Exploring the Universe Through Astroparticle Physics

What can we learn from particles reaching Earth with energies far exceeding anything achievable in man-made laboratories?  On March 13, high-school students from across the Czech Republic came to explore this question at the Pierre Auger International Masterclasses.

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Organized for the fourth time by the Institute of Physics of the Czech Academy of Sciences (FZU) and the Faculty of Nuclear Sciences and Physical Engineering at CTU (FNSPE), the event offered a full day of lectures and hands-on activities.

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The programme was opened by the Dean of FJFI, Václav Čuba, and the Director of FZU, and FORTE member, Michael Prouza, who encouraged students to follow their curiosity and consider a future career in science. The morning lectures introduced both the broader context and the experimental side of the field of astroparticle physics. FORTE researcher Margita Kubátová (FNSPE, FZU) gave a lecture about astroparticle physics and cosmic rays, explaining what happens when these particles enter the Earth’s atmosphere, what we know and do not know about them and why they are so interesting to study.

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This was followed by FORTE WP2 leader Petr Trávníček (FZU), who introduced the Pierre Auger Observatory in Argentina, showcasing how scientists detect these extremely energetic cosmic rays and highlighting key discoveries from over 20 years of the observatory's operation. In the afternoon, the students transitioned from listeners to researchers.

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Using real data from the observatory’s surface detectors, they analyzed cosmic-ray events and reconstructed their energies and arrival directions. Their results demonstarted both the steepness of the energy spectrum and showed a minor trend towards an unisotropic nature of the arrival directions 8 EeV.

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This hands-on experience provided a unique glimpse into the daily work of an international scientific collaboration.The day concluded with a joint videoconference, connecting participants in Prague with students at other sites across Europe and with researchers at the Pierre Auger Observatory.

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Students discussed their results, gained insight into how a global scientific collaboration operates, and had the opportunity to ask questions, not only about science, but also about what a career in research looks like. Once again, the Pierre Auger Masterclasses proved to be an inspiring experience, combining real science, active learning, and lively discussion.

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An international team led by Jakub Vícha from the Institute of Physics of the Czech Academy of Sciences has proposed a revolutionary "heavy metal" scenario that may change the view on the composition of the most energetic particles arriving from space. The theory, which the physicist built together with his team based on the analysis of unique data from the Pierre Auger Observatory, will contribute to answering the question of what these particles are made of and where they originate. A crucial role might be played by iron.

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Cosmic rays are charged particles arriving from space. Most of these are low to medium energy particles that originate from the Sun and other objects in our Galaxy. The highest energy particles are very rare, and can be observed, for example, by the giant Pierre Auger Observatory in Argentina. The question of how and where they can be created remains one of the greatest mysteries of physics. These particles reach energies more than a million times higher than can be created at the largest LHC accelerator at CERN. They thus also reveal the limits of physical knowledge regarding particle interactions.

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The composition of the most energetic particles is inferred indirectly – from measurements of so-called cosmic ray showers, which are created in a cascade after these particles interact with nuclei in the atmosphere. Jakub Vícha noticed that these showers penetrate deeper into the atmosphere than current models predict. This suggests that the models of particle interactions in the shower, known as hadronic, are not sufficiently accurate.

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"If we adjust the model predictions regarding the penetration of the showers so that they all shift towards higher values, the measured data then correspond to higher metallicity, which means a greater proportion of nuclei of elements heavier than hydrogen and helium, and all the data then start to make better sense." Jakub Vícha, author of the article

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It has now been published by The Astrophysical Journal Letters. "Previously, this was considered more of a fringe theory, but it's precisely this possibility that offers a consistent explanation for the data we now have available: cosmic rays at the highest energies could be composed solely of the nuclei of heavy elements, such as iron," the scientist states.

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Iron is actually quite common in the universe.  It's the heaviest element formed in nuclear processes at the end of life of stars, it's very stable, and there's still a relatively abundant supply of it in the universe. "The development of new hypotheses suggests that in extreme processes, such as a merger of two neutron stars, even heavier nuclei could appear – but that currently remains in the realm of speculation," adds Jakub Vícha.

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Deflected trajectories and the muon problem

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If the incoming highest-energy nuclei are indeed this heavy, they're more strongly deflected by both galactic and intergalactic magnetic fields, arriving at Earth on significantly curved trajectories. This complicates efforts to pinpoint where the particles are coming from.  As a result, it is also hard to find their sources.

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For some physicists, this is an unwelcome outcome because it reduces the chance of definitively linking particles to specific objects in space, which would be possible with very light particles.

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"This extreme scenario – meaning pure iron at the highest energies – can actually fit the observed data well, including the long-standing problems with identifying cosmic ray sources from their arrival directions," notes Alena Bakalova, co-author of the article.

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The proposed "heavy metal" scenario also significantly mitigates the so-called muon problem, which is the discrepancy between the measured number of muons and model predictions.  Muons are particles generated in cosmic ray showers that can reach the ground.

The publication of the article about heavy metals at the highest energies in cosmic rays has sparked debate within the scientific teams of the Pierre Auger Observatory regarding data evaluation. "Cosmic ray analyses based on predictions from current models of hadronic interactions will have to be re-evaluated.  This won't be a minor correction, but a fundamental change to the basic framework of how measured data is interpreted," emphasizes Jakub Vícha.

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Source: FZU