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

LHCC 2017 POSTER. Light-by-light scattering ($\gamma\gamma\rightarrow\gamma\gamma$) 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 (Pb) ions. Using 480$\mu \text{b}^{-1}$ of Pb+Pb collision data recorded at a centre-of-mass energy per nucleon pair of $5.02$ TeV by the ATLAS detector, the ATLAS Collaboration reports evidence for the $\gamma\gamma\rightarrow\gamma\gamma$ reaction. A total of $13$ candidate events are observed with an expected background of $2.6\pm0.7$ events. After background subtraction and analysis corrections, the fiducial cross section of the process $\textrm{Pb+Pb}\,(\gamma\gamma)\rightarrow \textrm{Pb}^{(\ast)}\textrm{+}\textrm{Pb}^{(\ast)}\,\gamma\gamma$, for photon transverse energy $E_{\text{T}}>3$ GeV, photon absolute pseudorapidity $|\eta|<2.4$, diphoton invariant mass greater than $6$ GeV, diphoton transverse momentum lower than 2 GeV and diphoton acoplanarity below 0.01, is measured to be $70 \pm 24~\textrm{(stat.)} \pm 17~\textrm{(syst.)}$ nb, which is in agreement with Standard Model predictions.