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For centuries, scientists argued whether light was waves or particles. Light scattering with other light would favor the particle concept. Today we know both models are wrong, but quantum electrodynamics also predicts this scattering - just with an incredibly tiny rate, so it has never been observed before. Lead-lead collisions at the LHC allow a search for it: if the nuclei just pass each other without a direct collision, the intense electromagnetic fields around them can lead to photon-photon interactions. Typically electron/positron pairs are produced, but sometimes the product are photons again.
ATLAS searched for events with two photons and nothing else in the detector (meaning the lead nuclei stayed intact). The expected number of background events (other processes looking like the signal) was 2.7, the expected number of signal events was 7.3, and ATLAS saw 13 events. The significance is 4.4 sigma - the probability of getting 13 background events with just 2.7 expected is very small.
As comparison: Those 13 events were the needle in a haystack of a few billion more violent nucleus-nucleus collisions.
If you think the Higgs took long to discover: 60 years is short compared to the centuries needed to see light-by-light scattering.
CMS should have a similar dataset, but no public result yet. Apart from that, improving the measurements will need more lead-lead collisions, currently scheduled for the end of 2018.CERN Courier article
ATLAS note
ATLAS searched for events with two photons and nothing else in the detector (meaning the lead nuclei stayed intact). The expected number of background events (other processes looking like the signal) was 2.7, the expected number of signal events was 7.3, and ATLAS saw 13 events. The significance is 4.4 sigma - the probability of getting 13 background events with just 2.7 expected is very small.
As comparison: Those 13 events were the needle in a haystack of a few billion more violent nucleus-nucleus collisions.
If you think the Higgs took long to discover: 60 years is short compared to the centuries needed to see light-by-light scattering.
CMS should have a similar dataset, but no public result yet. Apart from that, improving the measurements will need more lead-lead collisions, currently scheduled for the end of 2018.CERN Courier article
ATLAS note