  We report on the first detection of GeV high-energy gamma-ray emission from a
young supernova remnant with the Large Area Telescope aboard the Fermi
Gamma-ray Space Telescope. These observations reveal a source with no
discernible spatial extension detected at a significance level of 12.2$\sigma$
above 500 MeV at a location that is consistent with the position of the remnant
of the supernova explosion that occurred around 1680 in the Cassiopeia
constellation - Cassiopeia A. The gamma-ray flux and spectral shape of the
source are consistent with a scenario in which the gamma-ray emission
originates from relativistic particles accelerated in the shell of this
remnant. The total content of cosmic rays (electrons and protons) accelerated
in Cas A can be estimated as $W_{\mathrm{CR}} \approx (1-4) \times 10^{49}$ erg
thanks to the well-known density in the remnant assuming that the observed
gamma-ray originates in the SNR shell(s). The magnetic field in the
radio-emitting plasma can be robustly constrained as B $\gt 0.1$ mG, providing
new evidence of the magnetic field amplification at the forward shock and the
strong field in the shocked ejecta.
