When charged particles from outer space called cosmic rays collide with particles in the Earth’s atmosphere, they create a shower of secondary particles (air showers) that cascade down to Earth. These secondary particles include photons, electrons, and muons. Some of these secondary particles reach the IceCube Neutrino Observatory at the South Pole, a detector consisting of a surface array, IceTop, and a cubic kilometer in-ice detector.
Using IceCube data, scientists are able to infer properties of the primary cosmic ray (before it entered the Earth’s atmosphere), such as energy and mass, by comparing detailed simulations of the air shower development with experimental observations. This is important for an overall understanding of what cosmic rays are made of and ultimately, where they come from.
In a study submitted to Physical Review D, the IceCube Collaboration presents a new measurement of the high-energy muon component of cosmic-ray-induced air showers based on events detected in coincidence at IceTop and in the in-ice detector. The mean number of muons with energies above 500 GeV in the air shower was in agreement with all of the hadronic interaction models.
Like other extensive air shower detectors, IceTop’s measurements of secondary particles rely on simulations and models that describe particle interactions in the atmosphere. However, previous studies have reported significant discrepancies between the measurements of the muon component of air showers and data from simulations, known as the “muon puzzle.”
“By performing a variety of muon measurements under different conditions, we can test these models and work towards resolving the muon puzzle,” says Stef Verpoest, a postdoctoral researcher at the University of Delaware and study lead. “With IceCube, we have the opportunity to perform not only low-energy muon measurements at the surface with IceTop, but also to measure the high-energy muons penetrating deep into the ice.”
Because the thick ice sheet covering the in-ice detector absorbs all muons with energies below several hundred GeV, low-energy muons, or GeV muons, are detected at the surface by IceTop while a bundle of high-energy muons, or TeV muons, from the shower can subsequently produce a track-like event in the in-ice array. The TeV muons are particularly interesting since they are predominantly produced in the early stages of air shower development.
For the study, researchers estimated the energy of the primary cosmic ray from the particles that triggered IceTop, while the number of high-energy muons from the same shower was estimated from the bright tracks they produced in the ice. By reconstructing the energy loss of the muons based on their light deposit and feeding this into a neural network, the researchers were able to estimate the average number of muons with energies above 500 GeV in the air shower as a function of the primary cosmic-ray energy in the energy range between 2.5 and 100 PeV.
They also compared the result to a previous measurement of the low-energy muon content in air showers determined with IceTop only and found that one of the models did not consistently describe the experimental data.
“These measurements will inform model builders about shortcomings in current models and may lead to improved future iterations,” says Verpoest. “These models are crucial for understanding the properties of cosmic rays at the highest energies, an important ingredient towards understanding their sources.”
Currently, the high-energy muon measurement can only be performed with the unique setup of IceCube, with the IceCube Collaboration looking to capitalize on this by repeating the analyses with updated versions of the hadronic models used in the current study.
+ info “Measurement of the mean number of muons with energies above 500 GeV in air showers detected with the IceCube Neutrino Observatory,” IceCube Collaboration: R. Abbasi et al. Submitted to Physical Review D. arxiv.org/abs/2506.19241