[论文解读] First Search for Axion-Like Particles in a Storage Ring Using a Polarized Deuteron Beam

S. Karanth, E. J. Stephenson+61 more|arXiv (Cornell University)|Aug 15, 2022

Dark Matter and Cosmic Phenomena被引用 4

一句话总结

本论文首次在存储环中利用极化氘束探测轴子样粒子（ALPs），通过自旋动力学与ALP耦合引起的极化跃迁，测量有效电子电偶极矩（EDM）。该方法实现了 $ d_{\text{AC}} \sim 10^{-22} \, \text{e} \cdot \text{cm} $ 的灵敏度极限，基于于德国于希尔斯堡研究中心（Forschungszentrum Jülich）的COSY环中束流极化测量，设定了ALP耦合的新约束。

ABSTRACT

Based on the notion that the local dark-matter field of axions or axion-like particles (ALPs) in our Galaxy induces oscillating couplings to the spins of nucleons and nuclei (via the electric dipole moment of the latter and/or the paramagnetic axion-wind effect), we establish the feasibility of a new method to search for ALPs in storage rings. Based on previous work that allows us to maintain the in-plane polarization of a stored deuteron beam for a few hundred seconds, we performed a first proof-of-principle experiment at the Cooler Synchrotron COSY to scan momenta near 970 MeV/c. This entailed a scan of the spin precession frequency. At resonance between the spin precession frequency of deuterons and the ALP-induced EDM oscillation frequency there will be an accumulation of the polarization component out of the ring plane. Since the axion frequency is unknown, the momentum of the beam and consequently the spin precession frequency were ramped to search for a vertical polarization change that would occur when the resonance is crossed. At COSY, four beam bunches with different polarization directions were used to make sure that no resonance was missed because of the unknown relative phase between the polarization precession and the axion/ALP field. A frequency window of 1.5-kHz width around the spin precession frequency of 121 kHz was scanned. We describe the experimental procedure and a test of the methodology with the help of a radiofrequency Wien filter located on the COSY ring. No ALP resonance was observed. As a consequence an upper limit of the oscillating EDM component of the deuteron as well as its axion coupling constants are provided.

研究动机与目标

• 通过在存储环环境中探测ALPs与氘核自旋的耦合，寻找轴子样粒子（ALPs）。
• 开发一种检测旋转束流中ALP引起的极化跃迁的校准方法。
• 设定由ALP相互作用诱导的有效电子EDM的新实验极限。
• 通过在不同扫描速率下对自旋动力学进行数值模拟，验证存储环装置对ALP诱导EDM的灵敏度。
• 校准COSY环中轴子诱导自旋振荡与可观测极化跃迁之间的关系。

提出的方法

• 在COSY存储环中使用极化氘束，通过控制回旋频率的斜坡扫描，诱导自旋进动。
• 采用每圈15步的3×3密度矩阵演化数值模拟，以高精度（0.1%收敛性）建模自旋动力学。
• 通过施加两个正交的轴子相位输入（$\phi_a = 0$ 和 $\pi/2$）校准极化跃迁响应，并将结果进行平方和开方处理，以确定最大跃迁幅度。
• 利用关系式 $ d_{\text{AC}} = \frac{1}{2\pi} \frac{\hbar}{B\rho} \frac{w}{0.958} \left| \frac{\psi_{\text{AC}}}{\Delta p_y} \right|_{\text{calib.}} A $ 推导有效电子EDM，其中 $ A $ 为观测到的跃迁幅度。
• 应用扫描速率（$ w $）和环几何因子（0.958）的修正，将校准结果扩展至实际实验条件。
• 该方法经轴子风效应验证，与径向自旋旋转模型一致。

实验结果

研究问题

• RQ1 能否通过极化氘核自旋耦合在存储环中探测轴子样粒子？
• RQ2 COSY存储环装置对ALP诱导的有效电子EDM的灵敏度如何？
• RQ3 极化跃迁幅度如何随轴子诱导自旋振荡幅度和扫描速率变化？
• RQ4 自旋动力学的数值模拟在多大程度上能准确预测可观测的极化跃迁？
• RQ5 该方法可检测到的最大有效电子EDM是多少？与现有极限相比如何？

主要发现

• 校准方法对快速扫描的灵敏度达到 $ 9.35 \times 10^{-7} $ rad/turn，对慢速扫描达到 $ 8.48 \times 10^{-7} $ rad/turn，实现了轴子诱导自旋振荡与可观测极化跃迁之间的关联。
• 对于典型观测到的跃迁幅度，有效电子EDM估计低于 $ 10^{-22} \, \text{e} \cdot \text{cm} $。
• 在相关范围内，极化跃迁与轴子诱导自旋振荡幅度近似呈线性关系，支持可靠校准。
• 通过修正因子 $ w $ 和 0.958，方法考虑了环几何结构和扫描速率的影响，提升了灵敏度估计的准确性。
• 每圈15步的数值模拟收敛精度达0.1%，验证了跃迁预测模型的可靠性。
• 校准结果在不同扫描速率和相位配置下保持一致，证实了该方法的鲁棒性。