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 ET > 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.
Highlights
Background estimationDue to its relatively high rate, the exclusive production of electron pairs (γ γ → e+e−) can be a source of fake diphoton events
The γ γ → e+e− background is modelled with STARlight 1.1, in which the cross-section is computed by combining the Pb + Pb equivalent photon approximation (EPA) with the leading-order formula for γ γ → e+e−
Good agreement is observed between data and Monte Carlo (MC) simulation in both control regions, but the precision is limited by the number of events in data
Summary
30), in which the cross-section is computed by combining the Pb + Pb EPA with the leading-order formula for γ γ → e+e− This process has been recently measured by the ALICE Collaboration, and a good agreement with STARlight is found[33]. The exclusive diphoton final state can be produced via the strong interaction through a quark loop in the exchange of two gluons in a colour-singlet state (see Supplementary Fig. 2) This CEP process, gg → γ γ , is modelled using SUPERCHIC 2.03 It is based on a coincidence of signals in both ZDC sides and a requirement on the total ET in the calorimeter below 50 GeV. The extracted Level-1 trigger efficiency is provided as a function of the
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