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We study the production of a beyond the Standard Model (BSM) free-streamingrelativistic particles which contribute to $N_{eff}$ and investigate how muchthe predictions for the inflationary analysis change. We consider inflatondecay as the source of this dark radiation (DR) and use the Cosmic MicrowaveBackground (CMB) data from $\textit{Planck}$-2018 to constrain the scenariosand identify the parameter space involving couplings and masses of the inflatonthat will be within the reach of next-generation CMB experiments like SPT-3G,CMB-S4, $\text{CMB-Bh$\overline{a}$rat}$, PICO, CMB-HD, etc. We find that ifthe BSM particle is produced only from the interaction with inflaton along withStandard Model (SM) relativistic particles, then its contribution to $N_{eff}$is a monotonically increasing function of the branching fraction, $B_X$ of theinflaton to the BSM particle $X$; $\textit{Planck}$ bound on $N_{eff}$ rulesout such $B_X \gtrsim 0.09$. Considering two different analyses of$\textit{Planck}$+BICEP data together with other cosmological observations,$N_{eff}$ is treated as a free parameter, which relaxes the constraints onscalar spectral index ($n_s$) and tensor-to-scalar ratio ($r$). The firstanalysis leads to predictions on the inflationary models like Hilltop inflationbeing consistent with the data. Second analysis rules out the possibility thatBSM particle $X$ producing from the inflaton decay in Coleman-WeinbergInflation or Starobinsky Inflation scenarios. To this end, we assume that SMHiggs is produced along with the BSM particle. We explore the possibilitiesthat $X$ can be either a scalar or a fermion or a gauge boson and considerpossible interactions with inflaton and find out the permissible range on theallowed parameter space Planck and those which will be within the reaches offuture CMB observations.

Measuring inflaton couplings via dark radiation as $\Delta N_{\rm eff}$ in CMB