发表机构
INAF, Osservatorio d’Abruzzo; Dipartimento di Fisica, Università Sapienza; INFN, Sezione di Roma; Theoretical Physics Department, CERN（意大利国家天体物理研究所阿布鲁佐天文台; 罗马大学物理系; 意大利国家核物理研究所罗马分部; 欧洲核子研究中心理论物理部）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究利用红巨星支尖端与隆起的热星等差作为距离无关观测量，结合三个球状星团数据，通过最大似然分析得出轴子-电子耦合上限g13<1.49，并排除g13>7.5，验证了该方法的有效性。
AI 中文摘要
对轴子和轴子类粒子的兴趣再度兴起，这源于它们在解决强CP问题中的作用以及作为暗物质候选者的吸引力。红巨星支（TRGB）尖端的亮度为探测这些粒子性质提供了一个关键观测量。本研究旨在通过采用一种对距离、星际消光、热改正（BCs）零点以及其他系统不确定性较不敏感的微分观测量，来改进现有的轴子-电子耦合约束。我们使用TRGB与红巨星支隆起（RGBB）之间的热星等差异作为距离无关的约束，应用于三个中等至高金属丰度的球状星团：NGC 104（47 Tuc）、NGC 362和NGC 5904（M5）。利用RGB恒星的光度星表，我们确定了这两个特征的V波段和I波段星等，并将其转换为热星等。在验证我们的恒星模型能够重现观测到的RGBB光度之后，我们进行了最大似然分析，并辅以蒙特卡洛模拟来传播不确定性，从而得出新的耦合约束。对三个星团的联合分析得出最大似然值为g13 = gae/10^-13 = 0.8，95%置信水平的上限为1.49。尽管略逊于近期来自更大多星团样本的约束，但该上限更为稳健。在合理的质量损失假设下，g13 > 7.5被排除，因为它预测了球状星团中常规观测到的水平分支（HB）和渐近巨星支（AGB）阶段的消失，这一限制比当前直接实验约束（如XENONnT）强一个数量级以上。我们证明了这种微分、距离无关的方法在约束标准模型之外物理方面的有效性。将其应用于更广泛的球状星团样本将进一步细化对gae的约束。
英文摘要
Interest in axions and axion-like particles has resurged, driven by their role in solving the strong CP problem and their appeal as dark matter candidates. The luminosity of the tip of the red giant branch (TRGB) offers a key observable for probing these particle properties. This work aims to improve existing bounds on the axion-electron coupling by adopting a differential observable that is less sensitive to distance, interstellar extinction, the zero-point of bolometric corrections (BCs), and other systematic uncertainties. We use the bolometric magnitude difference between the TRGB and the RGB bump (RGBB) as a distance-independent constraint, applied to three globular clusters of intermediate-to-high metallicity: NGC 104 (47 Tuc), NGC 362, and NGC 5904 (M5). Using photometric catalogs of RGB stars, we determine V- and I-band magnitudes of both features and convert them to bolometric values. After validating that our stellar models reproduce the observed RGBB luminosity, we perform a maximum likelihood analysis with Monte Carlo simulations to propagate uncertainties and derive new bounds on the coupling. A combined analysis of the three clusters yields a maximum likelihood at $g_{13} = g_{ae}/10^{-13} = 0.8$ and a 95% C.L. upper limit of 1.49. Although slightly less stringent than recent bounds from larger multi-cluster samples, this limit is substantially more robust. Under reasonable mass-loss assumptions, $g_{13} \gtrsim 7.5$ is ruled out, as it predicts the disappearance of the HB and AGB phases routinely observed in globular clusters, a limit more than an order of magnitude stronger than current direct experimental bounds such as XENONnT. We demonstrate the effectiveness of this differential, distance-independent method for constraining physics beyond the Standard Model. Applying it to a wider sample of globular clusters would further refine the constraint on $g_{ae}$.
