Tachyonic Preheating in Palatini $R^2$ Inflation

Understand

We study preheating in the Palatini formalism with a quadratic inflaton potential and an added $\alpha R^2$ term.

• In such models, the oscillating inflaton field repeatedly returns to the plateau of the Einstein frame potential, on which the tachyonic instability fragments the inflaton condensate within less than an e-fold.
• We find that tachyonic preheating takes place when $\alpha \gtrsim 10^{13}$ and that the energy density of the fragmented field grows with the rate $\Gamma/H \approx 0.011 \times \alpha^{0.31}$.
• The model extends the family of plateau models with similar preheating behaviour.