ATLAS observes direct evidence of light-by-light scattering

August 15, 2017 by Katarina Anthony, CERN
A light-by-light scattering event measured in the ATLAS detector. Credit: ATLAS/CERN

Physicists from the ATLAS experiment at CERN have found the first direct evidence of high energy light-by-light scattering, a very rare process in which two photons – particles of light – interact and change direction. The result, published today in Nature Physics, confirms one of the oldest predictions of quantum electrodynamics (QED).

"This is a milestone result: the first direct of light interacting with itself at high energy," says Dan Tovey(University of Sheffield), ATLAS Physics Coordinator. "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."

Direct evidence for light-by-light scattering at high energy had proven elusive for decades – until the Large Hadron Collider's second run began in 2015. As the accelerator collided at unprecedented rates, obtaining evidence for light-by-light scattering became a real possibility. "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," says Peter Steinberg (Brookhaven National Laboratory), ATLAS Heavy Ion Physics Group Convener.

Heavy-ion collisions provide a uniquely clean environment tostudy light-by-light scattering. As bunches of lead ions are accelerated, an enormous flux of surrounding photons is generated. When ions meet at the centre of the ATLAS detector, very few collide, yet their surrounding photons can interact and scatter off one another. These interactions are known as 'ultra-peripheral collisions'.

Studying more than 4 billion events taken in 2015, the ATLAS collaboration found 13 candidates for light-by-light scattering. This result has a significance of 4.4 standard deviations, allowing the ATLAS collaboration to report the first direct evidence of this phenomenon at .

"Finding evidence of this rare signature required the development of a sensitive new 'trigger' for the ATLAS detector," says Steinberg. "The resulting signature—two photons in an otherwise empty detector—is almost the diametric opposite of the tremendously complicated eventstypically expected from lead nuclei collisions. The new trigger's success in selecting these events demonstrates the power and flexibility of the system, as well as the skill and expertise of the analysis and trigger groups who designed and developed it."

ATLAS physicists will continue to study light-by-light scattering during the upcoming LHC heavy-ion run, scheduled for 2018. More data will further improve the precision of theresult and may open a new window to studies of new physics. In addition, the study of ultra-peripheral collisions should play a greater role in the LHC heavy-ion programme, as collision rates further increase in Run 3 and beyond.

Explore further: Scientists find evidence for light-by-light scattering, long standing prediction of the Standard Model

More information: M. Aaboud et al. Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC, Nature Physics (2017). DOI: 10.1038/nphys4208

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not rated yet Aug 15, 2017
How different is this photon-to-photon collision when compared to the 'photon-to-virtual-particle collision' theorised for EmDrive? I read that this photon-to-photon collision also involves virtual-particles, so I wonder if photons can just collide with virtual-particles at low energy environment, as with EmDrive?
Whydening Gyre
3.7 / 5 (3) Aug 15, 2017
EMdrive does NOT provide thrust via photon/virtual particle interaction.
Aug 15, 2017
This comment has been removed by a moderator.
3 / 5 (2) Aug 15, 2017
Don't high energy photons have relatively high equivalent mass? Wouldn't that mass have a gravitational effect on other high energy photons with their own relatively high equivalent mass?
Da Schneib
3.7 / 5 (3) Aug 16, 2017
This is really important. This type of collision is no longer theoretical (assuming it pans out to 5 sigma, which it's very close to doing at 4.4). It is however worth noting that it has been seen at lower significance at both the LEP and LHC, as well as other accelerators, over the past decade.

Don't high energy photons have relatively high equivalent mass? Wouldn't that mass have a gravitational effect on other high energy photons with their own relatively high equivalent mass?
Gravity is too weak for this to be measured with current technology. The interactions are charge interactions depending on the charges of the virtual particles, or interactions of a photon with one of the virtual particles the other photon oscillates into.
Whydening Gyre
5 / 5 (2) Aug 16, 2017
Photons interact with electrons, which are BOUND to massive entities, protons.
I realize that may seem to simple an explanation for you...
Whydening Gyre
3 / 5 (2) Aug 16, 2017
Photons interact with electrons, which are BOUND to massive entities, protons.
I realize that may seem to simple an explanation for you...

oops, I hit edit instead of quote....
I did not say EMdrive does not provide thrust. I said it does not provide thrust via photon/virtual particle interaction...
Read the whole quote.
3 / 5 (2) Aug 19, 2017
There was no direct light-by-light scattering in the experiment. The heavy ions scattered with changing of initial directions together with their surronding photons.
Aug 20, 2017
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