[General] Atlas observes light scattering off light

[DataPacRat]

>From my newsfeed, an article that seems likely to be of interest to
this group: https://atlas.cern/updates/physics-briefing/atlas-observes-light-scattering-light
(via https://news.ycombinator.com/item?id=19417619 ).


Quoting the article for future reference, in case it falls into the memory hole:


ATLAS observes light scattering off light
New result studies photons interacting at high energies
By ATLAS Collaboration, 17th March 2019

Figure 1: ATLAS event display showing the energy deposits of two
photons in the electromagnetic calorimeter (green) on opposite sides
and no other activity in the detector, which is the clean signature of
light-by-light scattering. The Feynman diagram of this process is
shown in the lower right corner. (Image: ATLAS Collaboration/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. Indeed, the coherent action from the large number of 82
protons in a lead atom with all the electrons stripped off (as is the
case for the lead ions in the LHC) give rise to an electromagnetic
field of up to 1025 Volt per metre. 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.

In a result published in Nature Physics 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 (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.




Thank you for your time,
--
DataPacRat
"Does aₘᵢₙ=2c²/Θ ? I don't know, but wouldn't it be fascinating if it were?"