Saxion Decay and Dark Radiation in the KSVZ Axion Model
Abstract
We calculate the relativistic axion energy density produced by the decay of a curvature-displaced Peccei--Quinn condensate in a quartic KSVZ model. We follow the radial PQ field (the saxion) as it relaxes from its inflationary minimum and determine the surviving condensate abundance from the adiabatic action of its oscillations. We then evolve the condensate through its decay into axions and visible particles to calculate the resulting dark-radiation contribution, expressed as ΔNeff. With one heavy coloured KSVZ fermion and no additional portal interactions included, the axion channel dominates the two tree-level decay modes. Two reference histories yield ΔNeff≃0.033 and $0.12$, with maximum saxion energy fractions below 1% and 4%, respectively. At fixed quartic coupling, increasing the duration of curvature release reduces the predicted axion radiation density relative to visible radiation by a factor of approximately $28$ while preserving the primordial isocurvature amplitude.
Related
- Enabling Polyspectra via Stochastic Realizations of Inverse Decay Curvature Perturbations in Axion Inflation
- Coherent versus incoherent amplification: from parametric resonance to the kinetic regime in stimulated axion decay
- From Axion Detection to Dark Matter Astronomy
- Gravitational Instability of Cold Axion Dark Matter Beyond Minimal Coupling
- Enhancing Constraints on Ultralight Axion Dark Matter from Gravitational Capture
BibTeX
@misc{samtani2026saxion,
title = {{Saxion Decay and Dark Radiation in the KSVZ Axion Model}},
author = {Sidhartha Samtani and Amartya Sengupta},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.31101},
eprint = {2610.31101},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.31101},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Saxion Decay and Dark Radiation in the KSVZ Axion Model
AU - Sidhartha Samtani
AU - Amartya Sengupta
PY - 2026
DA - 2026/10/22
PB - arXiv
DO - 10.48550/arxiv.2610.31101
UR - https://arxiv.org/abs/2610.31101
AB - We calculate the relativistic axion energy density produced by the decay of a curvature-displaced Peccei--Quinn condensate in a quartic KSVZ model. We follow the radial PQ field (the saxion) as it relaxes from its inflationary minimum and determine the surviving condensate abundance from the adiabatic action of its oscillations. We then evolve the condensate through its decay into axions and visible particles to calculate the resulting dark-radiation contribution, expressed as ΔN_eff. With one heavy coloured KSVZ fermion and no additional portal interactions included, the axion channel dominates the two tree-level decay modes. Two reference histories yield ΔN_eff≃0.033 and $0.12$, with maximum saxion energy fractions below 1% and 4%, respectively. At fixed quartic coupling, increasing the duration of curvature release reduces the predicted axion radiation density relative to visible radiation by a factor of approximately $28$ while preserving the primordial isocurvature amplitude.
ER -