  Supernova remnants (SNRs) have long been considered the leading candidates
for the accelerators of cosmic rays within the Galaxy through the process of
diffusive shock acceleration. The connection between SNRs and cosmic rays is
supported by the detection of high energy (HE; 100 MeV to 100 GeV) and very
high energy (VHE; 100 GeV to 100 TeV) gamma rays from young and middle-aged
SNRs. However, the interpretation of the gamma-ray observations is not unique.
This is because gamma rays can be produced both by electrons through
non-thermal Bremsstrahlung and inverse Compton scattering, and by protons
through proton-proton collisions and subsequent neutral-pion decay. To
disentangle and quantify the contributions of electrons and protons to the
gamma-ray flux, it is necessary to measure precisely the spectra and morphology
of SNRs over a broad range of gamma-ray energies. Cassiopeia A (Cas A) is one
such young SNR (~ 350 years) which is bright in radio and X-rays. It has been
detected as a bright point source in HE gamma rays by Fermi-LAT and in VHE
gamma rays by HEGRA, MAGIC and VERITAS. Cas A has been observed with VERITAS
for more than 60 hours, tripling the published exposure. The observations span
2007-2013, and half of the data were taken at large zenith angles to boost the
effective area above few TeV. We will present the detailed spectral and
morphological results from the complete dataset.
