Czech physicist Jakub Vícha's method helps determine what cosmic rays consist of

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The highest energy cosmic ray particles are likely to penetrate much deeper into the atmosphere than previously thought. The incoming particles are therefore likely to be much heavier. New and fundamental insights emerge from a method that generalises the approach to predicting models of cosmic particle collisions with the Earth's atmosphere. The accuracy of Jakub Vícha's method has been confirmed by hundreds of international scientists at the Pierre Auger Observatory, as shown in a study published these days in Physical Review D.

A unique method is bringing science a step closer to solving one of the greatest mysteries of physics: what are the particles from space that bombard the Earth's atmosphere?

The composition of these particles is estimated indirectly from measurements of the secondary particles that cascade in the atmosphere after the primary cosmic particle collides with a nucleus. Some of these secondary particles, such as muons, reach the ground. There has been a large discrepancy between observations and model predictions of the number of muons hitting the ground.

The international team of scientists used unique data from the Pierre Auger Observatory's measurements and Jakub Vícha's unique method for a generalised approach to predictions of hadronic interactions that take place in showers.

"Our interpretation of the measurements showed that the showers of ultra-high-energy particles probably penetrate much deeper into the atmosphere than we thought," explains Jakub Vícha. "At the same time, it turns out that the composition of cosmic rays, which is most often determined by the aforementioned shower penetration, may also be significantly heavier than previously thought and therefore contain more heavy nuclei," adds the scientist.

The more revolutionary the idea, the more resistance

Jakub Vícha's revolutionary method significantly refined the description of measured data. At the same time, it was the first time to clearly demonstrate the inability of previous models to reliably describe the measured data. As a result, astroparticle physicists are now likely to reconsider the results of previously published work on the composition of ultra-high-energy cosmic rays.

Initially, the new method was met with suspicion and resistance from the scientific community. The Czech scientist's method was verified by hundreds of astroparticle physicists at the Pierre Auger Observatory, eventually on a total of 2239 particle showers detected simultaneously with fluorescence and surface detectors at energies between 3 and 10 EeV (exa electron volts).

"There were times when I felt very disappointed that my method was not accepted, but I was unable to find out what I was doing wrong. I was convinced that everything was as it should be, and no one really found anything wrong with my method," Jakub Vícha points out. He continued to promote the method with the support of some of the most respected colleagues in the community.

"They told me: the more revolutionary the result, the more resistance it will cause, it takes time," adds the astroparticle physicist and winner of the 2023 Lumina quaeruntur prize.

Extrapolation to higher energies introduces large systematic errors

To interpret the ultra-high-energy cosmic ray data, researchers rely on predictions of hadronic interaction models, which do not describe the measured properties of the showers reliably enough. These models have been built on the basis of observations from hadron colliders such as the LHC at CERN near Geneva, but the most energetic cosmic particles exceed even 100 EeV, well beyond the capabilities of ground-based colliders.

The properties of the hadronic interactions must therefore be extrapolated to many orders of magnitude higher energies in the models, introducing significant systematic uncertainty in the interpretation of cosmic ray measurements. It now appears that the models need to be refined much more comprehensively than simply generating more muons, which is itself quite problematic.

This means that heavier particles, such as iron nuclei, are likely to be found in ultra-high-energy cosmic rays, which has important implications for the search for their sources at the highest energies. In fact, the more charged the particles are, the more they will bend in our Galaxy's magnetic field, and thus the farther away from their source in the sky their direction of arrival will be.

"Nature is simply much more complicated than we would like it to be, which makes it difficult to finally discover where these particles come from. However, we are gradually narrowing down the range of possibilities, and one day we may be able to discover how and where these most extreme processes take place in the Universe," says Jakub Vícha.

Messengers of the most extreme processes in the Universe

High-energy cosmic rays are a stream of charged particles coming from space. Some of them come from the Sun, some from our Galaxy, and the rarest ones, the messengers of the most extreme processes in the Universe, as astroparticle physicist Jakub Vícha calls them, come even from other galaxies.

Where the primary particle came from before triggering the shower of secondary particles, what its energy was and what kind of particle it was, scientists are trying to deduce from the signals produced by the shower's secondary particles in detectors on the Earth's surface. The best detector in the world today is the Pierre Auger Observatory in Argentina, which is being built, operated and analysed by scientists from the Czech Republic.

Testing hadronic-model predictions of depth of maximum of air-shower profiles and ground-particle signals using hybrid data of the Pierre Auger Observatory

A. Abdul Halim et al. (The Pierre Auger Collaboration) Phys. Rev. D 109, 102001 – Published 2 May 2024

DOI: 10.1103/PhysRevD.109.102001