Neutrino self-interactions in light of DESI and CMB-SPA

We present an updated analysis of non-standard neutrino self-interactions, where the interaction is effectively mediated by a heavy particle, using the latest available data from DESI and the CMB. The considered data include combinations of DESI DR1 full-shape galaxy clustering (FS), BAO distance measurements, Lyman-$\alpha$ 1D power spectra (Ly-$\alpha$), and a combined CMB likelihood (CMB-SPA) using data from Planck, ACT, and SPT. First, we probe two DESI DR1-only datasets, where the effective interaction parameter $G_{\mathrm{e}\mathrm{f}\mathrm{f}}$ is multimodal in the profile likelihood, leading us to analyze two different regimes: moderately interacting (MI${}_{\nu }$) and strongly interacting (SI${}_{\nu }$) neutrinos. With FS+BAO, the evidence disfavors the SI${}_{\nu }$ mode, a trend that becomes slightly stronger with the inclusion of Ly-$\alpha$. However, in the latter dataset, the MI${}_{\nu }$ region shows a bimodality in which the intermediate mode is very sharp and peaks around $G_{\mathrm{e}\mathrm{f}\mathrm{f}}\simeq {10}^{-3}{\mathrm{M}\mathrm{e}\mathrm{V}}^{-2}$; interestingly, this mode is not penalized versus the weaker standard-like mode. Furthermore, we test CMB and DESI joint data and show that the SI${}_{\nu }$ mode is more disfavored by CMB-SPA than by Planck-only data. In particular, the evidence against this mode becomes the strongest thus far (with odds of 5:100000) in the FS+BAO+CMB-SPA combination.