Abstract: This paper describes the measurements of flow harmonics v(2)-v(6) in 3 μb(-1) of Xe Xe collisions at root S-NN = 5.44 TeV performed using the ATLAS detector at the Large Hadron Collider (LHC). Measurements of the centrality, multiplicity, and p(T) dependence of the v(n) obtained using two-particle correlations and the scalar product technique are presented. The measurements are also performed using a template-fit procedure, which was developed to remove nonflow correlations in small collision systems. This nonflow removal is shown to have a significant influence on the measured v(n) at high p(T), especially in peripheral events. Comparisons of the measured v(n) with measurements in Pb + Pb collisions and p + Pb collisions at root S-NN = 5.02 TeV are also presented. The v(n) values in Xe + Xe collisions are observed to be larger than those in Pb + Pb collisions for n = 2, 3, and 4 in the most central events. However, with decreasing centrality or increasing harmonic order n, the v(n) values in Xe + Xe collisions become smaller than those in Pb + Pb collisions. The v(n) in Xe + Xe and Pb + Pb collisions are also compared as a function of the mean number of participating nucleons, < N-part >, and the measured charged-particle multiplicity in the detector. The v(3) values in Xe + Xe and Pb + Pb collisions are observed to be similar at the same < N-part > or multiplicity, but the other harmonics are significantly different. The ratios of the measured v(n) in Xe + Xe and Pb + Pb collisions, as a function of centrality, are also compared to theoretical calculations.

Abstract: Exclusive dimuon production in ultraperipheral collisions (UPC), resulting from photon-photon interactions in the strong electromagnetic fields of colliding high-energy lead nuclei, PbPb(gamma gamma) -> mu(+) mu(-) (Pb-(*Pb-)(()*())), is studied using L-int = 0.48 nb(-1) of root S-NN = 5.02 TeV lead-lead collision data at the LHC with the ATLAS detector. Dimuon pairs are measured in the fiducial region p(T,mu) > 4 GeV, vertical bar eta(mu)vertical bar < 2.4, invariant m(mu mu) > 10 GeV, and p(T,mu mu) <2 GeV. The primary background from single-dissociative processes is extracted from the data using a template fitting technique. Differential cross sections are presented as a function of m(mu mu), absolute pair rapidity (vertical bar y(mu mu)vertical bar), scattering angle in the dimuon rest frame (vertical bar cos v*(mu mu)vertical bar), and the colliding photon energies. The total cross section of the UPC gamma gamma -> mu(+) mu(-) process in the fiducial volume is measured to be sigma(mu mu)(fid) = 34.1 +0.3(stat.)+0.7(syst.) μb. Generally good agreement is found with calculations from STARlight, which incorporate the leading-order Breit-Wheeler process with no final-state effects, albeit differences between the measurements and theoretical expectations are observed. In particular, the measured cross sections at larger vertical bar y(mu mu)vertical bar are found to be about 10-20% larger in data than in the calculations, suggesting the presence of larger fluxes of photons in the initial state. Modification of the dimuon cross sections in the presence of forward and/or backward neutron production is also studied and is found to be associated with a harder incoming photon spectrum, consistent with expectations.

Abstract: Leptons with essentially the same properties apart from their mass are grouped into three families (or flavours). The number of leptons of each flavour is conserved in interactions, but this is not imposed by fundamental principles. Since the formulation of the standard model of particle physics, the observation of flavour oscillations among neutrinos has shown that lepton flavour is not conserved in neutrino weak interactions. So far, there has been no experimental evidence that this also occurs in interactions between charged leptons. Such an observation would be a sign of undiscovered particles or a yet unknown type of interaction. Here the ATLAS experiment at the Large Hadron Collider at CERN reports a constraint on lepton-flavour-violating effects in weak interactions, searching for Z-boson decays into a tau lepton and another lepton of different flavour with opposite electric charge. The branching fractions for these decays are measured to be less than 8.1 x 10(-6) (e tau) and 9.5 x 10(-6) (mu tau) at the 95% confidence level using 139 fb(-1) of proton-proton collision data at a centre-of-mass energy of root s = 13 TeV and 20.3 fb(-1) at root s = 8 TeV. These results supersede the limits from the Large Electron-Positron Collider experiments conducted more than two decades ago.

Abstract: The standard model of particle physics(1-4) describes the known fundamental particles and forces that make up our Universe, with the exception of gravity. One of the central features of the standard model is a field that permeates all of space and interacts with fundamental particles(5-9). The quantum excitation of this field, known as the Higgs field, manifests itself as the Higgs boson, the only fundamental particle with no spin. In 2012, a particle with properties consistent with the Higgs boson of the standard model was observed by the ATLAS and CMS experiments at the Large Hadron Collider at CERN10,11. Since then, more than 30 times as many Higgs bosons have been recorded by the ATLAS experiment, enabling much more precise measurements and new tests of the theory. Here, on the basis of this larger dataset, we combine an unprecedented number of production and decay processes of the Higgs boson to scrutinize its interactions with elementary particles. Interactions with gluons, photons, and W and Z bosons-the carriers of the strong, electromagnetic and weak forces-are studied in detail. Interactions with three third-generation matter particles (bottom (b) and top (t) quarks, and tau leptons (tau)) are well measured and indications of interactions with a second-generation particle (muons, mu) are emerging. These tests reveal that the Higgs boson discovered ten years ago is remarkably consistent with the predictions of the theory and provide stringent constraints on many models of new phenomena beyond the standard model.

Abstract: Silicon pixel detectors are at the core of the current and planned upgrade of the ATLAS experiment at the LHC. Given their close proximity to the interaction point, these detectors will be exposed to an unprecedented amount of radiation over their lifetime. The current pixel detector will receive damage from non-ionizing radiation in excess of 10(15) 1 MeV n(eq)/cm(2), while the pixel detector designed for the high-luminosity LHC must cope with an order of magnitude larger fluence. This paper presents a digitization model incorporating effects of radiation damage to the pixel sensors. The model is described in detail and predictions for the charge collection efficiency and Lorentz angle are compared with collision data collected between 2015 and 2017 (<= 10(15) 1 MeV n(eq)/cm(2)).