100 research outputs found
Gravitational waves from Higgs domain walls
The effective potential for the Standard Model Higgs field allows two
quasi-degenerate vacua; one is our vacuum at the electroweak scale, while the
other is at a much higher scale. The latter minimum may be at a scale much
smaller than the Planck scale, if the potential is lifted by new physics. This
gives rise to a possibility of domain wall formation after inflation. If the
high-scale minimum is a local minimum, domain walls are unstable and disappear
through violent annihilation processes, producing a significant amount of
gravitational waves. We estimate the amount of gravitational waves produced
from unstable domain walls in the Higgs potential and discuss detectability
with future experiments.Comment: 15 pages, 3 figures; v2: title changed, comments and references
added; v3: accepted for publication in PL
Cosmological abundance of the QCD axion coupled to hidden photons
We study the cosmological evolution of the QCD axion coupled to hidden
photons. For a moderately strong coupling, the motion of the axion field leads
to an explosive production of hidden photons by tachyonic instability. We use
lattice simulations to evaluate the cosmological abundance of the QCD axion. In
doing so, we incorporate the backreaction of the produced hidden photons on the
axion dynamics, which becomes significant in the non-linear regime. We find
that the axion abundance is suppressed by at most for the
decay constant GeV, compared to the case without the coupling.
For a sufficiently large coupling, the motion of the QCD axion becomes strongly
damped, and as a result, the axion abundance is enhanced. Our results show that
the cosmological upper bound on the axion decay constant can be relaxed by a
few hundred for a certain range of the coupling to hidden photons.Comment: 5 pages, 3 figures, v2: accepted for publication in PL
Relaxing Isocurvature Bounds on String Axion Dark Matter
If inflation scale is high, light scalars acquire large quantum fluctuations
during inflation.If sufficiently long-lived, they will give rise to CDM
isocurvature perturbations, which are highly constrained by the Planck data.
Focusing on string axions as such light scalars, we show that thermal inflation
can provide a sufficiently large entropy production to dilute the CDM
isocurvature perturbations. Importantly, efficient dilution is possible for the
string axions, because effectively no secondary coherent oscillations are
induced at the end of thermal inflation, in contrast to the moduli fields. We
also study the viability of the axion dark matter with mass of about 7 keV as
the origin of the 3.5 keV X-ray line excess, in the presence of large entropy
production.Comment: 18 pages, 2 figure
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