  We have measured the cosmic momentum power spectrum from the peculiar
velocities of galaxies in the SFI sample. The SFI catalog contains field spiral
galaxies with radial peculiar velocities derived from the I-band Tully-Fisher
relation. As a natural measure of the large-scale peculiar velocity field, we
use the cosmic momentum field that is defined as the peculiar velocity field
weighted by local number of galaxies. We have shown that the momentum power
spectrum can be derived from the density power spectrum for the constant linear
biasing of galaxy formation, which makes it possible to estimate \beta_S =
\Omega_m^{0.6} / b_S parameter precisely where \Omega_m is the matter density
parameter and b_S is the bias factor for optical spiral galaxies. At each
wavenumber k we estimate \beta_S(k) as the ratio of the measured to the derived
momentum power over a wide range of scales (0.026 h^{-1}Mpc <~ k <~ 0.157
h^{-1}Mpc) that spans the linear to the quasi-linear regimes. The estimated
\beta_S(k)'s have stable values around 0.5, which demonstrates the constancy of
\beta_S parameter at scales down to 40 h^{-1}Mpc. We have obtained
\beta_S=0.49_{-0.05}^{+0.08} or \Omega_m = 0.30_{-0.05}^{+0.09} b_S^{5/3}, and
the amplitude of mass fluctuation as
\sigma_8\Omega_m^{0.6}=0.56_{-0.21}^{+0.27}. The 68% confidence limits include
the cosmic variance. We have also estimated the mass density power spectrum.
For example, at k=0.1047 h Mpc^{-1} (\lambda=60 h^{-1}Mpc) we measure
\Omega_m^{1.2} P_{\delta}(k)=(2.51_{-0.94}^{+0.91})\times 10^3 (h^{-1}Mpc)^3,
which is lower compared to the high-amplitude power spectra found from the
previous maximum likelihood analyses of peculiar velocity samples like Mark
III, SFI, and ENEAR.
