Tree-Level Bispectrum in the Effective Field Theory of Large-Scale Structure extended to Massive Neutrinos

Abstract: We compute the tree-level bispectrum of dark matter in the presence of massive neutrinos in the mildly non-linear regime in the context of the effective field theory of large-scale structure (EFTofLSS). For neutrinos, whose typical free streaming wavenumber ($k_{\rm fs}$) is longer than the non-linear scale ($k_{\mathrm{NL}}$), we solve a Boltzmann equation coupled to the effective fluid equation for dark matter. We solve perturbatively the coupled system by expanding in powers of the neutrino density fraction ($f_{\nu}$) and the ratio of the wavenumber of interest over the non-linear scale ($k/k_{\mathrm{NL}}$) and add suitable counterterms to remove the dependence from short distance physics. For equilateral configurations, we find that the total-matter tree-level bispectrum is approximately $16f_{\nu}$ times the dark matter one on short scales ($k > k_{\rm fs}$). The largest contribution stems from the back-reaction of massive neutrinos on the dark matter growth factor. On large scales ($k < k_{\rm fs}$) the contribution of neutrinos to the bispectrum is smaller by up to two orders of magnitude.