Scientists working on the Compact Muon Solenoid (CMS) experiment at CERN have published the latest data in their search for exotic, long-lived particles known as dark photons.
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Dark photons (also called hidden photons) differ from ordinary photons – particles of light – in that they are thought to have mass, making them prime candidates to explain dark matter. Dark matter is a general term to describe something seemingly invisible in space that has only been observed through gravitational effects, but has never been directly detected, and no one is sure what it actually is.
Physicists at CMS are trying to change that. Like the particles produced in other experiments at CERN, the hypothetical dark photon would result from the decay of another particle: the Higgs boson, proposed in the 1960s and famously observed in 2012. The Higgs boson is thought to break down into dark photons, which would then decay into the muon is displaced. The CMS Collaborative is trying to limit the parameters within which that process will occur.
CERN’s Large Hadron Collider begins its third run in July 2022, with a larger particle collision capacity than previous runs. This means the CMS experiment algorithm – or “trigger” – that detects interesting collisions has more events to filter through, and thus more opportunities to see displaced muons resulting from dark photons.
Graph showing how muon signals can be traced back to the decay point of a long-lived particle. Graphics: CMS/CERN
“We really improved our ability to detect displaced muons,” said Juliette Alimena of the CMS experiment in a statement. “This allows us to collect far more events than before with muons displaced from the impact point by distances from several hundred micrometers to several meters. Thanks to these improvements, if dark photons are present, CMS is now much more likely to find them.”
Dark photons are considered long-lived, by particle standards: they exist for ten-billionths of a second. Despite their longevity, they are difficult to see – which is why no one has done it. In fact, the search for dark photons has been going on for years. “The search for dark photons is simultaneously easy and challenging,” physicist James Beacham told Gizmodo in 2018. “It’s easy because the concept is general and simple enough that designing an experimental search is easy enough, but challenging because we really don’t know where in the parameter space dark photons can exist.”
Some scientists are searching for dark matter using tiny mirrors, while others are trying to capture its frequencies with “dark matter radio.” At CMS, physicists try to look at particles as they break down into pairs of muons.
To support CMS, the Large Hadron Collider will be upgraded soon. The upcoming High Luminosity-LHC will increase the facility’s luminosity by a factor of 10 and increase the number of Higgs bosons that physicists have to study by one degree. HL-LHC is expected to be ready for operation in 2029. Meanwhile, Run 3 LHC will continue until 2026.
Data from particle collisions continues to reveal new subatomic particles for study, but some – thought to be responsible for the dark matter in the universe – remain invisible. At least for now.
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2023-12-31 09:29:59
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