The recent electron recoil excess observed by XENON1T has a possible interpretation in terms of solar axions coupled to electrons. If such axions are still relativistic at recombination they would also leave a cosmic imprint in the form of an additional radiation component, parameterized by an effective neutrino number $\Delta N_\text{eff}$. We explore minimal scenarios with a detectable signal in future CMB surveys: axions coupled democratically to all fermions, axion-electron coupling generated radiatively, the DFSZ framework for the QCD axion. The predicted $\Delta N_\text{eff}$ is larger than $0.03-0.04$ for all cases, close to the $2\sigma$ forecasted sensitivity of CMB-S4 experiments. This opens the possibility of testing with cosmological observations the solar axion interpretation of the XENON1T excess.

中文翻译：

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XENON1T 轴子的宇宙印记

最近由 XENON1T 观察到的电子反冲过剩可能解释为太阳轴子与电子耦合。如果这样的轴子在复合时仍然是相对论的，它们也会以附加辐射分量的形式留下宇宙印记，由有效中微子数 $\Delta N_\text{eff}$ 参数化。我们在未来的 CMB 调查中探索具有可检测信号的最小场景：轴子与所有费米子民主耦合、辐射生成的轴子-电子耦合、QCD 轴子的 DFSZ 框架。对于所有情况，预测的 $\Delta N_\text{eff}$ 大于 $0.03-0.04$，接近 CMB-S4 实验的 $2\sigma$ 预测灵敏度。这开启了用宇宙学观测测试 XENON1T 过剩的太阳轴子解释的可能性。