JEM-EUSO will address basic problems of fundamental physics and high-energy astrophysics studying the nature and origin of the Extreme Energy Cosmic Rays (E > 5×10¹⁹ eV).Â

JEM-EUSO will pioneer Space observations of UHECR-induced Extensive Air Showers making measurements with unprecedented statistics at energies above 5×10¹⁹ eV. It will observe the primary energy and arrival direction of UHECRs, using a target volume far greater than it is possible from the ground. It will also be able to distinguish photon and neutrino induced showers from hadronic showers. Such data will shed light on the origin of the UHECRs, on the sources that are producing them, on the propagation environment from the source to the Earth and, possibly, on particle physics interactions at energies well beyond the ones achievable in man-made accelerators.

JEM-EUSO science also includes constraining the Galactic and extragalactic magnetic fields, the detection of extreme energy neutrinos and gamma rays, the verification of special relativity at extremely large Lorentz factors, the examination of possible quantum gravity effects at extreme energies, and the systematic surveillance of atmospheric phenomena.

Main Science Objectives:

• identification of UHE sources
• measurement of the energy spectra
of individual sources
• measurement of the trans-GZK spectrum

Exploratory objectives:

• discovery of UHE neutrinos
• discovery of UHE Gamma-rays
• study of the galactic and local 
extragalactic magnetic field

JEM-EUSO observes extreme energy cosmic rays (EECRs) by measuring the fluorescence and Cherenkov photons produced in the extensive air shower (EAS). When a EECR reaches the Earth’s atmosphere it interacts with the atmospheric nuclei producing secondary particles that in turn collide with the air atoms giving rise to a propagating cascade of hundreds of billions of particles. The number of the secondary particles in an EAS is related to the energy of the primary UHECR. The most dominant particles in EAS are electrons moving through the atmosphere, which excite metastable energy levels in atmospheric atoms and molecules, in particular nitrogen, that return to the ground state by emitting characteristic fluorescence light in the ultraviolet (UV) band with wavelengths between 330 and 400 nm. The emitted light is isotropic and its intensity is proportional to the energy deposited in the atmosphere. An EECR-induced EAS then forms a significant streak of fluorescence light along its passage in the atmosphere, depending on the energy and zenith angle of the primary EECR. In addition, numerous secondary particles have velocities higher than that of the light in the atmosphere and therefore they emit Cherenkov photons. These Cherenkov photons are highly beamed within a cone of < 1.3° radius along the trajectory and may be scattered by the molecular and aerosol content in the atmosphere. A fraction of these photons will be isotropically diffused when reaching land, sea or clouds.

Looking downward at the dark Earth atmosphere, JEM-EUSO will record such fluorescence light streaks as sketched in the Figure. EAS appear as a small disc-shaped luminous object which, when viewed continuously, moves on a straight path at the speed of light. The recorded amount of light is nearly proportional to the shower size at the various depths in the atmosphere. By imaging the motion of the fluorescence emission every few microseconds the arrival direction of the primary UHECR can be determined. The total amount of light recorded determines the energy of the primary UHECR. The cascade shape (especially the position of the shower maximum in the traversed slant depth) gives an indication of the nature of the primary.