Extraterrestrial neutrinos provide a unique window to probe the
deepest reaches of stars, quasars, and exotic structures in the
cosmos. In contrast to all other SM particles, they can escape
from dense astrophysical environments and propagate to the Earth
unscathed, arriving aligned with their source. Even at the highest
energies the 
annihilation mean
free path on the cosmic neutrino background,
 |
(69) |
is somewhat above the present size of the horizon (recall that H0-1 ~ 1028 cm) [465].
In addition, the fluxes of ultra-high energy cosmic neutrinos offer
clues to the properties of neutrinos themselves and provide an important
probe of new ideas in particle physics. Namely,
their known interactions are so weak that new physics may easily alter
neutrino properties. This is especially relevant for extraterrestrial
neutrinos with energies
106
GeV that interact with nucleons in the Earth atmosphere with
center-of-mass energies above the electroweak scale, where the SM is
expected to be modified by new physics.
In summary, ultra high energy extraterrestrial neutrinos are inextricably linked with the physics processes discussed throughout this review. This chapter will therefore focus on the main features of high energy neutrino astronomy. For comprehensive reviews on cosmic neutrinos the reader is referred to [466, 467, 468, 469].