GRAPES-3 Experiment Redefines Laws of Cosmic Ray Physics

The Tata Institute of Fundamental Research's GRAPES-3 experiment, situated in Ooty, India, has revealed a new facet in the realm of cosmic ray physics. Researchers have identified a distinctive characteristic in the cosmic-ray proton spectrum, particularly at an energy level of approximately 166 tera-electron-volt (TeV). This revelation spans a measurement range from 50 TeV to slightly over one peta-electron-volt (PeV), sparking potential shifts in our comprehension of cosmic ray origins, their acceleration processes, and their movement within the Milky Way.

Cosmic rays, first discovered over a century ago, constitute the most energetic particles in the universe. These particles uniformly bombard Earth from all directions, penetrating the atmosphere and generating a cascade of secondary particles such as electrons, photons, muons, protons, and neutrons, which reach the ground at nearly light speed.

The study of cosmic rays encompasses a wide energy spectrum, ranging from 10^8 to 10^20 eV. Their flux experiences a steep decline at higher energies, conventionally described by a single power-law up to the "knee" at approximately 3 PeV. This knee represents a notable alteration in the energy spectrum, indicating a limit to Galactic cosmic-ray acceleration.

Contrary to this long-standing model, the recent findings from the GRAPES-3 experiment identify a new feature in the cosmic-ray spectrum between 100 TeV and the knee. This breakthrough became possible through the inventive utilization of a dense array of plastic scintillator detectors combined with a large-area muon detector. This experimental configuration enabled the team to amass data from around eight million cosmic ray shower events, a scale of observation several thousand times larger than achievable with space-based detectors, which often lack precision in this energy range due to limited data.

Pravata K. Mohanty, the Principal Investigator of the GRAPES-3 experiment and faculty member at the Tata Institute of Fundamental Research in Mumbai, the team's analysis, coupled with extensive computer simulations, has opened new avenues for comprehending cosmic ray phenomena. Published in Physical Review Letters, the study implies that the standard model of cosmic rays may necessitate reevaluation to incorporate this newly observed feature.

This research not only enhances our understanding of cosmic rays but also underscores the pivotal role played by ground-based observatories in astrophysical research. By expanding the observable spectrum of cosmic rays with unprecedented precision, the GRAPES-3 experiment contributes to a deeper comprehension of the most energetic processes in the universe.

The discovery signifies a significant step forward in cosmic ray research, unveiling complexities in cosmic ray production and propagation within our galaxy that are yet to be fully comprehended. As the scientific community delves deeper into this revelation, it may pave the way for innovative theories regarding cosmic ray origins and their role in the cosmic ecosystem.