Light Scatters Light

ATLAS event display showing the energy deposits of two photons in the electromagnetic calorimeter on opposite sides and no other activity in the detector, which is the clean signature of light-by-light scattering. (Source: ATLAS Coll., CERN)

Light-by-light scattering is a very rare pheno­menon in which two photons interact, producing another pair of photons. This process was among the earliest predictions of quantum electro­dynamics (QED), the quantum theory of electromagnetism, and is forbidden by classical physics theories such as Maxwell’s theory of electro­dynamics. Direct evidence for light-by-light scattering at high energy had proven elusive for decades, until the Large Hadron Collider (LHC) began its second data-taking period.

Collisions of lead ions in the LHC provide a uniquely clean environ­ment to study light-by-light scattering. Bunches of lead ions that are accelerated to very high energy are surrounded by an enormous flux of photons. Indeed, the coherent action from the large number of 82 protons in a lead atom with all the electrons stripped off give rise to an electro­magnetic field of up to 1025 Volt per meter. When two lead ions pass close by each other at the center of the ATLAS detector, but at a distance greater than twice the lead ion radius, those photons can still interact and scatter off one another without any further inter­action between the lead ions, as the reach of the much stronger strong force is bound to the radius of a single proton. These inter­actions are known as ultra-peri­pheral collisions.

In 2017, the ATLAS Colla­boration found thirteen candidate events for light-by-light scattering in lead-lead collision data recorded in 2015, for 2.6 events expected from background processes. The correspon­ding signi­ficance of this result was 4.4 standard deviations – making it the first direct evidence of high-energy light-by-light scattering. Today, at the Rencontres de Moriond conference in 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 run 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, 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 – 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 of 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. (Source: CERN)

Reference: ATLAS Collaboration: Observation of light-by-light scattering in ultraperipheral Pb+Pb collisions with the ATLAS detector, ATLAS-CONF-2019-002, 17 March 2019

Link: ATLAS Experiment, CERN, Geneva, Switzerland

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