Radiative θ in Left-Right Universal Seesaw
Abstract
We investigate radiative corrections to the QCD θ̅ parameter in the minimal left-right symmetric model with a universal seesaw mechanism, where parity symmetry ensures θ̅=0 at tree level. Assuming Hermitian vector-like quark mass matrices, we identify the CP-odd rephasing invariants contributing to θ̅ and evaluate the leading two-loop corrections to the light-quark mass matrices induced by soft parity breaking. We find that the dominant contributions do not decouple and depend sensitively on the mass splittings as well as flavor mixings of the vector-like quarks. For hierarchical vector-like quark masses, the resulting θ̅ typically exceeds the current experimental bound from the neutron electric dipole moment. Satisfying this bound requires either nearly degenerate vector-like quark masses or strongly suppressed additional flavor mixing angles. In the former case, the radiative corrections are suppressed by a GIM-like mechanism. For generic mixing parameters, the dimensionless relative mass splittings are constrained to be below approximately 10−2 and 10−4 in the up- and down-type vector-like quark sectors, respectively. These results severely restrict the inverted vector-like quark mass hierarchy motivated by the universal seesaw mechanism.
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BibTeX
@misc{banno2026radiative,
title = {{Radiative $θ$ in Left-Right Universal Seesaw}},
author = {Tatsuya Banno and Junji Hisano and Teppei Kitahara and Naohiro Osamura},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.39697},
eprint = {2610.39697},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.39697},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Radiative θ in Left-Right Universal Seesaw
AU - Tatsuya Banno
AU - Junji Hisano
AU - Teppei Kitahara
AU - Naohiro Osamura
PY - 2026
DA - 2026/10/29
PB - arXiv
DO - 10.48550/arxiv.2610.39697
UR - https://arxiv.org/abs/2610.39697
AB - We investigate radiative corrections to the QCD θ̅ parameter in the minimal left-right symmetric model with a universal seesaw mechanism, where parity symmetry ensures θ̅=0 at tree level. Assuming Hermitian vector-like quark mass matrices, we identify the CP-odd rephasing invariants contributing to θ̅ and evaluate the leading two-loop corrections to the light-quark mass matrices induced by soft parity breaking. We find that the dominant contributions do not decouple and depend sensitively on the mass splittings as well as flavor mixings of the vector-like quarks. For hierarchical vector-like quark masses, the resulting θ̅ typically exceeds the current experimental bound from the neutron electric dipole moment. Satisfying this bound requires either nearly degenerate vector-like quark masses or strongly suppressed additional flavor mixing angles. In the former case, the radiative corrections are suppressed by a GIM-like mechanism. For generic mixing parameters, the dimensionless relative mass splittings are constrained to be below approximately 10^−2 and 10^−4 in the up- and down-type vector-like quark sectors, respectively. These results severely restrict the inverted vector-like quark mass hierarchy motivated by the universal seesaw mechanism.
ER -