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 for the first time at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead (Pb) ions. Using 480μb−1 of Pb+Pb collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, we report evidence for the γγ→γγ process. A total of 13 candidate events are observed with a background expectation 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 has been measured to be 70±24 (stat.)±17 (syst.) nb, which is in agreement with the Standard Model predictions.
Highlights
EstimationsContributions from different background sources are taken into account: central exclusive production (CEP) gg → γγ, γγ → e+ e−, gg → qq, exclusive two-meson production and other backgrounds that could fake the diphoton signatures.Electron final statesDue to its relatively high rate, the exclusive production of electron pairs could be a source of fake diphoton events
Contributions from different background sources are taken into account: CEP gg → γγ, γγ → e+ e−, gg → qq, exclusive two-meson production and other backgrounds that could fake the diphoton signatures
This mostly corresponds to a scenario when one photon is induced by the presence of one of the electrons and the other photon is emitted in hard bremsstrahlung
Summary
One of the key features of Maxwell’s Equations is their linearity, from which follows the superposition principle. In addition to improving signal efficiency it is designed to maximise expected signal significance To reduce real photon background from central exclusive production (CEP) gg → γγ reactions, an addi∆φ tional requirement on acoplanarity, Aco = (1 − πγγ ), is used. The final acoplanarity requirement is optimised to retain high signal efficiency and reduce the CEP background significantly. Nsig / Nsig + Ngg→γγ , where Nsig is the expected number of signal events and Ngg→γγ is the expected background This is obtained for Acomax = 0.01 and this value is taken as a final diphoton acoplanarity requirement. The fiducial volume of the diphoton final state is defined using the following requirements: Normalized counts
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