  We study the production of a beyond the Standard Model (BSM) free-streaming
relativistic particles which contribute to $N_{eff}$ and investigate how much
the predictions for the inflationary analysis change. We consider inflaton
decay as the source of this dark radiation (DR) and use the Cosmic Microwave
Background (CMB) data from $\textit{Planck}$-2018 to constrain the scenarios
and identify the parameter space involving couplings and masses of the inflaton
that 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 if
the BSM particle is produced only from the interaction with inflaton along with
Standard Model (SM) relativistic particles, then its contribution to $N_{eff}$
is a monotonically increasing function of the branching fraction, $B_X$ of the
inflaton to the BSM particle $X$; $\textit{Planck}$ bound on $N_{eff}$ rules
out 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 on
scalar spectral index ($n_s$) and tensor-to-scalar ratio ($r$). The first
analysis leads to predictions on the inflationary models like Hilltop inflation
being consistent with the data. Second analysis rules out the possibility that
BSM particle $X$ producing from the inflaton decay in Coleman-Weinberg
Inflation or Starobinsky Inflation scenarios. To this end, we assume that SM
Higgs is produced along with the BSM particle. We explore the possibilities
that $X$ can be either a scalar or a fermion or a gauge boson and consider
possible interactions with inflaton and find out the permissible range on the
allowed parameter space Planck and those which will be within the reaches of
future CMB observations.
