Abstract
We investigate the production of axion quanta during the early universe evolution of an axion-like field rolling down a wiggly potential. We compute the growth of quantum fluctuations and their back-reaction on the homogeneous zero-mode. We evaluate the transfer of kinetic energy from the zero mode to the quantum fluctuations and the conditions to decelerate the axion zero-mode as a function of the Hubble rate, the slope of the potential, the size of the barriers and the initial field velocity. We discuss how these effects impact the relaxion mechanism.
Highlights
Are well-motivated candidates to explain e.g. dark matter [3,4,5], inflation [6,7,8,9], and baryogenesis [10, 11]
We investigate the production of axion quanta during the early universe evolution of an axion-like field rolling down a wiggly potential
We evaluate the transfer of kinetic energy from the zero mode to the quantum fluctuations and the conditions to decelerate the axion zero-mode as a function of the Hubble rate, the slope of the potential, the size of the barriers and the initial field velocity
Summary
We discuss how the axion field φ evolution is affected by the axion fragmentation phenomenon. We assume that φ0 is large enough to overcome the barrier, i.e., φ0 Λ2b , otherwise φ is trapped in the first valley This marks a crucial difference with the wellstudied case of parametric resonance due to a scalar field which oscillates coherently at the minimum of its potential [16, 25,26,27,28,29,30,31,32,33,34]. The homogeneous mode gradually looses its kinetic energy because of back-reaction, and the instability band moves towards the region of small values of k (see figure 3). The timescale that the mode with wave number kc0r spends inside the instability band can be estimated combining eqs. The correct numerical factors are the ones of eqs. (3.26) and (3.27) below, which reproduce the parametric dependence of eqs. (2.22) and (2.23)
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