Physicists with the CMS Collaboration at CERN’s Large Hadron Collider (LHC) have presented their first search for new physics using data from LHC Run 3.
Illustration of two types of long-lived particles decaying into a pair of muons, showing how the signals of the muons can be traced back to the long-lived particle decay point using data from the tracker and muon detectors. Image credit: CMS / CERN.
Dark photons are hypothetical long-lived dark sector particles proposed as a force carrier similar to the photon of electromagnetism.
‘Long-lived’ because they have an average lifetime of more than a tenth of a billionth of a second.
This may seem like an incredibly short amount of time, but in terms of the particles produced in LHC collisions it is actually quite long.
The Higgs boson, for example, has a lifetime ten billion times shorter!
In fact, particles with lifetimes longer than a thousandth of a billionth of a second are already classed ‘long-lived.’
In terms of the CMS detector, this means that a dark photon would travel a measurable distance before decaying, making it potentially detectable.
‘Exotic’ because they are not part of the Standard Model of particle physics, which is the leading theory guiding our understanding of the fundamental blocks of the Universe.
However, the Standard Model does not answer all the questions within particle physics, and so searches for phenomena ‘beyond the Standard Model’ continue.
The new CMS result defines more constrained limits on the parameters of the decay of Higgs bosons to dark photons, further narrowing down the area in which physicists can search for them.
In theory, dark photons would travel a measurable distance in the CMS detector before they decay into ‘displaced muons.’
If scientists were to retrace the tracks of these muons, they would find that they don’t reach all the way to the collision point, because the tracks come from a particle that has already moved some distance away, without any trace.
Run 3 of the LHC began in July 2022 and has a higher instantaneous luminosity than previous LHC runs, meaning there are more collisions happening at any one moment for researchers to analyze.
The LHC produces tens of millions of collisions every second, but only a few thousand of them can be stored, as recording every collision would quickly consume all the available data storage.
This is why CMS is equipped with a real-time data selection algorithm called the trigger, which decides whether or not a given collision is interesting.
Therefore, it is not only a higher volume of data that could help to reveal evidence of the dark photon, but also the way in which the trigger system is fine-tuned to look for specific phenomena.
“We have really improved our ability to trigger on displaced muons,” said Dr. Juliette Alimena, a member of the CMS Collaboration.
“This allows us to collect much more events than before with muons that are displaced from the collision point by distances from a few hundred micrometers to several meters.”
“Thanks to these improvements, if dark photons exist, CMS is now much more likely to find them.”
The CMS physicists will continue using the most powerful techniques to analyze all data taken in the remaining years of Run 3 operations, with the aim of further exploring physics beyond the Standard Model.
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CMS Collaboration. 2023. Search for long-lived particles decaying to a pair of muons in pp collisions at √s= 13.6 TeV with 2022 data. CMS-PAS-EXO-23-014
