Observation of light-by-light scattering 
in lead-lead collisions in the ATLAS experiment

Light-by-light (LbyL) scattering, $\gamma\gamma\rightarrow\gamma\gamma$, is a very rare process allowed in Quantum Electrodynamics via a loop diagram. The precise measurement of this process is potentially sensitive to contributions from Beyond Standard Model, for example from axion-like particles. Despite the small cross-section, the LbyL scattering can be observed in ultra-peripheral high energy heavy-ion collisions. Based on 2018 Pb+Pb dataset, corresponding to integrated luminosity of 1.73 nb$^{-1}$, the first direct observation of LbyL scattering was reported by ATLAS Collaboration in 2019. 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.) $\pm$ 7 (syst.) $\pm$ 3 (lumi.) nb and agrees with Standard Model prediction within two standard deviations.