A new distortion in the proton spectrum to improve our knowledge of the origin of cosmic rays

The GRAPES-3 experiment in Ooty, India, operated by the Tata Institute of Fundamental Research, has discovered a new feature in the cosmic ray proton spectrum at an energy of about 166 tera-electron volts (TeV) by measuring the spectrum extending from 50 TeV. at just over 1 peta-electron-volt (PeV). The observed feature suggests a potential reassessment of our understanding of cosmic ray sources, acceleration mechanisms and their propagation within our galaxy.

The study was led by Pravata K. Mohanty, principal investigator of the GRAPES-3 experiment and a faculty member at the Tata Institute of Fundamental Research in Mumbai. The results were published in Physical Examination Letters

Cosmic rays were discovered more than a century ago. They are considered the most energetic particles in the universe. Our planet is constantly bombarded by them from space, almost uniformly from all directions and at a constant rate. They enter the Earth’s atmosphere and cause a shower of particles that travel toward the ground at almost the speed of light. The shower particles constitute electrons, photons, muons, protons, neutrons, etc.

Cosmic rays have been observed over a remarkably wide energy range (10⁸ at 10²⁰ eV). The flux of cosmic ray particles decreases sharply with energy according to the power law.

A cosmic ray proton bend at around 3 PeV, known as the “knee”, which was discovered about seven decades ago, is considered the maximum energy for cosmic ray acceleration in galactic sources . A single power law description of the cosmic ray spectrum down to the knee energy has been proposed for a long time and explained by various models. This observation from the GRAPES-3 experiment shows a new feature above 100 TeV and below the knee.

Using a combination of a dense array of plastic scintillator detectors and a large-area muon detector, GRAPES-3 experiment researchers collected data over a collection area several thousand times larger than that of space detectors, allowing more detailed examination of cosmic rays above 100 TeV, where space measurements lack precision due to low statistics.

A subset of about 8 million cosmic ray shower events recorded by these detectors was analyzed by Fahim Varsi and other team members, who used CPU-intensive computer simulations to measure the spectrum cosmic rays.