Speaker
Description
Light-by-light (LbyL) scattering, $\gamma\gamma\rightarrow\gamma\gamma$, is a quantum-mechanical process, forbidden by the classical theory of electrodynamics, but possible in Quantum Electrodynamics via a loop diagram. Despite the small cross-section, it is theoretically possible to observe this process in ultra-peripheral high energy heavy-ion collisions. Based on 0.48 nb$^{-1}$ of 2015 Pb+Pb data, a first direct evidence of LbyL scattering was established by the ATLAS Collaboration in 2017 with 4.4$\sigma$ significance over the background-only hypothesis. The observation of LbyL scattering was reported by ATLAS Collaboration in 2019, based on 2018 Pb+Pb dataset corresponding to integrated luminosity of 1.73 $\mathrm{nb}^{-1}$. In total, 59 events were found in the signal region with a background expectation of 12$\,\pm$ 3 events. The observed signal significance over the background-only hypothesis amounts to 8.2$\sigma$. The measured fiducial cross-section is 78$\,\pm\,$13 (stat.)$\,\pm\,$8 (syst.) nb.
