Publication
We present a clustering analysis of the BOSS DR12 CMASS galaxy sample, combining measurements of the galaxy two-point correlation function and density-split clustering down to a scale of 1 h^{-1} Mpc. Our theoretical framework is based on emulators trained on high-fidelity mock galaxy catalogues that forward model the cosmological dependence of the clustering statistics within an extended-Lambda CDM framework, including redshift-space and Alcock-Paczynski distortions. Our base-Lambda CDM analysis finds omega cdm = 0.1201 +/- 0.0022, sigma 8 = 0.792 +/- 0.034, and ns = 0.970 +/- 0.018, corresponding to f sigma 8 = 0.462 +/- 0.020 at z approximate to 0.525, which is in agreement with Planck 2018 predictions and various clustering studies in the literature. We test single-parameter extensions to base-Lambda CDM, varying the running of the spectral index, the dark energy equation of state, and the density of mass-less relic neutrinos, finding no compelling evidence for deviations from the base model. We model the galaxy-halo connection using a halo occupation distribution framework, finding signatures of environment-based assembly bias in the data. We validate our pipeline against mock catalogues that match the clustering and selection properties of CMASS, showing that we can recover unbiased cosmological constraints even with a volume 84 times larger than the one used in this study.