Kavli Affiliate: Masahiro Kawasaki
| First 5 Authors: Masahiro Kawasaki, Kai Murai, Fuminobu Takahashi, ,
| Summary:
We study the evolution of Q-balls under a spontaneously broken global $U(1)$
symmetry. Q-balls are stabilized by the conservation of $U(1)$ charge, but when
the symmetry is spontaneously broken, the resulting Nambu-Goldstone (NG) boson
can carry charge away from the Q-ball, potentially leading to charge leakage.
To study this process in a controlled setting, we consider a scenario where
Q-balls first form under an unbroken $U(1)$ symmetry, which is then
spontaneously broken. We introduce two complex scalar fields: one responsible
for forming the Q-ball, and the other for spontaneously breaking the $U(1)$
symmetry, allowing us to clearly separate the formation and symmetry-breaking
phases. Using numerical simulations in a spherically symmetric system, we find
that the evolution of Q-balls depends sensitively on the structure of the
interaction between the two fields and the magnitude of symmetry breaking.
Depending on parameters, Q-balls can completely decay, evaporate into smaller,
stable Q-balls, or transition into oscillons/I-balls. In particular, we find
that stable, localized remnants often survive the evolution over long
timescales, especially when the symmetry-breaking scale is small. These results
demonstrate that, even though spontaneous $U(1)$ breaking can lead to
significant energy and charge loss from Q-balls, stable localized objects with
reduced or no charge can frequently survive and potentially contribute to
cosmological relics.
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