Results of a search for supersymmetry via direct production of third-generation squarks are reported, using 20.3 fb −1 of proton-proton collision data at ffiffi ffi s p ¼ 8 TeV recorded by the ATLAS experiment at the LHC in 2012. Two different analysis strategies based on monojetlike and c-tagged event selections are carried out to optimize the sensitivity for direct top squark-pair production in the decay channel to a charm quark and the lightest neutralino (t 1 → c þχ 0 1 ) across the top squark-neutralino mass parameter space. No excess above the Standard Model background expectation is observed. The results are interpreted in the context of direct pair production of top squarks and presented in terms of exclusion limits in the (m~t 1 , m~χ0 1 ) parameter space. A top squark of mass up to about 240 GeV is excluded at 95% confidence level for arbitrary neutralino masses, within the kinematic boundaries. Top squark masses up to 270 GeV are excluded for a neutralino mass of 200 GeV. In a scenario where the top squark and the lightest neutralino are nearly degenerate in mass, top squark masses up to 260 GeV are excluded. The results from the monojetlike analysis are also interpreted in terms of compressed scenarios for top squark-pair production in the decay channelt 1 → b þ ff 0 þχ 0 1 and sbottom pair production withb 1 → b þχ 0 1 , leading to a similar exclusion for nearly mass-degenerate third-generation squarks and the lightest neutralino. The results in this paper significantly extend previous results at colliders.
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...
A determination of the top-quark mass is presented using 20.2 fb −1 of 8 TeV proton-proton collision data produced by the Large Hadron Collider and collected by the ATLAS experiment. The normalised differential cross section of top-quark pair production in association with an energetic jet is measured in the lepton+jets final state and unfolded to parton and particle levels. The unfolded distribution at parton level can be described using next-to-leading-order QCD predictions in terms of either the top-quark pole mass or the running mass as defined in the (modified) minimal subtraction scheme. A comparison between the experimental distribution and the theoretical prediction allows the top-quark mass to be extracted in the two schemes. The value obtained for the pole-mass scheme is: m pole t = 171.1 ± 0.4 (stat) ± 0.9 (syst) +0.7 −0.3 (theo) GeV. The extracted value in the running-mass scheme is: m t (m t) = 162.9 ± 0.5 (stat) ± 1.0 (syst) +2.1 −1.2 (theo) GeV. The results for the top-quark mass using the two schemes are consistent, when translated from one scheme to the other.
The ATLAS CollaborationThis paper presents the electron and photon energy calibration obtained with the ATLAS detector using about 36 fb −1 of LHC proton-proton collision data recorded at √ s = 13 TeV in 2015 and 2016. The different calibration steps applied to the data and the optimization of the reconstruction of electron and photon energies are discussed. The absolute energy scale is set using a large sample of Z boson decays into electron-positron pairs. The systematic uncertainty in the energy scale calibration varies between 0.03% to 0.2% in most of the detector acceptance for electrons with transverse momentum close to 45 GeV. For electrons with transverse momentum of 10 GeV the typical uncertainty is 0.3% to 0.8% and it varies between 0.25% and 1% for photons with transverse momentum around 60 GeV. Validations of the energy calibration with J/ψ → e + e − decays and radiative Z boson decays are also presented.
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