# On Scalaron Decay via the Trace of Energy-Momentum Tensor

## TL;DR

This paper investigates how quantum effects, specifically trace anomalies, influence the decay of scalarons into gauge bosons in inflation models, revealing that heavy particles decouple due to anomaly cancellations.

## Contribution

It demonstrates the role of trace anomalies in scalaron decay processes and clarifies decoupling of heavy degrees of freedom using both Fujikawa and dimensional renormalization methods.

## Key findings

- Trace anomaly cancels classical contributions from heavy particles.
- Scalaron decay amplitude can be evaluated at all orders in dimensional renormalization.
- Heavy degrees of freedom decouple from low-energy scalaron interactions.

## Abstract

In some inflation scenarios such as $R^{2}$ inflation, a gravitational scalar degrees of freedom called scalaron is identified as inflaton. Scalaron linearly couples to matter via the trace of energy-momentum tensor. We study scenarios with a sequestered matter sector, where the trace of energy-momentum tensor predominantly determines the scalaron coupling to matter. In a sequestered setup, heavy degrees of freedom are expected to decouple from low-energy dynamics. On the other hand, it is non-trivial to see the decoupling since scalaron couples to a mass term of heavy degrees of freedom. Actually, when heavy degrees of freedom carry some gauge charge, the amplitude of scalaron decay to two gauge bosons does not vanish in the heavy mass limit. Here the quantum contribution to the trace of energy-momentum tensor plays an essential role. This quantum contribution is known as trace anomaly or Weyl anomaly. The trace anomaly contribution from heavy degrees of freedom cancels with the contribution from the ${\it classical}$ scalaron coupling to a mass term of heavy degrees of freedom. We see how trace anomaly appears both in the Fujikawa method and in dimensional renormalization. In dimensional renormalization, one can evaluate the scalaron decay amplitude in principle at all orders, while it is unclear how to process it beyond the one-loop level in the Fujikawa method. We consider scalaron decay to two gauge bosons via the trace of energy-momentum tensor in quantum electrodynamics with scalars and fermions. We evaluate the decay amplitude at the leading order to demonstrate the decoupling of heavy degrees of freedom.