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

J. Chudoba1, J.Hejbal1, O.Hladík1,Jakoubek1, O. Kepka1, A. Kupčo1, V. Kús1, M. Lokajíček1, R. Lysák1, M. Marčišovský1, M. Mikeštíková1, S. Němeček1, O. Penc1, P. Šícho1, P. Staroba1, M. Svatoš1, M. Taševský.1 M. Aaboud et al.2

Light-by-light scattering (γγ right arrow γγ) 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 ions. Using 480 μb−1 of lead–lead collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, here we report evidence for light-by-light scattering. A total of 13 candidate events were observed with an expected background of 2.6 ± 0.7 events. After background subtraction and analysis corrections, the fiducial cross-section of the process Pb + Pb (γγ) right arrow Pb(∗) + Pb(∗)γγ, for photon transverse energy ET > 3 GeV, photon absolute pseudorapidity |η| < 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 ± 24 (stat.) ± 17 (syst.) nb, which is in agreement with the standard model predictions.

a, Diphoton acoplanarity before applying the Aco < 0.01 requirement. b, Diphoton invariant mass after applying the Aco < 0.01 requirement. Data (points) are compared to MC predictions (histograms). The statistical uncertainties on the data are shown as vertical bars.

1Institute of Physics, Czech Academy of Sciences, Na Slovance 1992/2, 1822 Prague, Czech Republic.
2Atlas Collaboration