21 cm Angular Power Spectrum from Minihalos as a Probe of Primordial
  Spectral Runnings
  Measurements of 21 cm line fluctuations from minihalos have been discussed as
a powerful probe of a wide range of cosmological models. However, previous
studies have taken into account only the pixel variance, where contributions
from different scales are integrated. In order to sort out information from
different scales, we formulate the angular power spectrum of 21 cm line
fluctuations from minihalos at different redshifts, which can enhance the
constraining power enormously. By adopting this formalism, we investigate
expected constraints on parameters characterizing the primordial power
spectrum, particularly focusing on the spectral index $n_s$ and its runnings
$\alpha_s$ and $\beta_s$. We show that future observations of 21 cm line
fluctuations from minihalos, in combination with cosmic microwave background,
can potentially probe these runnings as $\alpha_s \sim {\cal O}(10^{-3})$ and
$\beta_s \sim {\cal O}(10^{-4})$. Its implications to the test of inflationary
models are also discussed.
