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19 March, 2019
The ATLAS Collaboration has reported the observation of light-by-light scattering with a significance beyond eight standard deviations
Image above: An ATLAS event with energy deposits of two photons in the electromagnetic calorimeter (green) on opposite sides and no other activity in the detector, a clean signature of light-by-light scattering. The Feynman diagram of this process is also shown (Image: CERN).
Light-by-light scattering is a very rare phenomenon in which two photons – particles of light – interact, producing again a pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of electromagnetism, and is forbidden in classical physics (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 (Run 2). Collisions of lead ions in the LHC provide a uniquely clean environment to study light-by-light scattering. The bunches of lead ions that are accelerated to very high energy are surrounded by an enormous flux of photons. When two lead ions pass close by each other at the centre of the ATLAS detector, but with 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.
Yesterday, 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 uses data from the most recent heavy-ion operation of the LHC, which took place in November 2018. 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.
Read more on the ATLAS website: https://atlas.cern/updates/physics-briefing/atlas-observes-light-scattering-light
Note:
CERN, the European Organization for Nuclear Research, is one of the world’s largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. At CERN, the world’s largest and most complex scientific instruments are used to study the basic constituents of matter — the fundamental particles. By studying what happens when these particles collide, physicists learn about the laws of Nature.
The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies before they are made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions.
Founded in 1954, the CERN Laboratory sits astride the Franco–Swiss border near Geneva. It was one of Europe’s first joint ventures and now has 22 Member States.
Related links:
Large Hadron Collider (LHC): https://home.cern/science/accelerators/large-hadron-collider
ATLAS detector: https://home.cern/science/experiments/atlas
Rencontres de Moriond: http://moriond.in2p3.fr/2019/EW/
Observation of light-by-light scattering: http://cdsweb.cern.ch/record/2667214
For more information about European Organization for Nuclear Research (CERN), Visit: https://home.cern/
Image (mentioned), Text, Credit: European Organization for Nuclear Research (CERN).
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