Photon Interaction:

https://phys.org/news/2017-08-atlas-evidence-light-by-light.html

ATLAS observes direct evidence of light-by-light scattering:

August 15, 2017 by Katarina Anthony

Physicists from the ATLAS experiment at CERN have found the first direct evidence of high energy light-by-light scattering, a very rare process in which two photons – particles of light – interact and change direction. The result, published today in Nature Physics, confirms one of the oldest predictions of quantum electrodynamics (QED).

"This is a milestone result: the first direct evidence of light interacting with itself at high energy," says Dan Tovey(University of Sheffield), ATLAS Physics Coordinator. "This phenomenon is impossible in classical theories of electromagnetism; hence this result provides a sensitive test of our understanding of QED, the quantum theory of electromagnetism."


Read more at: https://phys.org/news/2017-08-atlas-evidence-light-by-light.html#jCp

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys4208.html?foxtrotcallback=true

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

Abstract:

Light-by-light scattering (γγ  γγ) 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 μb⁻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 ± 0.7 events. After background subtraction and analysis corrections, the fiducial cross-section of the process Pb + Pb (γγ)  Pb^(∗) +  Pb^(∗)γγ, for photon transverse energy E_T > 3 GeV, photon absolute pseudorapidity |η| < 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 ± 24 (stat.) ± 17 (syst.) nb, which is in agreement with the standard model predictions.