Quantifying the Redshift Space Distortion of the Bispectrum I:
  Primordial Non-Gaussianity
  The anisotropy of the redshift space bispectrum contains a wealth of
cosmological information. This anisotropy depends on the orientation of three
vectors ${\bf k_1,k_2,k_3}$ with respect to the line of sight. Here we have
decomposed the redshift space bispectrum in spherical harmonics which
completely quantify this anisotropy. To illustrate this we consider linear
redshift space distortion of the bispectrum arising from primordial
non-Gaussianity. In the plane parallel approximation only the first four even
$\ell$ multipoles have non-zero values, and we present explicit analytical
expressions for all the non-zero multipoles {\it i.e.} upto $\ell=6,m=4$. The
ratio of the different multipole moments to the real space bispectrum depends
only on $\beta_1$ the linear redshift distortion parameter and the shape of the
triangle. Considering triangles of all possible shapes, we have studied how
this ratio depends on the shape of the triangle for $\beta_1=1$. We have also
studied the $\beta_1$ dependence for some of the extreme triangle shapes. If
measured in future, these multipole moments hold the potential of constraining
$\beta_1$. The results presented here are also important if one wishes to
constrain $f_{\text{NL}}$ using redshift surveys.
