Chow and augmented Chow polynomials as evaluations of Poincaré-extended ab-indices

We show that Chow polynomials and augmented Chow polynomials of matroids, and more generally of finite graded posets admitting R -labelings, are obtained as evaluations of their Poincaré-extended a b -indices. This implies in particular explicit combinatorial γ -positive expansions for both, providing the first proof of the γ -positivity not relying on the Kähler package for the Chow ring. We then evaluate this expansion to obtain an explicit closed formula for the braid arrangement.