Axion Kinetic Misalignment Mechanism

In the conventional misalignment mechanism, the axion field has a constant initial field value in the early Universe and later begins to oscillate. We present an alternative scenario where the axion field has a nonzero initial velocity, allowing an axion decay constant much below the conventional prediction from axion dark matter. This axion velocity can be generated from explicit breaking of the axion shift symmetry in the early Universe, which may occur as this symmetry is approximate.

Figure 1

The schematics of the (kinetic) misalignment mechanism. Initial conditions are labeled, shadings from light to dark indicate the time sequence of the motion, and arrows with different relative lengths denote instantaneous velocities.

Figure 2

The parameter space of the QCD axion decay constant $f_a$ (or mass $m_a$) and the saxion vacuum mass $m_S$ compatible with the observed dark matter abundance. The blue line excludes high $m_S$ for the quartic potential. Applicable to both quartic and quadratic potentials, the gray region is ruled out for a maximal thermalization rate, while the constraint is the gray dashed line for thermalization via gluons only.