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A search for nonresonant Higgs boson pair production in the final state is performed using 140 fb −1 of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. This analysis supersedes and expands upon the previous nonresonant ATLAS results in this final state based on the same data sample. The analysis strategy is optimised to probe anomalous values not only of the Higgs ( H ) boson self-coupling modifier κ λ but also of the quartic HHVV ( V = W, Z ) coupling modifier κ 2V . No significant excess above the expected background from Standard Model processes is observed. An observed upper limit μ HH < 4.0 is set at 95% confidence level on the Higgs boson pair production cross-section normalised to its Standard Model prediction. The 95% confidence intervals for the coupling modifiers are −1.4 < κ λ < 6.9 and −0.5 < κ 2V < 2.7, assuming all other Higgs boson couplings except the one under study are fixed to the Standard Model predictions. The results are interpreted in the Standard Model effective field theory and Higgs effective field theory frameworks in terms of constraints on the couplings of anomalous Higgs boson (self-)interactions.

This paper reports the observation of top-quark pair production in proton-lead collisions in the ATLAS experiment at the Large Hadron Collider. The measurement is performed using 165 nb − 1  of  p +Pb data collected at √s NN  = 8 . 16 TeV in 2016. Events are categorised in two analysis channels, consisting of either events with exactly one lepton (electron or muon) and at least four jets, or events with two opposite-charge leptons and at least two jets. In both channels at least one  b -tagged jet is also required. Top-quark pair production is observed with a significance over five standard deviations in each channel. The top-quark pair production cross-section is measured to be nb, with a total uncertainty of 9%. In addition, the nuclear modification factor is measured to be . The measurements are found to be in good agreement with theory predictions involving nuclear parton distribution functions.

The ratio of branching ratios of the W boson tomuons and electrons, R μ/eW = B(W → μν)/B(W → eν), has been measured using 140 fb−1 of pp collision data at√s = 13 TeV collected with the ATLAS detector at the LHC, probing the universality of lepton couplings. The ratiois obtained from measurements of the tt¯ production crosssection in the ee, eμ and μμ dilepton final states. To reduce systematic uncertainties, it is normalised by the square rootof the corresponding ratio R μμ/eeZ for the Z boson measured in inclusive Z → ee and Z → μμ events. By using the precise value of R μμ/eeZ determined from e+e− colliders, the ratio R μ/eW is determined to be R μ/eW = 0.9995 ± 0.0022 (stat) ± 0.0036 (syst)± 0.0014 (ext). The three uncertainties correspond to data statistics, experimental systematics and the external measurement of R μμ/eeZ , giving a total uncertainty of 0.0045, and confirming the Standard Model assumption of lepton flavour universality in W-boson decays at the 0.5% level

The inclusive Higgs boson production cross-section is measured in the di-photon and the ZZ* → 4ℓ decay channels using 31.4 and 29.0 fb −1 of pp collision data respectively, collected with the ATLAS detector at a centre-of-mass energy of √ s =13.6 TeV. To reduce the model dependence, the measurement in each channel is restricted to a particle-level phase space that closely matches the channel’s detector-level kinematic selection, and it is corrected for detector effects. These measured fiducial cross-sections are σ fid,γγ = fb, and σ fid,4ℓ = 2.80±0.74 fb, in agreement with the corresponding Standard Model predictions of 67.6±3.7 fb and 3.67±0.19 fb. Assuming Standard Model acceptances and branching fractions for the two channels, the fiducial measurements are extrapolated to the full phase space yielding total cross-sections of σ ( pp → H )= pb and 46±12 pb at 13.6 TeV from the di-photon and ZZ *→ 4ℓ measurements respectively. The two measurements are combined into a total cross-section measurement of σ ( pp → H )=58.2±8.7 pb, to be compared with the Standard Model prediction of σ ( pp → H ) SM =59.9±2.6 pb.

Exclusive production of dielectron pairs, γγ → e + e − , is studied using = 1.72 nb −1 of data from ultraperipheral collisions of lead nuclei at √s NN = 5.02 TeV recorded by the ATLAS detector at the LHC. The process of interest proceeds via photon–photon interactions in the strong electromagnetic fields of relativistic lead nuclei. Dielectron production is measured in the fiducial region defined by following requirements: electron transverse momentum > 2.5 GeV, absolute electron pseudorapidity | η e | < 2.5, dielectron invariant mass m ee > 5 GeV, and dielectron transverse momentum < 2 GeV. Differential cross-sections are measured as a function of m ee , average , absolute dielectron rapidity | y ee |, and scattering angle in the dielectron rest frame, |cos θ * |, in the inclusive sample, and also with a requirement of no activity in the forward direction. The total integrated fiducial cross-section is measured to be 215±1(stat.)(syst.)±4(lumi.) μb. Within experimental uncertainties the measured integrated cross-section is in good agreement with the QED predictions from the Monte Carlo programs STARLIGHT and SUPERCHIC, confirming the broad features of the initial photon fluxes. The differential cross-sections show systematic differences from these predictions which are more pronounced at high | y ee | and |cos θ * | values.

This paper reports the first ATLAS measurement of the → μμ effective lifetime. The measurement is based on the data collected in 2015–2016, amounting to 26.3 fb −1 of 13 TeV LHC proton-proton collisions. The proper decay-time distribution of 58 ± 13 background-subtracted signal candidates is fit with simulated signal templates parameterised as a function of the effective lifetime, with statistical uncertainties extracted through a Neyman construction. The resulting effective measurement of the → μμ lifetime is (stat.) ± 0.17 (syst.) ps and it is found to be consistent with the Standard Model.

The total and differential Higgs boson production cross-sections are measured through a combined statistical analysis of the H → ZZ * → 4ℓ and H → γγ decay channels. The results are based on a dataset of 139 fb −1 of proton–proton collisions at a centre-of-mass energy of 13 TeV, recorded by the ATLAS detector at the Large Hadron Collider. The measured total Higgs boson production cross-section is pb, consistent with the Standard Model prediction of 55.6 ± 2.5 pb. All results from the two decay channels are compatible with each other, and their combination agrees with the Standard Model predictions. A combined statistical interpretation of the measured fiducial cross-sections as a function of the Higgs boson transverse momentum is performed in order to probe the Yukawa couplings to the bottom and charm quarks. A similar interpretation is performed by including also the constraints from the measurements of Higgs boson production in association with a W or Z boson in the and decay channels.

A search for non-resonant Higgs boson pair ( HH ) production is presented, in which one of the Higgs bosons decays to a  b -quark pair () and the other decays to WW * ,  ZZ * , or  τ + τ − , with in each case a final state with  ℓ + ℓ − + neutrinos ( ℓ  =  e, μ ). The analysis targets separately the gluon-gluon fusion and vector boson fusion production modes. Data recorded by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV at the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb − 1 , are used in this analysis. Events are selected to have exactly two  b -tagged jets and two leptons with opposite electric charge and missing transverse momentum in the final state. These events are classified using multivariate analysis algorithms to separate the  HH  events from other Standard Model processes. No evidence of the signal is found. The observed (expected) upper limit on the cross-section for non-resonant Higgs boson pair production is determined to be 9.7 (16.2) times the Standard Model prediction at 95% confidence level. The Higgs boson self-interaction coupling parameter  κ λ  and the quadrilinear coupling parameter  κ 2 V  are each separately constrained by this analysis to be within the ranges [ − 6 . 2 ,  13 . 3] and [ − 0 . 17 ,  2 . 4], respectively, at 95% confidence level, when all other parameters are fixed.

This paper presents measurements of charged-hadron spectra obtained in  pp ,  p +Pb, and Pb+Pb collisions at √s or √s NN = 5 . 02 TeV, and in Xe+Xe collisions at √s NN = 5 . 44 TeV. The data recorded by the ATLAS detector at the LHC have total integrated luminosities of 25 pb − 1 , 28 nb − 1 , 0.50 nb − 1 , and 3  μ b − 1 , respectively. The nuclear modification factors  R p Pb  and  R AA  are obtained by comparing the spectra in heavy-ion and  pp  collisions in a wide range of charged-particle transverse momenta and pseudorapidity. The nuclear modification factor  R p Pb  shows a moderate enhancement above unity with a maximum at  p T  ≈ 3 GeV; the enhancement is stronger in the Pb-going direction. The nuclear modification factors in both Pb+Pb and Xe+Xe collisions feature a significant, centrality-dependent suppression. They show a similar distinct  p T -dependence with a local maximum at  p T  ≈ 2 GeV and a local minimum at  p T  ≈ 7 GeV. This dependence is more distinguishable in more central collisions. No significant | η |-dependence is found. A comprehensive comparison with several theoretical predictions is also provided. They typically describe  R AA  better in central collisions and in the  p T  range from about 10 to 100 GeV.

A study of the polarisation and CP properties in ZZ production is presented. The used data set corresponds to an integrated luminosity of 140 fb −1 of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. The ZZ candidate events are reconstructed using two same-flavour opposite-charge electron or muon pairs. The production of two longitudinally polarised Z bosons is measured with a significance of 4.3 standard deviations, and its cross-section is measured in a fiducial phase space to be 2.45 ± 0.60 fb, consistent with the next-to-leading-order Standard Model prediction. The inclusive differential cross-section as a function of a CP-sensitive angular observable is also measured. The results are used to constrain anomalous CP-odd neutral triple gauge couplings.