Contribution of new light particles beyond the Standard Model to ΔN_eff as a function of their decoupling temperature. Assuming natural units, we report the temperatures in GeV. As a reference we show the contribution of a Goldstone boson (solid brown line), a Weyl fermion (solid red line), and a massless Gauge boson (solid purple line). The grey vertical bands denote the range of temperatures of the QCD phase transition (around 200 MeV) and neutrino decoupling (around 1 MeV), while the thin grey vertical lines mark the annihilation temperature of Standard Model particles. The grey horizontal dot-dashed line represents the current 95% CL limit from Planck TT,TE,EE+lensing+BAO (Planck Collaboration VI 2020). The horizontal dashed lines mark the 95% upper bounds on ΔN_eff from Euclid only (in blue) and in combination with current (Planck) and future (CMB-S4+LiteBIRD) CMB surveys (in orange and green, respectively). As explained in Sect. 4.2, we choose to adopt more conservative assumptions regarding the CMB-S4 sensitivity and foreground removal than in a similar figure in the CMB-S4 white paper, Fig. 23 of Abazajian et al. (2016).