Reanalyzing DESI DR1: 1. $\Lambda$CDM Constraints from the Power Spectrum and Bispectrum

We present the first independent re-analysis of the galaxy clustering data from DESI Data Release 1, utilizing an effective field theory full-shape model. We analyze the power spectra and bispectra of the public catalogs using a custom-built pipeline based on window-deconvolved quasi-optimal estimators, accounting for a number of systematic effects. Compared to the official collaboration analysis, we add the galaxy power spectrum hexadecapole and the bispectrum monopole, and also introduce a novel stochastic estimator for fiber collisions, which facilitates robust bispectrum analyses. As a first application, we perform a full-shape analysis of the DESI power spectra and bispectra in the context of the standard cosmological model, $\Lambda$CDM. Using external priors on the physical baryon density and the primordial power spectrum tilt, we constrain the matter density fraction to $\Omega_m=0.284\pm 0.011$, the Hubble constant to $H_0=70.7\pm 1.1$ km/s/Mpc, and the mass fluctuation amplitude to $\sigma_8=0.811\pm 0.030$. The bispectrum sharpens constraints on $\sigma_8$ and $\Omega_m$ by $\approx 10\%$ and shifts $\Omega_m$ by $\approx 1\sigma$ towards the \textit{Planck} $\Lambda$CDM value. Combining our full-shape likelihood with the official DESI DR2 BAO measurements, cosmological parameters shift further towards the \textit{Planck} values, with $\Omega_m=0.296\pm 0.007$, $H_0=68.8\pm 0.6$ km/s/Mpc, $\sigma_8=0.818\pm 0.029$ (with tighter constraints obtained in joint analyses). Similar results are obtained in a joint analysis with DR1 BAO, accounting for the cross-covariance. Finally, the bispectrum data improves measurements of quadratic bias parameters, which are consistent with predictions from halo occupation distribution models. Our work highlights the importance of higher-order statistics and sets the stage for upcoming full-shape analyses of non-minimal cosmological models.