Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC
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Aaboud, M., Aad, G., Abbott, B. et al. Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC. Nature Phys 13, 852–858 (2017). [https://doi.org/10.1038/nphys4208]
SponsorshipBCKDF; Canada Council; CANARIE; Australian Government; Department of Industry, Innovation and Science; Cooperative Research Centres (CRC) Programme; Compute Canada; FQRNT; Ontario Innovation Trust, Canada; EPLANET; European Research Council (ERC); European Union (EU); FP7; Horizon 2020; French National Research Agency (ANR); Fondation Partager le Savoir, France; German Research Foundation (DFG); Alexander von Humboldt Foundation; Thales programme; Aristeia programme; Greek Ministry of Development-GSRT; BSF, Israel; German-Israeli Foundation for Scientific Research and Development; Minerva, Israel; BRF, Norway; Generalitat Valenciana; Royal Society of London; Leverhulme Trust
Light-by-light scattering (γγ → 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) → 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.