How ATLAS Scientists use CERN LHC to make Photons collide from Lead Nuclei

“This is a very basic process. It’s never been observed before, and here it is finally emerging from the data”

Theoretical physicist John Ellis of King’s College London commenting on the discovery by CERN

Looks like it the month for massless particles to interact. This time the latest discover comes to us thanks to CERN, host of the LHC (Large Hadron Collider).

Researchers with the ATLAS experiment at the LHC reported that photons do bump into each other as reported in the article “Normally aloof particles of light seen ricocheting off each other”, published August 14 2017 by Emily Conover, Science News.

ATLAS Researchers published their results in August 14^th 2017 in Nature Physics. The discovery is a huge achievement for the ATLAS Scientists, as it proves that light does collide with itself at high -energies to quote  of the ATLAS Physics Coordinator and Physicist from the University of Sheffield, Dr. Dan Tovey: “This is a milestone result: the first direct evidence of light interacting with itself at high energy. This phenomenon is impossible in classical theories of electromagnetism; hence this result provides a sensitive test of our understanding of QED, the quantum theory of electromagnetism”

 Photon scattering is extremely rare and difficult to spot. However, Photons with more energy interact more often, providing additional chances to spot scattering. Enter the LHC.

ATLAS Scientists make Photons collide - How Heavy Lead reveals Light Collisions

A quick primer on the quantum mechanics of photons.

This process is called pair-production and is the result of ultra-peripheral collisions in the otherwise empty space of the LHC as pointed out in “ATLAS observes direct evidence of light-by-light scattering”, published August 15, 2017 by Katarina Anthony, Physorg.

According to quantum mechanics, photons can briefly transform into transient pairs of electrically charged particles and antiparticles.  This is extremely important in high energy physics, as the result is an electron and a positron, as predicted by the Feynman diagram shown below.

This stage lasts for an extremely short space of time before reverting back to photons. it is this transition that has been of great interest for years to heavy-ion and high-energy physics communities as pointed out by ATLAS Heavy Ion Physics Group Convener of the Brookhaven National Laboratory, Peter Steinberg, quote: “This measurement has been of great interest to the heavy-ion and high-energy physics communities for several years, as calculations from several groups showed that we might achieve a significant signal by studying lead-ion collisions in Run 2”.

Photons have no electric charge, they shouldn’t notice one another’s presence. As they are massless, however, they would require a high amount of energy to make them interact and ricochet away from one another. The ATLAS scientists used CERN's LHC to accelerate Lead (Pb) nuclei and smash them together.

The strong electromagnetic fields created by the massive nuclei would trap any photons produced and beam scattering could be observed. This took a lot of data gathering, as the scientists were looking for only two (2) photons that collided and scattered away. This means spotting only two photons  in their enormous, highly sensitive particle detector, and accounting for every possible particle produced during the collision.

Using a process of elimination and calculating probable paths, the researchers found 13 such events over 19 days of data collection.  The predictions agree with the standard model, physicists’ theory of particle physics, but more research is needed, as potentially, this soup of Pb nuclei fragments could hint at the existence of new, undiscovered particles.