Scientists of the NEVOD Research and Education Center of the MEPhI summed up the results of a ten-year experiment on the study of ultrahigh-energy cosmic rays, as a result of which they discovered the anisotropy of these rays and determined its parameters. The results of the study were published in the Astrophysical Journal (https://iopscience.iop.org/article/10.3847/1538-4357/acb1fc).

Cosmic rays are particles of matter, which include stable elementary particles (photons, neutrinos, electrons, protons) and nuclei of chemical elements flying towards us from space at a tremendous speed close to the speed of light. At energies of the order of 1015 electron volts, the trajectories of charged particles become entangled in the magnetic fields of our Galaxy and conditions are created for their diffusion, said Mikhail Amelchakov, Candidate of Physical and Mathematical Sciences, Senior Researcher at the NEVOD Research and Education Center of the MEPhI.

“Cosmic rays behave like a drop of paint in a glass of water. Particles "run away" from the area with a high concentration. If we observe the effect of cosmic ray diffusion on the Earth, then, taking into account the daily rotation of the Earth, against a uniform background of the cosmic ray flux in the space surrounding the Earth, two poles will be present from two opposite sides. At one pole there will be an excess of particles, and at the other there will be a lack of them,” he said.

According to him, the theory of diffuse propagation of cosmic rays, developed by Soviet scientists, predicts that cosmic rays have anisotropy (the difference between particles from different directions). This anisotropy is determined by the vector of the dipole directed from the center of the Galaxy.

Previously, scientists have found that the flux of cosmic rays decreases rapidly with increasing energy. The flow intensity at ultrahigh energies drops so much that their registration on satellites (outside the Earth's atmosphere) and high-altitude balloons becomes irrational. But, getting into the atmosphere, cosmic rays give rise to cascades of secondary particles, including elementary particles muons, and these cascades, developing with a wide front, move towards the Earth's surface, keeping the direction of the primary particle.

“This phenomenon is called “wide air shower”. Reaching the Earth's surface, a wide air shower can cover vast areas, and therefore studies of cosmic rays at energies over 1015 eV are carried out on ground-based facilities,” said Mikhail Amelchakov.

He noted that quite a lot of such installations have already been created in different places on the globe, and each is unique in its own way. The use of various conditions and registration methods allows scientists to identify unknown physical effects.

One of these unique installations is the DECOR coordinate-tracking detector, created in collaboration with Italian physicists at the NEVOD Experimental Complex (MEPhI). Due to its unique design, DECOR registers muon tracks in a wide range of zenith angles from 0° to 90°. Muons have good penetrating power and retain the direction of motion of primary cosmic rays with good accuracy.

Researchers at the NEVOD Experimental Complex have been collecting data to study the anisotropy of cosmic rays for 10 years, from 2012 to 2022, Mikhail Amelchakov said.

“We have created and developed a number of data processing methods for research, such as a method for taking into account the influence of meteorological effects on the intensity of muon groups, a method for taking into account the design features of the detector and the inhomogeneity of the detection efficiency in different directions, as well as a method for estimating the primary energies of cosmic rays. In the method for estimating the primary energy, we for the first time used the multiplicity of registered muon tracks depending on the zenith angle of their arrival,” he said.

As a result, the researchers discovered the presence of cosmic ray anisotropy at energies of about 1015 eV and determined its parameters (amplitude and direction). According to scientists, the data obtained are consistent with the results of studies at other facilities and with the theory of diffuse propagation of cosmic rays in the direction from the center of the Galaxy.