THE GRAND PROJECT HAS OBSERVED ITS FIRST COSMIC RAYS!
These atomic nuclei rain down from the sky with an energy a million times greater than that of the particles accelerated in the Large Hadron Collider at CERN. GRANDProto300, the main prototype of the GRAND project deployed in the Gobi Desert in China, detected them for the first time with radio antennas operating autonomously and systematically.
Figure 1: Arrival directions in the sky (in polar representation, zenith 𝜃 and azimuth 𝜙 angles where 𝜃 = 0 indicates up and 𝜙 = 0 indicates north) of the cosmic-ray candidates identified in the GRAND prototype data.
Credit: Grand Collaboration (from [2]).
The deployment, calibration, and optimization of the prototype was carried out mainly by partners from Xidian University and Purple Mountain Observatory[1]. Sixty-five antennas are now in stable operation in a low-noise, controlled environment.
An initial selection made by Jolan Lavoisier[2], PhD student at IAP, and Xishui Tian, PhD student at LPNHE, isolated 41 candidate astrophysical signals from more than 500,000 transient noise events detected between January and March 2025. Three independent analyses conducted within the GRAND collaboration, including those by Marion Guelfand[3], PhD student at LPNHE and IAP, and Arsène Ferrière[4], PhD student at CEA and LPNHE, yielded compatible results, with energies distributed between 1017 and 5x1018 eV, the range expected for GRANDProto300. At the same time, the network of 10 GRAND antennas deployed at the Pierre Auger Observatory site observed a cosmic ray detected by the Pierre Auger Cherenkov tank network[5].
Figure 2: A team bringing together members from Xidian University, Purple Mountain Observatory, IAP, LPNHE, and the local staff, in front of the Grand prototype's living quarters and data acquisition base in the Gobi Desert, China.Credit: Grand Collaboration.
These results confirm that autonomous radio detection of cosmic particles is possible. The GRANDProto300 experiment aims to demonstrate that the detection of horizontal atmospheric showers—such as those induced by cosmic rays or neutrinos—is possible in an efficient and pure manner, thereby validating the detection principle of the Giant Radio Array for Neutrino Detection project. Estimates made by Sei Kato[6], postdoctoral researcher at IAP, and Clément Prévotat, PhD student at IAP, based on numerical simulations, predict that around 100 cosmic rays should be observed per day in full operation phase.
This major milestone paves the way for the design of the next phase: a larger network aimed at searching for ultra-high-energy cosmic neutrinos. One option being considered[7][8], is an efficient hybrid network combining autonomous antennas spaced like GRAND and phased array stations like BEACON, which would be 20 times more sensitive for the same number of antennas. This network could be deployed in the province of San Juan in Argentina, with the support of Argentine partners such as ITeDA and CNEA.
Notes
[1] P. Ma, for the GRAND Collaboration, ICRC 2025 Proceedings, “Progress of the GRANDProto300 Project”.
[2] J. Lavoisier, X. Tian, K. Kotera, T. Sako, H. Wang, M. Bustamante, for the GRAND Collaboration, ICRC 2025 Proceedings, “Search for cosmic rays in GRANDProto300”.
[3] M. Guelfand, P. Fritsch, V. Decoene, O. Martineau-Huynh, and M. Bustamante, for the GRAND Collaboration, ICRC 2025 Proceedings, “Reconstruction of inclined cosmic-ray properties with GRAND data”.
[4] A. Ferrière and A. Benoit-Lévy, for the GRAND Collaboration, ICRC 2025 Proceedings, “Reconstruction of cosmic-ray properties with GNN in GRAND”.
[5] B. de Errico, J. de Mello Neto, C. Timmermans, for the GRAND Collaboration, ICRC 2025 Proceedings, “The GRAND@Auger Prototype for the Giant Radio Array for Neutrino Detection”.
[6] S. Kato, C. Prévotat, R. Alves Batista, for the GRAND Collaboration, ICRC 2025 Proceedings, “Calculation of the exposure of GRANDProto300 to cosmic rays”.
[7] K. Kotera et al., for the GRAND Collaboration, ICRC 2025 Proceedings, “The Hybrid Elevated Radio Observatory for Neutrinos (HERON) Project”.
[8] A. Zeolla et al., for the GRAND Collaboration, ICRC 2025 Proceedings, “Detection of Radio Emission by Cosmic Rays with the BEACON Prototype”.
Links
Science news, August 2025: “Neutrino astronomers set biggest traps ever for messengers from cosmic accelerators”.
O. Martineau-Huynh, for the GRAND Collaboration, ICRC 2025 Proceedings, “Status of the GRAND project”.
Contact
- Kumiko Kotera
Institut d'astrophysique de Paris, CNRS, Sorbonne Université
Kumiko [dot] Kotera [at] iap [dot] fr
Layout: Jean Mouette
September 2025