Web-halo model (WHM): accurate non-linear matter power spectrum predictions without free parameters

ABSTRACT We present a parameter-free variant of the halo model that significantly improves the precision of matter clustering predictions, particularly in the challenging 1-halo to 2-halo transition regime, where standard halo models often fail. Unlike HMcode-2020, which relies on 12 phenomenological parameters, our approach achieves comparable or superior accuracy without any free fitting parameters. This new web-halo model (WHM) extends the traditional halo model by incorporating structures that have collapsed along two dimensions (filaments) and one dimension (sheets), in addition to haloes, and combines these with 1-loop Lagrangian perturbation theory (1$\ell$-LPT) in a consistent framework. We show that WHM matches N-body simulation power spectra within the precision of state-of-the-art emulators at the 2-halo to 1-halo transition regime at all redshifts. Specifically, the WHM achieves better than 2 per cent accuracy up to scales of $k = 0.4\, h\, \mathrm{Mpc}^{-1}$, $0.7\, h\, \mathrm{Mpc}^{-1}$, and $1.3\, h\, \mathrm{Mpc}^{-1}$ at redshifts $z = 0.0$, 0.8, and 1.5, respectively, for both the baccoemu and EuclidEmu2 emulators, across their full $w_0w_a\mathrm{CDM} + \sum m_\nu$ cosmological parameter space. This marks a substantial improvement over 1$\ell$-LPT and HMcode-2020, the latter of which performs similarly at low redshift but deteriorates at higher redshifts despite its 12 tuned parameters. We publicly release WHM as WHMcode, integrated into existing HMcode implementations for CAMB and CLASS.