Particles smaller than an atom push through the universe almost at the speed of light, ejected into space from something, somewhere in space.
The scientific collaboration of the Pierre Auger Observatory, including researchers from the University of Delaware, measured with unprecedented accuracy the most powerful of these particles – cosmic rays of ultrahigh energy. In doing so, they discovered a “break” in the energy spectrum that sheds more light on the possible origins of these subatomic space travelers.
The group’s findings are based on an analysis of 215,030 cosmic ray events with energies above 2.5 quintillion electron volts (eV) recorded over the past decade by the Pierre Auger Observatory in Argentina. It is the largest observatory in the world for the study of cosmic rays.
The new spectral characteristic, the refraction of the energy spectrum of cosmic rays at approximately 13 quintillion electron volts, represents more than the points plotted on the graph. This brings humanity one step closer to unraveling the mysteries of the most energetic particles in nature, according to Frank Schroeder, an associate professor at the Bartol Research Institute at the Department of Physics and Astronomy at the University of Delaware. The study is published in Physical review letters and Review of physics D.
“Since cosmic rays were discovered 100 years ago, the long-standing question was, what accelerates these particles?” said Schroeder. “Pierre Auger Collaboration’s measurements provide important clues as to what we can exclude as a source. We know from previous work that the accelerator is not in our galaxy. Thanks to this latest analysis, we can further confirm our previous indications that ultrahigh energy cosmic rays are not just hydrogen protons, but also a mixture of nuclei of heavier elements, and this composition changes with energy. “
Between “ankle” and “toe”
Schroeder and UD doctoral researcher Alan Coleman, who facilitated the data analysis, have been members of the Pierre Auger Collaboration for several years. UD officially joined the cooperation as an institutional member in 2018. This group of more than 400 scientists from 17 countries operates an observatory that covers an area of 1,200 square miles, about the size of Rhode Island.
The observatory has more than 1,600 detectors, called water-Cherenkov stations, distributed on the high plains of the Pampa Amarilla (Yellow Prairie), which do not notice 27 fluorescent telescopes. Collectively, these devices measure the energy that a fraction of a cosmic beam emits into the atmosphere and provide an indirect estimate of its mass. All these data – energy, mass and direction, where these extraordinary particles came from – give important clues about their origin.
Scientists have previously thought that these ultra-high-energy cosmic ray particles are mostly hydrogen protons, but this latest analysis confirms that the particles have a mixture of nuclei – some heavier than oxygen or helium, such as silicon and iron, for example.
By plotting on a curved graph representing the energy spectrum of cosmic rays, you can see a fracture – a steep, flattened cross section – between an area that scientists call the “ankle” and the starting point of the graph, called the “toe.”
“We don’t have a specific name for this,” said Coleman, who was a member of a team of 20 people who wrote computer code and processed the amount needed for detailed data analysis. “We probably run out of parts of the anatomy to name it,” he joked.
Directly involved in the find, Coleman improved the reconstruction of the particle cascade that creates cosmic rays when exposed to the atmosphere to estimate energy. He also conducted detailed research to make sure that this new inflection point was genuine and not a detector artifact. The work of the data group took more than two years.
“Obviously, it’s quite insignificant,” Coleman said of the spectral break. “But every time you see a bump like that, it signals that the physics are changing, and it’s very exciting.”
It is very difficult to determine the mass of incoming cosmic rays, Coleman said. But measurement of collaboration is so reliable and accurate that a number of other theoretical models from which ultrahigh-energy cosmic rays come can now be eliminated, while other paths can be traced with greater energy.
Active galactic nuclei (AGNs) and star-exploding galaxies are now working as potential sources. Although their typical distance is about 100 million light-years, some candidates are within 20 million light-years.
“If we knew what the sources were, we could learn new details about what was happening,” Coleman said. What is happening that allows these incredibly high energies? These particles can come from something we don’t even know about. “
The current research by the UD team focuses on further improving the accuracy of ultrahigh energy cosmic ray measurements and extending accurate cosmic ray spectrum measurements to lower energies. This could be a better match for other experiments, Schroeder said, such as measuring the IceCube cosmic rays at the South Pole, another unique astroparticle observatory that involved mostly the University of Delaware.
New data show that cosmic rays are more complex than expected
A. Aab et al. Features of the energy spectrum of cosmic rays above 2.5 × 1018 eV using the Pierre Auger Observatory, Physical review letters (2020). DOI: 10.1103 / PhysRevLett.125.121106
A. Aab et al. Measurement of the energy spectrum of cosmic rays above 2.5 × 1018 eV using the Pierre Auger Observatory, Physical examination D (2020). DOI: 10.1103 / PhysRevD.102.062005
Provided by the University of Delaware
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