= Helicity selection for tensor CMB multipoles
{title2=$m=2\quad\text{for the specified complex helicity mode}$}
A complex helicity-$+2$ plane wave along the $z$ axis has polarization contraction $\sin^2\theta e^{2i\varphi}$. Its remaining <tensor CMB line-of-sight source> depends only on $\cos\theta$. Orthogonality of the azimuthal factors of <spherical harmonics> therefore restricts its temperature coefficients to $m=2$. A real perturbation also includes the conjugate mode.
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