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

February 28, 2017
As the result of light-by-light scattering, two low-energy photons are observed in the ATLAS detector. Credit: CERN/ATLAS experiment

Scientists from the ATLAS collaboration at the LHC have found evidence for light-by-light scattering, in which two photons interact and change their trajectory. Researchers from DESY, the Johannes Gutenberg University Mainz and the AGH University of Science and Technology in Krakow performed the study.

"According to , beams of light pass each other without being scattered. But if we take quantum physics into account, light can be scattered by light, even though this phenomenon seems very improbable", explains Mateusz Dyndal, a DESY scientist who played a major role in the data analysis. One of the oldest predictions of quantum electrodynamics says that photons, the carrier particles of the electromagnetic force, can interact and scatter off of each other. This process has been tested in different environments, but a direct observation of light-by-light has not previously been achieved.

In 2012, physicists proposed that light-by-light scattering could be observed in collisions at the LHC. Protons that are accelerated to nearly the speed of light produce a very strong electromagnetic field. The generated field is even stronger when Lead ions are used rather than protons . When two such ions pass each other in a so-called ultra-peripheral collision, two photons can scatter off one another while the ions themselves stay intact. The scientists then observe two low-energy photons with specific kinematic properties and no additional activity in the detector. Based on the data taken in 2015 at the LHC, physicists at the ATLAS experiment conducted a search for light-by-light scattering and found 4.4σ evidence for the phenomenon. The σ-value describes the statistical significance of a scientific result. Physicists usually speak of a "discovery" if they find a 5σ result and call a 3σ result "evidence" for something new. Light-by-light scattering has a very small cross-section, which means that it happens very rarely. Thus in four billionanalysed events, only 13 candidates for such diphoton events were observed.

Because the scientists observed only a few events attributed to light-by-light scattering, the statistical accuracy of their results is limited. When the next lead-lead run at the LHC starts (end of 2018), they hope to collect more data to test this phenomenon more precisely. Further studies could also provide an additional window into new physics at the LHC. "Maybe we can find evidence for physics beyond the standard model of particle physics, for example axion-like particles that are a possible candidate for dark matter. Different theoretical concepts predict that light-by-light scattering can be sensitive to such particles", says Dyndal.

Explore further: Researchers explore the billiard dynamics of photon collisions

More information: Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC. arxiv.org/pdf/1702.01625.pdf

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14 comments

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Da Schneib
5 / 5 (4) Feb 28, 2017
Now THIS is interesting. It's long been predicted and never observed. It will be even more interesting to see if Pb-p near misses will show this to a higher significance next year.
Kron
5 / 5 (4) Feb 28, 2017
Now THIS is interesting.

Quite. I'd add that 4.4 sigma confidence (99.81%) is quite indicative of the process. I don't doubt that 5 sigma will be hit given that a large enough data set is collected and that enough events are observed during the next run.
Hyperfuzzy
1 / 5 (3) Mar 01, 2017
This is not that! Charge motion, nothing else. I am concerned, do we apply known science or do we not know what we are doing? Calibration? Finding the impossible? This I would not publish. It's childish. Controls!

In order to deflect light, another charge or more is required, i.e. a media! To deflect charge, any field will do. This is what you see!
Kron
5 / 5 (3) Mar 02, 2017
This is not that! Charge motion, nothing else. I am concerned, do we apply known science or do we not know what we are doing? Calibration? Finding the impossible? This I would not publish. It's childish. Controls!

In order to deflect light, another charge or more is required, i.e. a media! To deflect charge, any field will do. This is what you see!

You have no idea what you are talking about. I'm sorry. The process is quantum vacuum fluctuation dependent, it involves charge. Could be EM (QED) in case of lepton creation or Strong force (QCD) and QED in case of quark creation. Point being that photons fluctuate into particle pairs allowing interaction. Do you honestly think you know more of this than these teams of researchers?
RNP
5 / 5 (4) Mar 02, 2017
@Hyperfuzzy
To reinforce what Kron has expalined to you, there is a simple explanation given on Wikipedia ( https://en.wikipe..._physics ). It says:

"From quantum electrodynamics it can be found that photons cannot couple directly to each other, since they carry no charge, but they can interact through higher-order processes: a photon can, within the bounds of the uncertainty principle, fluctuate into a virtual charged fermion–antifermion pair, to either of which the other photon can couple. This fermion pair can be leptons or quarks. Thus, two-photon physics experiments can be used as ways to study the photon structure, or, somewhat metaphorically, what is "inside" the photon."
Hyperfuzzy
1 / 5 (2) Mar 02, 2017
@Hyperfuzzy
To reinforce what Kron has expalined to you, there is a simple explanation given on Wikipedia ( https://en.wikipe..._physics ). It says:

"From quantum electrodynamics it can be found that photons cannot couple directly to each other, since they carry no charge, but they can interact through higher-order processes: a photon can, within the bounds of the uncertainty principle, fluctuate into a virtual charged fermion–antifermion pair, to either of which the other photon can couple. This fermion pair can be leptons or quarks. Thus, two-photon physics experiments can be used as ways to study the photon structure, or, somewhat metaphorically, what is "inside" the photon."

Is this supported by your non-theoretical theory, QM? My information is supported by the same physics used to create your instrumentation. I submit, you are in error!
SiaoX
not rated yet Mar 02, 2017
Now THIS is interesting. It's long been predicted and never observed. It will be even more interesting to see if Pb-p near misses will show this to a higher significance next year.

It was observed, for example here in 1997:

A signal of 106±14 positrons above background has been observed in collisions of a low-emittance 46.6 GeV electron beam with terawatt pulses from a Nd:glass laser at 527 nm wavelength in an experiment at the Final Focus Test Beam at SLAC. The positrons are interpreted as arising from a two-step process in which laser photons are backscattered to GeV energies by the electron beam followed by a collision between the high-energy photon and several laser photons to produce an electron-positron pair. These results are the first laboratory evidence for inelastic light-by-light scattering involving only real photons.
Captain Stumpy
5 / 5 (2) Mar 02, 2017
Is this supported by your non-theoretical theory, QM?
@hyper
quick question:
if QM = Quantum Mechanics

then... how, exactly, is QM a "non-theoretical theory"?

that makes absolutely no sense: it's one of the more validated scientific theories, for starters... so when you say
My information is supported by the same physics used to create your instrumentation
it makes even less sense

what physics are you, specifically, using to create instrumentation?

GoodElf
5 / 5 (1) Mar 02, 2017
@SiaoX:
It was observed, for example http://www.slac.s...ers.html in 1997

That was sort of "right", the abstract shows it happened only, in that case, the interaction was to produce electron-positron pairs not "simple" scattering of two "prosaic" photons off each other deflecting their mutual paths. In the case you mentioned this "trick" required very energetic photons to pull off this stunt and it is "easier" to measure the electron and positron in an accelerator than possibly correlated photon pairs. The "photon cluster" energy resolution is extracted from data using γγ → e+e− events to get this result. The process of scattering two photons off each other is a prerequisite to finding the pair production event... but what about low energy collisions of γ photons? Nit pickers will always have some other strange possibility to put forward such as "virtual photons" for example.

A bejillion workers at ATLAS appended their names. Job satisfaction.
Kron
not rated yet Mar 02, 2017
@SiaoX,
From article:
This process has been tested in different environments, but a direct observation of light-by-light scattering has not previously been achieved.

Key words here being "direct observation."
Hyperfuzzy
1 / 5 (1) Mar 03, 2017
Now THIS is interesting. It's long been predicted and never observed. It will be even more interesting to see if Pb-p near misses will show this to a higher significance next year.

It was observed, for example http://www.slac.s...ers.html in 1997:

A signal of 106±14 positrons above background has been observed in collisions of a low-emittance 46.6 GeV electron beam with terawatt pulses from a Nd:glass laser at 527 nm wavelength in an experiment at the Final Focus Test Beam at SLAC. The positrons are interpreted as arising from a two-step process in which laser photons are backscattered to GeV energies by the electron beam followed by a collision between the high-energy photon and several laser photons to produce an electron-positron pair. These results are the first laboratory evidence for inelastic light-by-light scattering involving only real photons.

Check how the beam is created. Those positrons are protons.
Hyperfuzzy
1 / 5 (1) Mar 03, 2017
Is this supported by your non-theoretical theory, QM?
@hyper
quick question:
if QM = Quantum Mechanics

then... how, exactly, is QM a "non-theoretical theory"?

that makes absolutely no sense: it's one of the more validated scientific theories, for starters... so when you say
My information is supported by the same physics used to create your instrumentation
it makes even less sense

what physics are you, specifically, using to create instrumentation?


Go back to school. It's a tool. No theory. No axioms. No Photons.

Please note, a photon is a field event; there are no particles. I don't know WTF the SM is. So please don't try to tell me gluons hold the nucleus together, until you describe the NXN Matrix with your defined stability of the objects you are trying to either control, measure a response, create magic! Whatever
jonesdave
5 / 5 (1) Mar 03, 2017
Check how the beam is created. Those positrons are protons


Err, yep. Those dumb scientists, and their even dumber equipment, couldn't differentiate between two particles, one of which is ~1800 times more massive than the other. Idiots. They really should head over to phys.org for an education.
Hyperfuzzy
not rated yet Mar 03, 2017
Check how the beam is created. Those positrons are protons


Err, yep. Those dumb scientists, and their even dumber equipment, couldn't differentiate between two particles, one of which is ~1800 times more massive than the other. Idiots. They really should head over to phys.org for an education.

Close, but The Field and charge do not have mass. I thought we settled this, either rewrite physics as we know it, or simply accept it with a smile!

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