Earth absorbs high-energy 'ghost particles'

Some high-energy neutrinos are absorbed as they pass through Earth.

IceCube consists of multiple optical sensors encased in pristine Antarctic ice. Photo: Jamie Yang.

Neutrinos are known as “ghost particles” because of their ability to pass through solid objects with ease. However, an international team of researchers working at IceCube, the world’s largest neutrino detector in Antarctica, found that some of these subatomic particles were stopped dead in their tracks when they encountered our planet. The results of the study were published in the journal Nature on November 22.

Neutrinos interact very weakly with matter. A neutrino can travel a light-year (10 trillion kilometers) through lead without colliding with any atoms. The neutrinos detected by the IceCube detector have very high energies. This is important because the higher the energy of a neutrino, the more likely it is to interact with matter and be absorbed by the Earth.

The IceCube machine consists of a basketball-sized array of 5,160 optical sensors - Digital Optical Modules (DOMs). They are wrapped in a 1 km³The ice is very clean in Antarctica, near Earth's South Pole. The sensors do not observe neutrinos directly but measure flashes of blue light, or Cherenkov radiation, emitted by other particles such as muons, which are created when neutrinos interact with the ice.

By measuring Cherenkov radiation in or near the detector, IceCube can assess and estimate the direction and energy of the neutrinos. The team found that the number of high-energy neutrinos that travel through the Earth to the IceCube detector is smaller than the number of neutrinos that come from less obstructed paths, such as those traveling more horizontally.

The path of neutrinos to the IceCube detector. Photo: Nature.

Most of the neutrinos recorded by the IceCube detector in this study were millions of times more energetic than those produced by familiar sources such as the Sun or nuclear power plants. They were formed in Earth's atmosphere through a process that originated from cosmic rays or unknown sources outside Earth's atmosphere.

"Understanding how neutrinos interact is key to IceCube's operation," said Francis Halzen, a physics professor at the University of Wisconsin-Madison (USA) who works on the IceCube project.

According to VNE