Gravitational Wave Echo of Relaxion Trapping

Understand

To solve the hierarchy problem, the relaxion must remain trapped in the correct minimum, even if the electroweak symmetry is restored after reheating.

• In this scenario, the relaxion starts rolling again until the backreaction potential, with its set of local minima, reappears.
• Depending on the time of barrier reappearance, Hubble friction alone may be insufficient to retrap the relaxion in a large portion of the parameter space.
• Thus, an additional source of friction is required, which might be provided by coupling to a dark photon.The dark photon experiences a tachyonic instability as the relaxion rolls, which slows down the relaxion by backreacting to its motion, and efficiently creates anisotropies in the dark photon energy-momentum tensor, sourcing gravitational waves.