发表机构
Università di Padova; INFN Sezione di Padova; University of California, Berkeley; Lawrence Berkeley National Laboratory; Università degli Studi di Torino（帕多瓦大学; 意大利国家核物理研究所帕多瓦分部; 加州大学伯克利分校; 劳伦斯伯克利国家实验室; 都灵大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本文研究受激轴子衰变中相干与不相干放大的差异，揭示参数共振（$s\propto g_{a\gamma}$）与动力学行为（$s\propto g_{a\gamma}^2$）源于轴子场的相干性，并推导出统一的有效演化方程及交叉描述。
AI 中文摘要
尽管轴子自发衰变为两个光子的过程在宇宙学时间尺度上极其缓慢，但在玻色子占据数较大的环境中，该过程可被显著增强。从经典场论的角度看，受激轴子衰变由修正的麦克斯韦方程组描述。对于相干轴子背景，这些方程表现出参数共振，其特征为光子增长指数$s\propto g_{a\gamma}$。相比之下，标准的动力学描述（例如基于玻尔兹曼方程）涉及衰变过程的矩阵元平方，因此自然预期特征速率标度为$s\propto g_{a\gamma}^2$。在本工作中，我们表明这些看似不同的行为源于轴子场的相干性特性。从具有有限动量弥散的轴子场出发，我们展示了其动量模式间的退相干如何修改光子不稳定性，并推导出一个有效演化方程，其中该效应被编码在一个核函数中。对于轴子速度的连续麦克斯韦-玻尔兹曼分布，我们获得了光子增长指数的隐式表达式，该表达式重现了两个渐近区域，并提供了交叉区域的近似描述。当放大发生在比轴子相干长度更短的尺度上时，恢复参数共振标度$s\propto g_{a\gamma}$；而在相反极限下，则出现具有$s\propto g_{a\gamma}^2$的有效动力学行为。尽管交叉区域的具体形式依赖于轴子动量分布，我们预期$g_{a\gamma}$的渐近标度在很大程度上对所假设的具体分布不敏感。
英文摘要
Although the spontaneous decay of axions into two photons is extremely slow on cosmological timescales, it can be strongly enhanced in environments with large bosonic occupation numbers. From a classical field perspective, stimulated axion decay is described by modified Maxwell equations. For a coherent axion background, these equations exhibit a parametric resonance characterized by a photon growth exponent $s\propto g_{aγ}$. By contrast, a standard kinetic description, for instance based on Boltzmann equations, involves the squared matrix element of the decay process and therefore would naturally predict a characteristic rate scaling as $s\propto g_{aγ}^2$. In this work, we show that these apparently different behaviors originate from the coherence properties of the axion field. Starting from an axion field with a finite momentum dispersion, we show how dephasing among its momentum modes modifies the photon instability and derive an effective evolution equation in which this effect is encoded in a kernel. For a continuous Maxwell-Boltzmann distribution of axion velocities, we obtain an implicit expression for the photon growth exponent that reproduces the two asymptotic regimes and provides an approximate description of the crossover. When amplification develops on scales shorter than the axion coherence length, the parametric resonance scaling $s\propto g_{aγ}$ is recovered, whereas in the opposite limit an effectively kinetic behavior with $s\propto g_{aγ}^2$ emerges. Although the detailed form of the crossover depends on the axion momentum distribution, we expect the asymptotic scaling with $g_{aγ}$ to be largely insensitive to the specific distribution assumed.
Comments35 pages, 3 figures
