupling combinations explored is 900 GeV. Limits on coupling strengths are also provided.

The correlations between flow harmonics v_n for n=2, 3, and 4 and mean transverse momentum [p_T] in ¹²⁹Xe+¹²⁹Xe and ²⁰⁸Pb+²⁰⁸Pb collisions at √s=5.44 and 5.02 TeV, respectively, are measured using charged particles with the ATLAS detector. The correlations are potentially sensitive to the shape and size of the initial geometry, nuclear deformation, and initial momentum anisotropy. The effects from nonflow and centrality fluctuations are minimized, respectively, via a subevent cumulant method and an event-activity selection based on particle production at very forward rapidity. The v_n−[p_T] correlations show strong dependencies on centrality, harmonic number n, p_T, and pseudorapidity range. Current models qualitatively describe the overall centrality- and system-dependent trends but fail to quantitatively reproduce all features of the data. In central collisions, where models generally show good agreement, the v₂−[p_T] correlations are sensitive to the triaxiality of the quadruple deformation. Comparison of the model with the Pb+Pb and Xe+Xe data confirms that the ¹²⁹Xe nucleus is a highly deformed triaxial ellipsoid that has neither a prolate nor oblate shape. This provides strong evidence for a triaxial deformation of the ¹²⁹Xe nucleus from high-energy heavy-ion collisions.

Cross-section measurements for a Z boson produced in association with high-transverse-momentum jets (p_T ≥ 100 GeV) and decaying into a charged-lepton pair (e⁺e⁻, μ⁺μ⁻) are presented. The measurements are performed using proton–proton collisions at √s = 13 TeV corresponding to an integrated luminosity of 139 fb⁻¹ collected by the ATLAS experiment at the LHC. Measurements of angular correlations between the Z boson and the closest jet are performed in events with at least one jet with p_T ≥ 500 GeV. Event topologies of particular interest are the collinear emission of a Z boson in dijet events and a boosted Z boson recoiling against a jet. Fiducial cross sections are compared with state-of-the-art theoretical predictions. The data are found to agree with next-to-next-to-leading-order predictions by NNLOᴊᴇᴛ and with the next-to-leading-order multi-leg generators MADGRAPH5_AMC@NLO and SHERPA.

Inclusive and differential measurements of the top–antitop () charge asymmetry and the leptonic asymmetry are presented in proton–proton collisions at √s = 13 TeV recorded by the ATLAS experiment at the CERN Large Hadron Collider. The measurement uses the complete Run 2 dataset, corresponding to an integrated luminosity of 139 fb⁻¹, combines data in the single-lepton and dilepton channels, and employs reconstruction techniques adapted to both the resolved and boosted topologies. A Bayesian unfolding procedure is performed to correct for detector resolution and acceptance effects. The combined inclusive charge asymmetry is measured to be = 0.0068 ± 0.0015, which differs from zero by 4.7 standard deviations. Differential measurements are performed as a function of the invariant mass, transverse momentum and longitudinal boost of the system. Both the inclusive and differential measurements are found to be compatible with the Standard Model predictions, at next-to-next-to-leading order in quantum chromodynamics perturbation theory with next-to-leading-order electroweak corrections. The measurements are interpreted in the framework of the Standard Model effective field theory, placing competitive bounds on several Wilson coefficients.

The electroweak production of  in association with two jets is studied in a regime with a photon of high transverse momentum above 150 GeV using proton–proton collisions at a centre-of-mass energy of 13 TeV at the Large Hadron Collider. The analysis uses a data sample with an integrated luminosity of 139 fb⁻¹collected by the ATLAS detector during the 2015–2018 LHC data-taking period. This process is an important probe of the electroweak symmetry breaking mechanism in the Standard Model and is sensitive to quartic gauge boson couplings via vector-boson scattering. The fiducial cross section for electroweak production is measured to be fb and is consistent with the Standard Model prediction. Evidence of electroweak production is found with an observed significance of 3.2σ for the background-only hypothesis, compared with an expected significance of 3.7σ. The combination of this result with the previously published ATLAS observation of electroweak production yields an observed (expected) signal significance of 6.3σ (6.6σ). Limits on anomalous quartic gauge boson couplings are obtained in the framework of effective field theory with dimension-8 operators.

A study of the charge conjugation and parity (CP) properties of the interaction between the Higgs boson and τ-leptons is presented. The study is based on a measurement of CP-sensitive angular observables defined by the visible decay products of τ-leptons produced in Higgs boson decays. The analysis uses 139 fb⁻¹ of proton–proton collision data recorded at a centre-of-mass energy of √s=13 TeV with the ATLAS detector at the Large Hadron Collider. Contributions from CP-violating interactions between the Higgs boson and τ-leptons are described by a single mixing angle parameter φ_τ in the generalised Yukawa interaction. Without constraining the H→ττ signal strength to its expected value under the Standard Model hypothesis, the mixing angle φ_τ is measured to be 9°±16°, with an expected value of 0°±28° at the 68% confidence level. The pure CP-odd hypothesis is disfavoured at a level of 3.4 standard deviations. The results are compatible with the predictions for the Higgs boson in the Standard Model.

Measurements of Higgs boson production cross-sections are carried out in the diphoton decay channel using 139 fb⁻¹ of pp collision data at √s = 13 TeV collected by the ATLAS experiment at the LHC. The analysis is based on the definition of 101 distinct signal regions using machine-learning techniques. The inclusive Higgs boson signal strength in the diphoton channel is measured to be 1.04. Cross-sections for gluon-gluon fusion, vector-boson fusion, associated production with a W or Z boson, and top associated production processes are reported. An upper limit of 10 times the Standard Model prediction is set for the associated production process of a Higgs boson with a single top quark, which has a unique sensitivity to the sign of the top quark Yukawa coupling. Higgs boson production is further characterized through measurements of Simplified Template Cross-Sections (STXS). In total, cross-sections of 28 STXS regions are measured. The measured STXS cross-sections are compatible with their Standard Model predictions, with a p-value of 93%. The measurements are also used to set constraints on Higgs boson coupling strengths, as well as on new interactions beyond the Standard Model in an effective field theory approach. No significant deviations from the Standard Model predictions are observed in these measurements, which provide significant sensitivity improvements compared to the previous ATLAS results

The inclusive top-quark pair (tt¯) production cross-section σ_tt¯ is measured inproton–proton collisions at a centre-of-mass energy √s = 5.02 TeV, using 257 pb⁻¹ of datacollected in 2017 by the ATLAS experiment at the LHC. The tt¯ cross-section is measuredin both the dilepton and single-lepton final states of the tt¯ system and then combined. Thecombination of the two measurements yields

σ_tt¯ = 67.5 ± 0.9 (stat.) ± 2.3 (syst.) ± 1.1 (lumi.) ± 0.2 (beam) pb,

where the four uncertainties reflect the limited size of the data sample, experimental andtheoretical systematic effects, and imperfect knowledge of both the integrated luminosityand the LHC beam energy, giving a total uncertainty of 3.9%. The result is in agreementwith theoretical quantum chromodynamic calculations at next-to-next-to-leading order inthe strong coupling constant, including the resummation of next-to-next-to-leading logarithmic soft-gluon terms, and constrains the parton distribution functions of the proton atlarge Bjorken-x.

Studies of the correlations of the two highest transverse momentum (leading) jets in individual Pb+Pb collision events can provide information about the mechanism of jet quenching by the hot and dense matter created in such collisions. In Pb+Pb and pp collisions at √^sNN=5.02TeV, measurements of the leading dijet transverse momentum (p_T) correlations are presented. Additionally, measurements in Pb+Pb collisions of the dijet pair nuclear modification factors projected along leading and subleading jet p_T are made. The measurements are performed using the ATLAS detector at the LHC with 260 pb⁻¹ of pp data collected in 2017 and 2.2 nb⁻¹ of Pb+Pb data collected in 2015 and 2018. An unfolding procedure is applied to the two-dimensional leading and subleading jet p_T distributions to account for experimental effects in the measurement of both jets. Results are provided for dijets with leading jet p_T greater than 100 GeV. Measurements of the dijet-yield-normalized x_J distributions in Pb+Pb collisions show an increased fraction of imbalanced jets compared to pp collisions; these measurements are in agreement with previous measurements of the same quantity at 2.76 TeV in the overlapping kinematic range. Measurements of the absolutely normalized dijet rate in Pb+Pb and pp collisions are also presented, and show that balanced dijets are significantly more suppressed than imbalanced dijets in Pb+Pb collisions. It is observed in the measurements of the pair nuclear modification factors that the subleading jets are significantly suppressed relative to leading jets with p_T between 100 and 316 GeV for all centralities in Pb+Pb collisions

A measurement of the production of vector bottomonium states, Υ(1S), Υ(2S), and Υ(3S), in Pb+Pb and pp collisions at a center-of-mass energy per nucleon pair of 5.02 TeV is presented. The data correspond to integrated luminosities of 1.38nb⁻¹ of Pb+Pb data collected in 2018, 0.44nb⁻¹ of Pb+Pb data collected in 2015, and 0.26fb⁻¹ of pp data collected in 2017 by the ATLAS detector at the Large Hadron Collider. The measurements are performed in the dimuon decay channel for transverse momentum <30GeV, absolute rapidity |y^μμ|<1.5, and Pb+Pb event centrality 0–80%. The production rates of the three bottomonium states in Pb+Pb collisions are compared with those in pp collisions to extract the nuclear modification factors as functions of event centrality, , and |y^μμ|. In addition, the suppression of the excited states relative to the ground state is studied. The results are compared with theoretical model calculations.

This paper presents a search for a new Z′ vector gauge boson with the ATLAS experiment at the Large Hadron Collider using pp collision data collected at √s = 13 TeV, corresponding to an integrated luminosity of 139 fb⁻¹. The new gauge boson Z′ is predicted by L_μ − L_τ models to address observed phenomena that can not be explained by the Standard Model. The search examines the four-muon (4μ) final state, using a deep learning neural network classifier to separate the Z′ signal from the Standard Model background events. The di-muon invariant masses in the 4μ events are used to extract the Z′ resonance signature. No significant excess of events is observed over the predicted background. Upper limits at a 95% confidence level on the Z′ production cross-section times the decay branching fraction of pp → Z′μμ → 4μ are set from 0.31 to 4.3 fb for the Z′ mass ranging from 5 to 81 GeV. The corresponding common coupling strengths, gZ′, of the Z′ boson to the second and third generation leptons above 0.003 – 0.2 have been excluded.

A search for diphoton resonances in the mass range between 10 and 70 GeV with the ATLAS experiment at the Large Hadron Collider (LHC) is presented. The analysis is based on pp collision data corresponding to an integrated luminosity of 138 fb⁻¹ at a centre-of-mass energy of 13 TeV recorded from 2015 to 2018. Previous searches for diphoton resonances at the LHC have explored masses down to 65 GeV, finding no evidence of new particles. This search exploits the particular kinematics of events with pairs of closely spaced photons reconstructed in the detector, allowing examination of invariant masses down to 10 GeV. The presented strategy covers a region previously unexplored at hadron colliders because of the experimental challenges of recording low-energy photons and estimating the backgrounds. No significant excess is observed and the reported limits provide the strongest bound on promptly decaying axion-like particles coupling to gluons and photons for masses between 10 and 70 GeV.

Several extensions of the Standard Model predict the production of dark matter particles at the LHC. A search for dark matter particles produced in association with a dark Higgs boson decaying into W⁺W⁻ in the , final states with ℓ = e, μ is presented. This analysis uses 139 fb⁻¹ of pp collisions recorded by the ATLAS detector at a centre-of-mass energy of 13 TeV. The W^± → decays are reconstructed from pairs of calorimeter-measured jets or from track-assisted reclustered jets, a technique aimed at resolving the dense topology from a pair of boosted quarks using jets in the calorimeter and tracking information. The observed data are found to agree with Standard Model predictions. Scenarios with dark Higgs boson masses ranging between 140 and 390 GeV are excluded.

This paper presents a search for dark matter, χ, using events with a single top quark and an energetic W boson. The analysis is based on proton–proton collision data collected with the ATLAS experiment at √s= 13 TeV during LHC Run 2 (2015–2018), corresponding to an integrated luminosity of 139 fb⁻¹. The search considers final states with zero or one charged lepton (electron or muon), at least one b-jet and large missing transverse momentum. In addition, a result from a previous search considering two-charged-lepton final states is included in the interpretation of the results. The data are found to be in good agreement with the Standard Model predictions and the results are interpreted in terms of 95% confidence-level exclusion limits in the context of a class of dark matter models involving an extended two-Higgs-doublet sector together with a pseudoscalar mediator particle. The search is particularly sensitive to on-shell production of the charged Higgs boson state, H^±, arising from the two-Higgs-doublet mixing, and its semi-invisible decays via the mediator particle, a: H^±→W^±a(→χχ). Signal models with H^± masses up to 1.5 TeV and a masses up to 350 GeV are excluded assuming a tan⁡ β value of 1. For masses of a of 150 (250) GeV, tan ⁡β values up to 2 are excluded for H^± masses between 200 (400) GeV and 1.5 TeV. Signals with tan⁡ β values between 20 and 30 are excluded for H^± masses between 500 and 800 GeV.

A search for the electroweak production of pairs of charged sleptons or charginos decaying into two-lepton final states with missing transverse momentum is presented. Two simplified models of R-parity-conserving supersymmetry are considered: direct pair-production of sleptons (), with each decaying into a charged lepton and a neutralino, and direct pair-production of the lightest charginos (), with each decaying into a W-boson and a . The lightest neutralino () is assumed to be the lightest supersymmetric particle (LSP). The analyses target the experimentally challenging mass regions where m()−m() and m()−m() are close to the W-boson mass (‘moderately compressed’ regions). The search uses 139 fb⁻¹ of √s = 13 TeV proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider. No significant excesses over the expected background are observed. Exclusion limits on the simplified models under study are reported in the (,) and () mass planes at 95% confidence level (CL). Sleptons with masses up to 150 GeV are excluded at 95% CL for the case of a mass-splitting between sleptons and the LSP of 50 GeV. Chargino masses up to 140 GeV are excluded at 95% CL for the case of a mass-splitting between the chargino and the LSP down to about 100 GeV.

A search is performed for delayed and nonpointing photons originating from the displaced decay of a neutral long-lived particle (LLP). The analysis uses the full run 2 dataset of proton-proton collisions delivered by the LHC at a center-of-mass energy of √s=13  TeV between 2015 and 2018 and recorded by the ATLAS detector, corresponding to an integrated luminosity of 139  fb⁻¹. The capabilities of the ATLAS electromagnetic calorimeter are exploited to precisely measure the arrival times and trajectories of photons. The results are interpreted in a scenario where the LLPs are pair produced in exotic decays of the 125 GeV Higgs boson, and each LLP subsequently decays into a photon and a particle that escapes direct detection, giving rise to missing transverse momentum. No significant excess is observed above the expectation due to Standard Model background processes. The results are used to set upper limits on the branching ratio of the exotic decay of the Higgs boson. A model-independent limit is also set on the production of photons with large values of displacement and time delay.

A search for pair production of doubly charged Higgs bosons (H^±±), each decaying into a pair of prompt, isolated, and highly energetic leptons with the same electric charge, is presented. The search uses a proton–proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 fb⁻¹ recorded by the ATLAS detector during Run 2 of the Large Hadron Collider (LHC). This analysis focuses on same-charge leptonic decays, H^±±→ℓ^±ℓ′^± where ℓ,ℓ′=e,μ,τ, in two-, three-, and four-lepton channels, but only considers final states which include electrons or muons. No evidence of a signal is observed. Corresponding upper limits on the production cross-section of a doubly charged Higgs boson are derived, as a function of its mass m(H^±±), at 95% confidence level. Assuming that the branching ratios to each of the possible leptonic final states are equal, , the observed (expected) lower limit on the mass of a doubly charged Higgs boson is 1080 GeV (1065 GeV) within the left-right symmetric type-II seesaw model, which is the strongest limit to date produced by the ATLAS Collaboration. Additionally, this paper provides the first direct test of the Zee–Babu neutrino mass model at the LHC, yielding an observed (expected) lower limit of m(H^±±) = 900 GeV (880 GeV).

A search for flavour-changing neutral current (FCNC) tqH interactions involving a top quark, another up-type quark (q = u, c), and a Standard Model (SM) Higgs boson decaying into a τ-lepton pair (H → τ⁺τ⁻) is presented. The search is based on a dataset of pp collisions at √s = 13 TeV that corresponds to an integrated luminosity of 139 fb⁻¹ recorded with the ATLAS detector at the Large Hadron Collider. Two processes are considered: single top quark FCNC production in association with a Higgs boson (pp → tH), and top quark pair production in which one of top quarks decays into Wb and the other decays into qH through the FCNC interactions. The search selects events with two hadronically decaying τ-lepton candidates (τ_had) or at least one τ_had with an additional lepton (e, μ), as well as multiple jets. Event kinematics is used to separate signal from the background through a multivariate discriminant. A slight excess of data is observed with a significance of 2.3σ above the expected SM background, and 95% CL upper limits on the t → qH branching ratios are derived. The observed (expected) 95% CL upper limits set on the t → cH and t → uH branching ratios are 9.4×10⁻⁴(4.8×10⁻⁴) and 6.9×10⁻⁴(3.5×10⁻⁴), respectively. The corresponding combined observed (expected) upper limits on the dimension-6 operator Wilson coefficients in the effective tqH couplings are C_cϕ < 1.35 (0.97) and C_uϕ < 1.16 (0.82).

This article presents a search for new resonances decaying into a Z or W boson and a 125 GeV Higgs boson h, and it targets the νν¯¯¯bb¯¯, ℓ⁺ℓ⁻bb¯, or ℓ^±νbb¯ final states, where ℓ = e or μ, in proton-proton collisions at √s = 13 TeV. The data used correspond to a total integrated luminosity of 139 fb⁻¹ collected by the ATLAS detector during Run 2 of the LHC at CERN. The search is conducted by examining the reconstructed invariant or transverse mass distributions of Zh or Wh candidates for evidence of a localised excess in the mass range from 220 GeV to 5 TeV. No significant excess is observed and 95% confidence-level upper limits between 1.3 pb and 0.3 fb are placed on the production cross section times branching fraction of neutral and charged spin-1 resonances and CP-odd scalar bosons. These limits are converted into constraints on the parameter space of the Heavy Vector Triplet model and the two-Higgs-doublet model.

A search for a WZ resonance, in the fully leptonic final state (electrons or muons), is performed using 139 fb⁻¹ of data collected at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. The results are interpreted in terms of a singly charged Higgs boson of the Georgi–Machacek model, produced by WZ fusion, and of a Heavy Vector Triplet, with the resonance produced by WZ fusion or the Drell–Yan process. No significant excess over the Standard Model prediction is observed and limits are set on the production cross-section times branching ratio as a function of the resonance mass for these processes.

A search for a new heavy scalar or pseudo-scalar Higgs boson (H/A) produced in association with a pair of top quarks, with the Higgs boson decaying into a pair of top quarks (H/A → ) is reported. The search targets a final state with exactly two leptons with same-sign electric charges or at least three leptons. The analysed dataset corresponds to an integrated luminosity of 139 fb⁻¹ of proton–proton collisions collected at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Two multivariate classifiers are used to separate the signal from the background. No significant excess of events over the Standard Model expectation is observed. The results are interpreted in the context of a type-II two-Higgs-doublet model. The observed (expected) upper limits at 95% confidence level on the H/A production cross-section times the branching ratio of H/A → range between 14 (10) fb and 6 (5) fb for a heavy Higgs boson with mass between 400 GeV and 1000 GeV, respectively. Assuming that only one particle, either the scalar H or the pseudo-scalar A, contributes to the final state, values of tan β below 1.2 or 0.5 are excluded for a mass of 400 GeV or 1000 GeV, respectively. These exclusion ranges increase to tan β below 1.6 or 0.6 when both particles are considered.

Searches for the exclusive decays of Higgs and Z bosons into a vector quarkonium state and a photon are performed in the μ⁺μ⁻γ final state with a proton–proton collision data sample corresponding to an integrated luminosity of 139 fb⁻¹ collected at √s=13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The observed data are compatible with the expected backgrounds. The 95% confidence-level upper limits on the branching fractions of the Higgs boson decays into J/ψγ, ψ(2S)γ, and Υ(1S,2S,3S)γ are found to be 2.0×10⁻⁴, 10.5×10⁻⁴, and (2.5,4.2,3.4)×10⁻⁴, respectively, assuming Standard Model production of the Higgs boson. The corresponding 95% CL upper limits on the branching fractions of the Z boson decays are 1.2×10⁻⁶, 2.4×10⁻⁶, and (1.1,1.3,2.4)×10⁻⁶. An observed 95% CL interval of (−133,175) is obtained for the κ_c/κ_γ ratio of Higgs boson coupling modifiers, and a 95% CL interval of (−37,40) is obtained for κ_b/κ_γ.

Jet quenching is the process of color-charged partons losing energy via interactions with quark-gluon plasma droplets created in heavy-ion collisions. The collective expansion of such droplets is well described by viscous hydrodynamics. Similar evidence of collectivity is consistently observed in smaller collision systems, including pp and p+Pb collisions. In contrast, while jet quenching is observed in Pb+Pb collisions, no evidence has been found in these small systems to date, raising fundamental questions about the nature of the system created in these collisions. The ATLAS experiment at the Large Hadron Collider has measured the yield of charged hadrons correlated with reconstructed jets in 0.36  nb⁻¹ of p+Pb and 3.6  pb⁻¹ of pp collisions at 5.02 TeV. The yields of charged hadrons with > 0.5 GeV near and opposite in azimuth to jets with >30 or 60 GeV, and the ratios of these yields between p+Pb and pp collisions, I_pPb, are reported. The collision centrality of p+Pb events is categorized by the energy deposited by forward neutrons from the struck nucleus. The I_pPb values are consistent with unity within a few percent for hadrons with >4  GeV at all centralities. These data provide new, strong constraints that preclude almost any parton energy loss in central p+Pb collisions.

The flavour-tagging algorithms developed by the ATLAS Collaboration and used to analyse its dataset of √s=13 TeV pp collisions from Run 2 of the Large Hadron Collider are presented. These new tagging algorithms are based on recurrent and deep neural networks, and their performance is evaluated in simulated collision events. These developments yield considerable improvements over previous jet-flavour identification strategies. At the 77% b-jet identification efficiency operating point, light-jet (charm-jet) rejection factors of 170 (5) are achieved in a sample of simulated Standard Model events; similarly, at a c-jet identification efficiency of 30%, a light-jet (b-jet) rejection factor of 70 (9) is obtained.