OBSERVATION OF LIGHT-BY-LIGHT SCATTERING AND MEASUREMENTS OF PHOTON-PHOTON COLLISIONS AT ATLAS

We present the observation of the light-by-light scattering process, $\gamma\gamma\rightarrow \gamma\gamma$, in lead-lead collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02~TeV. The analysis is conducted using 1.73~$\mathrm{nb}^{-1}$ of data collected in November 2018 by the ATLAS experiment at the LHC. Light-by-light scattering event candidates are selected in events with two photons produced exclusively, with small diphoton transverse momentum and small acoplanarity. After applying all selection criteria, 59 candidate events are observed for a background expectation of $12\pm 3$ events. An excess of events over the expected background is found with an observed significance of 8.2 standard deviations. The fiducial cross-section is also measured and compared to the theoretical predictions. In addition, we present the measurements of $\gamma\gamma\rightarrow W^+W^-$ and $\gamma\gamma\rightarrow \mu^+\mu^-$ in proton-proton collisions at ATLAS. The production of $\gamma\gamma\rightarrow \mu^+\mu^-$ is measured at a centre-of-mass energy of 13~TeV using 3.2~$\mathrm{fb}^{-1}$. Fiducial and differential cross-sections are compared to theoretical predictions both with and without corrections for absorptive effects. Exclusive production of $W^+W^-$ consistent with the Standard Model~(SM) prediction is found with a $3\sigma$ significance using 20.2~$\mathrm{fb}^{-1}$ of data at a centre-of-mass energy of 8 TeV. The fiducial cross-section is measured and found to be in agreement with SM predictions. Constraints are placed on anomalous quartic gauge boson interactions.