  We analyze clustering measurements of BOSS galaxies using a simulation-based
emulator of two-point statistics. We focus on the monopole and quadrupole of
the redshift-space correlation function, and the projected correlation
function, at scales of $0.1\sim60~h^{-1}$Mpc. Although our simulations are
based on $w$CDM with general relativity (GR), we include a scaling parameter of
the halo velocity field, $\gamma_f$, defined as the amplitude of the halo
velocity field relative to the GR prediction. We divide the BOSS data into
three redshift bins. After marginalizing over other cosmological parameters,
galaxy bias parameters, and the velocity scaling parameter, we find
$f\sigma_{8}(z=0.25) = 0.413\pm0.031$, $f\sigma_{8}(z=0.4) = 0.470\pm0.026$ and
$f\sigma_{8}(z=0.55) = 0.396\pm0.022$. Compared with Planck observations using
a flat $\Lambda$CDM model, our results are lower by $1.9\sigma$, $0.3\sigma$
and $3.4\sigma$ respectively. These results are consistent with other recent
simulation-based results at non-linear scales, including weak lensing
measurements of BOSS LOWZ galaxies, two-point clustering of eBOSS LRGs, and an
independent clustering analysis of BOSS LOWZ. All these results are generally
consistent with a combination of $\gamma_f^{1/2}\sigma_8\approx 0.75$. We note,
however, that the BOSS data is well fit assuming GR, i.e. $\gamma_f=1$. We
cannot rule out an unknown systematic error in the galaxy bias model at
non-linear scales, but near-future data and modeling will enhance our
understanding of the galaxy--halo connection, and provide a strong test of new
physics beyond the standard model.
