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      Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC

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          Abstract

          Light-by-light scattering (\(\gamma\gamma\rightarrow\gamma\gamma\)) is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics. This reaction is accessible at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead (Pb) ions. Using 480 \(\mu\)b\(^{-1}\) of Pb+Pb collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, the ATLAS Collaboration reports evidence for the \(\gamma\gamma\rightarrow\gamma\gamma\) reaction. A total of 13 candidate events are observed with an expected background of 2.6\(\pm\)0.7 events. After background subtraction and analysis corrections, the fiducial cross section of the process \(\textrm{Pb+Pb}\,(\gamma\gamma)\rightarrow \textrm{Pb}^{(\ast)}\textrm{+}\textrm{Pb}^{(\ast)}\,\gamma\gamma\), for photon transverse energy \(E_{\mathrm{T}}>\)3 GeV, photon absolute pseudorapidity \(|\eta|<\)2.4, diphoton invariant mass greater than 6 GeV, diphoton transverse momentum lower than 2 GeV and diphoton acoplanarity below 0.01, is measured to be 70 \(\pm\) 24 (stat.) \(\pm\) 17 (syst.) nb, which is in agreement with Standard Model predictions.

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          Asymptotic formulae for likelihood-based tests of new physics

          We describe likelihood-based statistical tests for use in high energy physics for the discovery of new phenomena and for construction of confidence intervals on model parameters. We focus on the properties of the test procedures that allow one to account for systematic uncertainties. Explicit formulae for the asymptotic distributions of test statistics are derived using results of Wilks and Wald. We motivate and justify the use of a representative data set, called the "Asimov data set", which provides a simple method to obtain the median experimental sensitivity of a search or measurement as well as fluctuations about this expectation.
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            Properties of heavy quarkonia and B_c mesons in the relativistic quark model

            The mass spectra and electromagnetic decay rates of charmonium, bottomonium and B_c mesons are comprehensively investigated in the relativistic quark model. The presence of only heavy quarks allows the expansion in powers of their velocities. All relativistic corrections of order v^2/c^2, including retardation effects and one-loop radiative corrections, are systematically taken into account in the computations of the mass spectra. The obtained wave functions are used for the calculation of radiative magnetic dipole (M1) and electric dipole (E1) transitions. It is found that relativistic effects play a substantial role. Their account and the proper choice of the Lorentz structure of the quark-antiquark interaction in a meson is crucial for bringing theoretical predictions in accord with experimental data. A detailed comparison of the calculated decay rates and branching fractions with available experimental data for radiative decays of charmonium and bottomonium is presented. The possibilities to observe the currently missing spin-singlet S and P states as well as D states in bottomonium are discussed. The results for B_c masses and decays are compared with other quark model predictions.
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              The analytical value of the electron light-light graphs contribution to the muon (g−2) in QED

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                Author and article information

                Journal
                2017-02-06
                Article
                1702.01625
                141f92fc-7d00-4658-9946-367eed8ea5dc

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

                History
                Custom metadata
                CERN-EP-2016-316
                31 pages in total, author list starting page 15, 3 figures, 2 tables. Submitted to Nature Physics. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2016-05/
                hep-ex nucl-ex

                High energy & Particle physics,Nuclear physics
                High energy & Particle physics, Nuclear physics

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