JEM-EUSO Collaboration: UHECR Observations

• JEM-EUSO Collaboration is an international consortium dedicated to detecting ultra-high-energy cosmic rays by observing atmospheric fluorescence and Cherenkov signals from diverse platforms.
• The collaboration utilizes advanced optical systems, modular focal surfaces, and hierarchical trigger algorithms to achieve high exposure, precise resolution, and efficient event discrimination.
• Its multi-tiered strategy, spanning ground prototypes to orbital missions, aims to unravel cosmic ray origins, test fundamental physics, and pioneer charged-particle astronomy.

The JEM-EUSO (Joint Experiment Missions for Extreme Universe Space Observatory) Collaboration is an international consortium dedicated to the detection and study of ultra-high-energy cosmic rays (UHECRs) and related phenomena through a hierarchy of innovative instruments deployed on ground, balloon, and space platforms. By observing atmospheric fluorescence and Cherenkov light emitted by extensive air showers (EAS) from vantage points ranging from the stratosphere to low-Earth orbit, JEM-EUSO aims to achieve exposures and statistics far surpassing those of existing ground-based observatories, thereby opening new avenues in particle astronomy, fundamental physics, and multi-messenger astrophysics (Ebisuzaki, 2011, Plebaniak, 21 Nov 2025, Fenu, 2017).

1. Scientific Motivation and Foundational Objectives

The primary impetus of the JEM-EUSO Collaboration is the measurement and characterization of UHECRs, with energies $E \gtrsim 5 \times 10^{19}$ eV (50 EeV), where the expected particle flux is $\lesssim 1$ km$^{-2}$ century$^{-1}$. The science case addresses two principal questions:

• Beyond the GZK Horizon: Above $\sim5 \times 10^{19}$ eV, cosmic ray protons and nuclei interact inelastically with the cosmic microwave background, producing the so-called Greisen-Zatsepin-Kuzmin (GZK) suppression. Mapping the spectrum and arrival directions of EECRs at these energies enables the identification of source populations (e.g., AGNs, radio galaxies within 100–200 Mpc), rigorous tests of source evolution scenarios, and discrimination between bottom-up and top-down production models.
• Fundamental Physics—Lorentz Invariance and Quantum Gravity: By observing cosmic rays with Lorentz factors $\gamma \sim 10^{11}$ and center-of-momentum energies far beyond terrestrial accelerators, JEM-EUSO is uniquely sensitive to departures from special relativity (e.g., modifications in particle dispersion or onset of exotic interactions). Detection of spectral features at and above the GZK cutoff yields constraints on Lorentz invariance up to unprecedented scales.

Secondary science objectives include searches for UHE neutrinos and photons, studies of the Galactic and intergalactic magnetic fields through UHECR deflections, investigation of transient luminous events (TLEs), meteors, and atmospheric phenomena, and global UV background mapping (Ebisuzaki, 2011, Plebaniak, 21 Nov 2025, Bertaina, 2021, Pastirčák et al., 2012).

2. Detection Principle and Instrumental Design

JEM-EUSO instruments utilize the detection of isotropic UV fluorescence ($\lambda \sim300$–$400$ nm) and beamed Cherenkov emission from EASs initiated in Earth’s atmosphere. The baseline orbital design employs a super-wide-field ($60^\circ$) refractive telescope realized with three large ($2.5$–$2.65$ m) double-sided PMMA Fresnel lenses, optimized for the near-UV band. The key features include:

• Optical System: Two aspheric Fresnel lenses plus an intermediate diffractive corrector (chromatic aberration minimized), with entrance diameters of $\sim$2.5–3 m and focal lengths $\sim$2.5–3.6 m. This configuration yields a ground footprint up to $1.9 \times 10^5$ km$^2$ from the ISS at 400 km altitude (Ebisuzaki, 2011, Klimov et al., 2022).
• Focal Surface: Spherical, radius $\sim$2.3–2.7 m, tiled with up to $\sim$137 Photo-Detector Modules (PDMs). Each PDM comprises nine Elementary Cells with Hamamatsu Multi-Anode PMTs (MAPMTs, 64 channels each), resulting in $3.2 \times 10^5$ channels for full-scale JEM-EUSO (Ebisuzaki, 2011, collaboration, 2013). K-EUSO's PDM architecture hosts 44 modules for $10^5$ channels (Klimov et al., 2022). Mini-EUSO pathfinder employs $2,304$ pixels (1 PDM) (Capel et al., 2017, Belov et al., 2017).
• Electronics and Trigger: Hierarchical multi-level triggering: (i) local pixel–wise persistency for fast events (GTU = 2.5 μs), (ii) pattern recognition for track-like structures, (iii) global direction-integrated trigger in cluster control boards, reducing raw rates ($\sim$10 GB/s) to sub-Mbps telemetry (collaboration, 2013, Fenu, 2017, Belov et al., 2017).
• Atmospheric Monitoring: Integrated IR cameras (dual-band, $\Delta T \leq 3$ K, $\Delta H \sim 0.5$ km) and steerable UV LIDAR (355 nm, range resolution $\sim$500 m, optical depth accuracy $\leq 0.15$) provide real-time corrections for clouds, aerosols, and atmospheric transmission (Toscano et al., 2014).

3. Observation Campaign: Multi-Platform Strategy

The Collaboration implements a tiered validation roadmap comprising:

• Ground-Based Prototypes — EUSO-TA: Located at the Telescope Array site, utilizing dual 1-m Fresnel lenses and 1 PDM, externally triggered by colocated fluorescence detectors. Demonstrated UHECR and calibration laser event detection, validated optics, triggering, and energy threshold at $\sim10^{18.5}$ eV with a $0.2^\circ$ pixel scale (Fenu, 2019, Fenu, 2017, Bertaina, 2021).
• Balloon-Borne Detectors — EUSO-Balloon, EUSO-SPB1/SPB2: Stratospheric missions (altitudes $30$–$38$ km) with Fresnel optics and 1–3 PDM focal planes, self-triggering electronics, and on-board IR/cloud monitoring. EUSO-Balloon achieved UV background mapping and laser-based calibration (Fenu, 2019); EUSO-SPB1 validated autonomous GTU triggers and data throughput, established an energy threshold $\sim3 \times 10^{18}$ eV, and set upper limits consistent with exposure (Eser et al., 2019). EUSO-SPB2 added the first Cherenkov channel and advanced dual-mission objectives (Plebaniak, 21 Nov 2025).
• Orbital Pathfinders and Missions — TUS, Mini-EUSO, K-EUSO: TUS (Lomonosov satellite) demonstrated the first PMT-based UV EAS imaging in orbit (Fenu, 2019). Mini-EUSO, installed on the ISS (Zvezda module), offers high-resolution mapping (6.11 km/pixel), multi-level triggering across $2.5$ μs–$41$ ms timescales, and detection of TLEs, meteors, and anthropogenic UV sources (Capel et al., 2017, Belov et al., 2017). K-EUSO, the first full-scale orbital UHECR telescope, will deploy after 2025 with a $3$ m$^2$ Fresnel system, $0.1^\circ$ pixels, and annual exposure $\sim 2 \times 10^4$ km$^2$ sr yr at $E>10^{20}$ eV (Klimov et al., 2022, Bertaina, 2021).
• Future and Next-Generation Missions — POEMMA, M-EUSO: Stereoscopic free-flyer satellite arrays with refractive or Schmidt optics, dual focal planes (MAPMT + SiPM), and enhanced capability for UHE neutrino detection (Earth-skimming $\nu_\tau$ technique) and multi-messenger astrophysics (Plebaniak, 21 Nov 2025).

4. Event Simulation, Data Analysis, and Reconstruction

JEM-EUSO developed a modular, extensible simulation and reconstruction software suite based on the Pierre Auger OffLine framework:

• Simulation: Modular pipeline architecture supports injection of EAS profiles (CORSIKA, CONEX), fluorescence/Cherenkov photon emission models, and detailed atmospheric propagation (Rayleigh, Mie scattering; ozone absorption). Detector responses are simulated using Geant4-based telescopic optics and custom electronics modules (Paul, 2023).
• Trigger, Reconstruction Algorithms: Real-time FPGA/ASIC interfaces implement multi-level triggers, including spatial cluster searching and temporal persistency (EECRs, TLEs, meteors). Offline, background-suppressed track finding, shower–detector plane fitting, Gaisser-Hillas energy profile extraction, and $X_{\max}$ reconstruction are performed on calibrated events (Belov et al., 2017, Paul, 2023).
• Configuration Management: All instrument geometry, detector conditions, and atmospheric state are abstracted by a Detector Description (DD) interface, with run steering and provenance ensured via XML configuration (Paul, 2023). Benchmarks indicate $\sim$1 s/event processing and $\sim$1 GB memory footprint per full simulation instance.

5. Performance Metrics and Scientific Impact

The JEM-EUSO instrument suite achieves:

• Energy Thresholds: 50% trigger efficiency at $E \sim 4$–$5 \times 10^{19}$ eV in nadir mode; lower thresholds in central FoV and at steeper EAS inclinations. K-EUSO expects $\sim65$ events/year above $5 \times 10^{19}$ eV and $\sim4$ events/year above $10^{20}$ eV (Klimov et al., 2022).
• Aperture and Exposure: From a 400 km orbit, annual exposures of $>1 \times 10^6$ km$^2$ sr yr (JEM-EUSO full mission), or $1.8\times 10^4$ km$^2$ sr yr (K-EUSO), with a duty cycle of $20$–$25$\%, including cloud and background corrections (Ebisuzaki, 2011, Klimov et al., 2022, collaboration, 2013, Plebaniak, 21 Nov 2025).
• Angular and Energy Resolution: Angular resolution $\sim0.1^\circ$ (JEM-EUSO, K-EUSO), corresponding to $\sim600$–$1000$ m ground pixels; energy resolution $\Delta E/E \lesssim 15$–$25$\% for vertical to inclined showers; $X_{\max}$ RMS error $\sim 50$–$90$ g/cm$^2$ (collaboration, 2013, Klimov et al., 2022).
• Event Discrimination and Secondary Science: Differentiation between hadronic and gamma/neutrino-induced showers is realized via profile fitting and directional triggers. Side science includes precise UV background maps, atmospheric TLE catalogs, meteor/space debris statistics, and constraints on nuclearite fluxes (Capel et al., 2017, Fenu, 2019).

6. Collaborating Institutions, Programmatic Structure, and Future Directions

The JEM-EUSO Collaboration brings together over 200 researchers from 20+ countries and is structured into instrument, calibration, simulation, and secondary science working groups.

• Key contributors: RIKEN (system integration), INFN Torino (project coordination, optics), University of Rome Tor Vergata (electronics/calibration), Aoyama Gakuin and Konan Universities (optics), Max-Planck-Institut für Physik, Moscow State University (detector/data analysis), EWHA Womans University & ETH Zurich (atmosphere monitoring), UNAM Mexico & University of Alcalá (simulation) (Ebisuzaki, 2011, Bertaina, 2021, collaboration et al., 2012, collaboration, 2013).
• Future Outlook: Post-K-EUSO, the strategic focus is on stereoscopic multi-instrument constellations (e.g., M-EUSO, POEMMA) for full-sky, high-statistics UHECR and UHE neutrino mapping with even lower thresholds and enhanced source discrimination (Plebaniak, 21 Nov 2025). Strong synergies are anticipated with ground-based particle observatories, next-generation gamma and neutrino missions, and atmospheric monitoring satellites.
• Data Policy: Internal data validation precedes staged public releases of UV background maps, event catalogs, and reconstructed EAS observables complementing the multi-messenger landscape (Bertaina, 2021).

7. Significance Within Cosmic-Ray Research

JEM-EUSO’s systematic multi-platform approach—from ground through balloon to full orbital deployment—establishes the technical basis for space-based observation of extreme-energy cosmic rays. By delivering uniform all-sky coverage, large effective apertures, and high event statistics, the collaboration aims to resolve the long-standing origin question of UHECRs, probe fundamental interactions at energies unattainable in terrestrial laboratories, and inaugurate a new era of “charged-particle astronomy” above the GZK scale (Ebisuzaki, 2011, Plebaniak, 21 Nov 2025, collaboration, 2013).