The ATLAS CollaborationThis letter describes the observation of the light-by-light scattering process, γγ → γγ, in Pb+Pb collisions at √ s NN = 5.02 TeV. The analysis is conducted using a data sample corresponding to an integrated luminosity of 1.73 nb −1 , collected in November 2018 by the ATLAS experiment at the LHC. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy E γ T > 3 GeV and pseudorapidity |η γ | < 2.4, diphoton invariant mass above 6 GeV, and small diphoton transverse momentum and acoplanarity. After applying all selection criteria, 59 candidate events are observed for a background expectation of 12 ± 3 events. The observed excess of events over the expected background has a significance of 8.2 standard deviations. The measured fiducial cross section is 78 ± 13 (stat.) ± 7 (syst.) ± 3 (lumi.) nb.Light-by-light scattering, γγ → γγ, is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics [1, 2]. In the Standard Model (SM), the γγ → γγ reaction proceeds at one-loop level at order α 4 (where α is the fine-structure constant) via virtual box diagrams involving electrically charged fermions (leptons and quarks) or W ± bosons. However, in various extensions of the SM, extra contributions are possible, making the measurement of γγ → γγ scattering sensitive to new physics. Relevant examples are magnetic monopoles [3], vector-like fermions [4] and axion-like particles [5,6]. The light-by-light cross section is also sensitive to the effect of possible non-SM operators in an effective field theory [7][8][9]. Light-by-light scattering graphs with electron loops also contribute to the anomalous magnetic moment of the electron and muon [10,11].Strong evidence for this process in relativistic heavy-ion (Pb+Pb) collisions at the Large Hadron Collider (LHC) has been reported by the ATLAS [12] and CMS [13] collaborations with observed significances of 4.4 and 4.1 standard deviations, respectively. Exclusive light-by-light scattering can occur in these collisions at impact parameters larger than about twice the radius of the ions, as demonstrated for the first time in Ref. [14]. The strong interaction becomes less significant and the electromagnetic (EM) interaction becomes more important in these ultraperipheral collision (UPC) events. In general, this allows to study processes involving nuclear photoexcitation, photoproduction of hadrons, and two-photon interactions [15,16]. The EM fields produced by the colliding Pb nuclei can be described as a beam of quasi-real photons with a small virtuality of Q 2 < 1/R 2 , where R is the radius of the charge distribution and so Q 2 < 10 −3 GeV 2 [17, 18]. The cross section for the elastic reaction Pb+Pb (γγ) → Pb+Pb γγ can then be calculated by convolving the appropriate photon flux with the elementary cross section for the process γγ → γγ. Since the photon flux associated with each nucleus scales with the square of the number of protons, the cross section is strongl...
Electron reconstruction and identification in the ATLAS experiment using the 2015 and 2016 LHC proton-proton collision data at √ s = 13 TeVThe ATLAS Collaboration Algorithms used for the reconstruction and identification of electrons in the central region of the ATLAS detector at the Large Hadron Collider (LHC) are presented in this paper; these algorithms are used in ATLAS physics analyses that involve electrons in the final state and which are based on the 2015 and 2016 proton-proton collision data produced by the LHC at √ s = 13 TeV. The performance of the electron reconstruction, identification, isolation, and charge identification algorithms is evaluated in data and in simulated samples using electrons from Z → ee and J/ψ → ee decays. Typical examples of combinations of electron reconstruction, identification, and isolation operating points used in ATLAS physics analyses are shown.
Measurement of the tt Z and ttW cross sections in proton-proton collisions at √ s = 13 TeV with the ATLAS detectorThe ATLAS Collaboration A measurement of the associated production of a top-quark pair (tt) with a vector boson (W, Z) in proton-proton collisions at a center-of-mass energy of 13 TeV is presented, using 36.1 fb −1 of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. Events are selected in channels with two same-or opposite-sign leptons (electrons or muons), three leptons or four leptons, and each channel is further divided into multiple regions to maximize the sensitivity of the measurement. The tt Z and ttW production cross sections are simultaneously measured using a combined fit to all regions. The best-fit values of the production cross sections are σ tt Z = 0.95 ± 0.08 stat. ± 0.10 syst. pb and σ ttW = 0.87 ± 0.13 stat. ± 0.14 syst. pb in agreement with the Standard Model predictions. The measurement of the tt Z cross section is used to set constraints on effective field theory operators which modify the tt Z vertex.
A search for new physics with non-resonant signals in dielectron and dimuon final states in the mass range above 2 TeV is presented. This is the first search for non-resonant signals in dilepton final states at the LHC to use a background estimate from the data. The data, corresponding to an integrated luminosity of 139 fb−1, were recorded by the ATLAS experiment in proton-proton collisions at a center-of-mass energy of $$ \sqrt{s} $$ s = 13 TeV during Run 2 of the Large Hadron Collider. The benchmark signal signature is a two-quark and two-lepton contact interaction, which would enhance the dilepton event rate at the TeV mass scale. To model the contribution from background processes a functional form is fit to the dilepton invariant-mass spectra in data in a mass region below the region of interest. It is then extrapolated to a high-mass signal region to obtain the expected background there. No significant deviation from the expected background is observed in the data. Upper limits at 95% CL on the number of events and the visible cross-section times branching fraction for processes involving new physics are provided. Observed (expected) 95% CL lower limits on the contact interaction energy scale reach 35.8 (37.6) TeV.
Jet substructure quantities are measured using jets groomed with the soft-drop grooming procedure in dijet events from 32.9 fb −1 of pp collisions collected with the ATLAS detector at ffiffi ffi s p ¼ 13 TeV. These observables are sensitive to a wide range of QCD phenomena. Some observables, such as the jet mass and opening angle between the two subjets which pass the soft-drop condition, can be described by a high-order (resummed) series in the strong coupling constant α S. Other observables, such as the momentum sharing between the two subjets, are nearly independent of α S. These observables can be constructed using all interacting particles or using only charged particles reconstructed in the inner tracking detectors. Trackbased versions of these observables are not collinear safe, but are measured more precisely, and universal nonperturbative functions can absorb the collinear singularities. The unfolded data are directly compared with QCD calculations and hadron-level Monte Carlo simulations. The measurements are performed in different pseudorapidity regions, which are then used to extract quark and gluon jet shapes using the predicted quark and gluon fractions in each region. All of the parton shower and analytical calculations provide an excellent description of the data in most regions of phase space.
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