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martyb writes:

ATLAS Observes Light Scattering off Light — New result studies photons interacting at high energies:

[...] Today, at the Rencontres de Moriond conference (La Thuile, Italy), the ATLAS Collaboration reported the observation of light-by-light scattering with a significance of 8.2 standard deviations. The result utilises data from the most recent heavy-ion operation of the LHC, which took place in November 2018. About 3.6 times more events (1.73 nb⁻¹) were collected compared to 2015. The increased dataset, in combination with improved analysis techniques, allowed the measurement of the scattering of light-by-light with greatly improved precision. A total of 59 candidate events were observed (see Figure 2), for 12 events expected from background processes. From these numbers, the cross section of this process, restricted to the kinematic region considered in the analysis, was calculated as 78 ± 15 nb.

Curiously, the signature of this process – two photons in an otherwise empty detector (see the event display in Figure 1) – is almost the opposite of the tremendously rich and complex events typically observed in high-energy collisions of two lead nuclei. Observing it required the development of improved trigger algorithms for fast online event selection, as well as a specifically-adjusted photon-identification algorithm using a neural network, as the studied photons have about ten times less energy than the lowest energetic photons usually measured with the ATLAS detector. Being able to record these events demonstrates the power and flexibility of the ATLAS detector and its event reconstruction, which was designed for very different event topologies.

This new measurement opens the door to further study the light-by-light scattering process, which is not only interesting in itself as a manifestation of an extremely rare QED phenomenon, but may be sensitive to contributions from particles beyond the Standard Model. It allows for a new generation of searches for hypothetical light and neutral particles.

[...]Links:

• Observation of light-by-light scattering in ultraperipheral Pb+Pb collisions with the ATLAS detector (ATLAS-CONF-2019-002, see figures)
• Moriond EW presentation by Mateusz Dyndal: Light-by-light scattering in ATLAS and CMS in Run 2
• Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC (Nature Physics 13 (2017) 852, see figures)
• Press Statement: ATLAS sees first direct evidence of light-by-light scattering at high energy
• ALP paper: Searching for axion-like particles with ultra-peripheral heavy-ion collisions (arXiv: 1607.06083)
• See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

So how does this fit in with the wave/particle duality of light?

Original Submission