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

Collaboration † Light-by-light scattering ( (cid:13) (cid:13) ! (cid:13) (cid:13) ) 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 (cid:22) b (cid:0) 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 (cid:6) 0.7 events. After background subtraction and analysis corrections, the fiducial cross-section of the process Pb C Pb ( (cid:13) (cid:13) ) ! Pb . (cid:3) / C Pb ( (cid:3) ) (cid:13) (cid:13) , for photon transverse energy E T > 3 GeV, photon absolute pseudorapidity j (cid:17) j < 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 (cid:6) 24 (stat.) (cid:6) 17 (syst.) nb, which is in agreement with the standard model predictions.

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