  Cosmic rays are particles (mostly protons) accelerated to relativistic
speeds. Despite wide agreement that supernova remnants (SNRs) are the sources
of galactic cosmic rays, unequivocal evidence for the acceleration of protons
in these objects is still lacking. When accelerated protons encounter
interstellar material, they produce neutral pions, which in turn decay into
gamma rays. This offers a compelling way to detect the acceleration sites of
protons. The identification of pion-decay gamma rays has been difficult because
high-energy electrons also produce gamma rays via bremsstrahlung and inverse
Compton scattering. We detected the characteristic pion-decay feature in the
gamma-ray spectra of two SNRs, IC 443 and W44, with the Fermi Large Area
Telescope. This detection provides direct evidence that cosmic-ray protons are
accelerated in SNRs.
