Observation of Light-by-Light Scattering in Ultraperipheral Pb+Pb Collisions with the ATLAS Detector

Area of Science:

• High-energy physics
• Quantum electrodynamics
• Particle physics

Background:

• Light-by-light scattering is a fundamental prediction of quantum electrodynamics.
• Observing this process provides a direct test of the Standard Model.
• Previous observations were limited by low statistics or indirect evidence.

Purpose of the Study:

• To directly observe and measure the light-by-light scattering process (γγ→γγ).
• To test predictions of quantum electrodynamics in ultra-peripheral lead-lead collisions.
• To provide precise measurements of the fiducial cross section for this rare process.

Main Methods:

• Analysis of lead-lead collision data collected by the ATLAS experiment at the Large Hadron Collider (LHC).
• Selection of exclusive two-photon events with specific kinematic criteria (transverse energy, pseudorapidity, invariant mass, acoplanarity).
• Statistical analysis to determine the significance of the observed signal over expected background.

Main Results:

• Observation of 59 candidate events for light-by-light scattering against an expected background of 12±3 events.
• The excess of events corresponds to a significance of 8.2 standard deviations.
• A measured fiducial cross section of 78±13(stat)±7(syst)±3(lumi) nb was determined.

Conclusions:

• The ATLAS experiment provides the first direct observation of light-by-light scattering in heavy-ion collisions.
• The results are in agreement with the predictions of quantum electrodynamics.
• This observation opens new avenues for precision tests of the Standard Model.