Axion Misalignment Driven to the Hilltop. (arXiv:1812.11192v1 [hep-ph])

Axion Misalignment Driven to the Hilltop. (arXiv:1812.11192v1 [hep-ph])
<a href="http://arxiv.org/find/hep-ph/1/au:+Co_R/0/1/0/all/0/1">Raymond T. Co</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Gonzalez_E/0/1/0/all/0/1">Eric Gonzalez</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Harigaya_K/0/1/0/all/0/1">Keisuke Harigaya</a>

The QCD axion serves as a well-motivated dark matter candidate and the
misalignment mechanism is known to reproduce the observed abundance with a
decay constant $f_a simeq mathcal{O}(10^{12})$ GeV for a misalignment angle
$theta_{rm mis} simeq mathcal{O}(1)$. While $f_a ll 10^{12}$ GeV is of
great experimental interest, the misalignment mechanism requires the axion to
be very close to the hilltop, i.e. $theta_{rm mis} simeq pi$. This
particular choice of $theta_{rm mis}$ has been understood as fine-tuning the
initial condition. We offer a dynamical explanation for $theta_{rm mis}
simeq pi$ in a class of models. The axion dynamically relaxes to the minimum
of the potential by virtue of an enhanced mass in the early universe. This
minimum is subsequently converted to a hilltop because the CP phase of the
theory shifts by $pi$ when one contribution becomes subdominant to another
with an opposite sign. We demonstrate explicit and viable examples in
supersymmetric models where the higher dimensional Higgs coupling with the
inflaton naturally achieves both criteria. Associated phenomenology includes a
strikingly sharp prediction of $3 times 10^9~{rm GeV} lesssim f_a lesssim
10^{10}$ GeV owing to anharmonic effects, the absence of isocurvature
perturbations, and possible formation of axion miniclusters due to attractive
self-interactions near the hilltop.

The QCD axion serves as a well-motivated dark matter candidate and the
misalignment mechanism is known to reproduce the observed abundance with a
decay constant $f_a simeq mathcal{O}(10^{12})$ GeV for a misalignment angle
$theta_{rm mis} simeq mathcal{O}(1)$. While $f_a ll 10^{12}$ GeV is of
great experimental interest, the misalignment mechanism requires the axion to
be very close to the hilltop, i.e. $theta_{rm mis} simeq pi$. This
particular choice of $theta_{rm mis}$ has been understood as fine-tuning the
initial condition. We offer a dynamical explanation for $theta_{rm mis}
simeq pi$ in a class of models. The axion dynamically relaxes to the minimum
of the potential by virtue of an enhanced mass in the early universe. This
minimum is subsequently converted to a hilltop because the CP phase of the
theory shifts by $pi$ when one contribution becomes subdominant to another
with an opposite sign. We demonstrate explicit and viable examples in
supersymmetric models where the higher dimensional Higgs coupling with the
inflaton naturally achieves both criteria. Associated phenomenology includes a
strikingly sharp prediction of $3 times 10^9~{rm GeV} lesssim f_a lesssim
10^{10}$ GeV owing to anharmonic effects, the absence of isocurvature
perturbations, and possible formation of axion miniclusters due to attractive
self-interactions near the hilltop.

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