We apply the Effective Field Theory of Large-Scale Structure (EFTofLSS) to analyze cosmological models with clustering quintessence, which allows us to consistently describe the parameter region in which the quintessence equation of state $w < - 1$. First, we extend the description of biased tracers in redshift space to the presence of clustering quintessence, and compute the one-loop power spectrum. We solve the EFTofLSS equations using the exact time dependence, which is relevant to obtain unbiased constraints. Then, fitting the full shape of BOSS pre-reconstructed power spectrum measurements, the BOSS post-reconstruction BAO measurements, BAO measurements from 6DF/MGS and eBOSS, the Supernovae from Pantheon, and a prior from BBN, we bound the clustering quintessence equation of state parameter $w=-1.011_{-0.048}^{+0.053}$ at $68%$ C.L.. Further combining with Planck, we obtain $w=-1.028_{-0.030}^{+0.037}$ at $68%$ C.L.. We also obtain constraints on smooth quintessence, in the physical regime $w geq -1$: combining all datasets, we get $-1leq w < - 0.979$ at $68%$ C.L.. These results strongly support a cosmological constant.
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