Tachyonic Preheating in Palatini $R^2$ Inflation

TL;DR

This paper investigates tachyonic preheating in Palatini $R^2$ inflation models, revealing rapid inflaton fragmentation driven by tachyonic instability for large $oldsymbol{ extit{ extalpha}}$ values, with implications for early universe dynamics.

Contribution

It demonstrates that tachyonic preheating occurs in Palatini $R^2$ inflation models when $oldsymbol{ extalpha extgreater 10^{13}}$, extending understanding of preheating in plateau inflation scenarios.

Findings

Tachyonic preheating occurs for $ extalpha extgreater 10^{13}$.

Fragmentation rate scales as $ extGamma/H extapprox 0.011 imes extalpha^{0.31}$.

Non-canonical kinetic terms can be neglected during preheating and inflation.

Abstract

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. Although it contains non-canonical quartic kinetic terms in the Einstein frame, we show that, in the first approximation, these can be neglected during both preheating and inflation.