Non-perturbative approach for scalar particle production in Higgs-$R^2$ inflation

We investigate the non-perturbative production of scalar particles within the Higgs-$R^2$ inflation model, focusing on a massive spectator field that interacts gravitationally via a non-minimal coupling to the Ricci scalar $R$. By transforming the model to the Einstein frame, where direct couplings between inflationary dynamics and the spectator field emerge, we analyze both perturbative and non-perturbative particle production. We solve the spectator field equation numerically, using the Bogolyubov transformation, to calculate the comoving particle density. Our findings indicate that particle production occurs for light particles with conformal coupling and for masses $m_\chi > M/2$ in the minimally coupled scenario.