EUSO or Extreme Universe Space Observatory is a space mission onboard the
future Manned Space Station to explore the domain of the highest energy
processes occurring in the Universe and its accessible boundaries. It
will detect cosmic rays and neutrinos of high energy (E > 5×10**19 eV)
by looking at the streak of fluorescence light produced when the particles
interact with the Earth’s atmosphere.

EUSO has just received the combined approval of the SPC (Science Programme
Committee) and the PB-MS (Program Board-Manned Spaceflight) of ESA for the
“PHASE A” study to be completed within June 2002.

EUSO will be accommodated onboard the International Space Station ISS,
with a goal for a three year flight starting in mid 2007. EUSO is a
collaboration effort of research groups from Europe, Japan and U.S.A,
chaired by Livio Scarsi from IFCAI, Istituto de Fisica Cosmica &
Informatica of Palermo University and CNR. The meeting for the preparation
report to the Phase A study took place in December 2000 in the framework
of the Chalonge School . In France, participants and proposers of EUSO are
the Observatoire de Paris, University of Paris VI and VII, CEA, CollËge de
France, and LAPP-Annecy.

More Detailed Issues

The Extreme Energy Cosmic Radiation (EECR) with energy >10**20 eV can be
considered as the “Particle channel complementing the Electromagnetic”
channel, specific of conventional Astronomy. EECRs present us with the
challenge of understanding their origin in connection with Fundamental
Physics, Cosmology and Astrophysics. The main characteristics of this
radiation are:

* the change in the spectral index at the “Ankle” (>> 5 x 10**18 eV).
This could correspond to either a change in the production mechanism
in the original sources; either a change in the primary elemental
composition connected with a different confinement region; or a change
in the interaction process in the first collision inducing the
extensive showers in the atmosphere.

* Evidence of the existence of Cosmic Rays (CRs) with energy > 10**20 eV.

* From the Astroparticle Physics point of view, the EECRs have energies
only a few decades below the Grand Unification Energy (10**24 –
10**25 eV), although still rather far from the Planck Mass of 10**28 eV.

What is the Maximum Cosmic Ray Energy, if there is any limit?

Two general prodution mechanisms have been proposed for the Extreme Energy
Cosmic Radiation (EECR):

* “bottom-up”, with acceleration in rapidly evolving processes such as the
Gamma Ray Bursts (GRBs). The observation of “direction of arrival and
time coincidences between the optical-radio transient and Extreme Energy
Neutrinos could provide a crucial identification of the EECR sources,
together with a unique test of the Relativity Principle.

* “top-down” processes with the cascading of ultrahigh energy particles
from the decay of Topological Defects (TD); these are predicted to be
the fossil remnants of the Grand Unification phase.They are cosmic
strings, monopoles, walls, necklaces and textures. Inside a topological
defect the vestiges of the early Universe may be preserved to the present
day. Topogical defects are expected to produce very heavy particles
(X-particles) that decay with production of ultrahigh-energy particles.
Relics of an early inflationary phase in the history of the Universe can
also lead to the production of extreme energy (EE) particles. These
particles may survive to the present as a part of dark matter. Their
decays can give origin to the highest-energy cosmic rays either by
emission of hadrons and photons or through production of EE neutrinos.
Observations of these neutrinos may teach us about the dark matter of
the Universe as well as its inflationary history.

Norma Sanchez

DÈpartement DEMIRM, Observatoire de Paris

TÈl: 33 1 40 51 20 75

Fax: 33 1 40 51 20 02