  We present the AGILE gamma-ray observations in the energy range 50 MeV - 10
GeV of the supernova remnant (SNR) W44, one of the most interesting systems for
studying cosmic-ray production. W44 is an intermediate-age SNR (20, 000 years)
and its ejecta expand in a dense medium as shown by a prominent radio shell,
nearby molecular clouds, and bright [SII] emitting regions. We extend our
gamma-ray analysis to energies substantially lower than previous measurements
which could not conclusively establish the nature of the radiation. We find
that gamma-ray emission matches remarkably well both the position and shape of
the inner SNR shocked plasma. Furthermore, the gamma-ray spectrum shows a
prominent peak near 1 GeV with a clear decrement at energies below a few
hundreds of MeV as expected from neutral pion decay. Here we demonstrate that:
(1) hadron-dominated models are consistent with all W44 multiwavelength
constraints derived from radio, optical, X-ray, and gamma-ray observations; (2)
ad hoc lepton-dominated models fail to explain simultaneously the
well-constrained gamma-ray and radio spectra, and require a circumstellar
density much larger than the value derived from observations; (3) the hadron
energy spectrum is well described by a power-law (with index s = 3.0 \pm 0.1)
and a low-energy cut-off at Ec = 6 \pm 1 GeV. Direct evidence for pion emission
is then established in an SNR for the first time.
