Kavli Affiliate: E. P. S. Shellard
| First 5 Authors: Amelia Drew, Tomasz Kinowski, E. P. S. Shellard, ,
| Summary:
In this paper, we perform an investigation into the effect of the string
radius of curvature $R_mathrm{,Gaussian}$ on the magnitude and relative
magnitude of the massive and massless radiation from axion (global) string
configurations, motivated by qualitative observations from string network
simulations. We construct initial conditions from travelling wave solutions on
a global string for two colliding Gaussians, performing parameter scans over
amplitude $A$ and standard deviation $sigma_mathrm{d}$. We show that the
energy emitted via massless radiation obeys a power law $E_mathrm{massless}
propto A^{gamma}$, where the coefficient $gamma$ depends on the curvature
regime. Massive radiation is exponentially suppressed approximately as
$E_{mathrm{massive}} propto e^{-zeta R_mathrm{,Gaussian}}$ in the
quasi-linear regime $sigma_mathrm{d} gg delta$ and exhibits power-law decay
$E_{mathrm{massive}} propto (R_mathrm{,Gaussian})^{-gamma}$ in the
nonlinear regime where $sigma_mathrm{d} lesssim 2delta$, with different
$gamma$ in different regimes of $R_mathrm{,Gaussian}$. In certain regions of
the nonlinear regime, massive particle radiation comprises up to 50% of the
total energy emitted. Drawing on a known parallel between axion radiation from
global strings and gravitational radiation from Abelian-Higgs strings, this
suggests that massive particle radiation channel may become of equal
significance to the massless (gravitational) channel for nonlinear burst
signals where $R < sigma_mathrm{d}$, unless we are in the regime where
additional loops are generated. We also estimate the spectral index $q$ of the
axion radiation for different amplitudes, showing that a higher proportion of
radiation is emitted in high frequency modes as the curvature increases,
bounded by $q gtrsim 1$ for the configurations studied.
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