Reexamining the Solar Axion Explanation for the XENON1T Excess

The XENON1T collaboration has observed an excess in electronic recoil events below 5 keV over the known background, which could originate from beyond-the-standard-model physics. The solar axion is a well-motivated model that has been proposed to explain the excess, though it has tension with astrophysical observations. The axions traveling from the Sun can be absorbed by the electrons in the xenon atoms via the axion-electron coupling. Meanwhile, they can also scatter with the atoms through the inverse Primakoff process via the axion-photon coupling, which emits a photon and mimics the electronic recoil signals. We found that the latter process cannot be neglected. After including the keV photon produced via the inverse Primakoff process in the detection, the tension with the astrophysical constraints can be significantly reduced. We also explore scenarios involving additional new physics to further alleviate the tension with the astrophysical bounds.

Figure 1

The solar axion induced photon signal through the inverse Primakoff process.

Figure 2

Fit to electronic recoil energy spectrum with $g_{a\gamma }$ only (top) and both $g_{a\gamma }$ and $g_{ae}$ allowed (bottom).

Figure 3

he 2D axion couplings parameter fit for the XENON1T excess after including the inverse Primakoff process. Our best fit (90% C.L.) to the XENON1T excess is shown in the red shaded region with the solid boundary. In comparison, a “XENON-like” analysis with only the electron recoil included as the signal yields is shown in the region with the dashed boundary. The main difference is that the inclusion of the inverse Primakoff process allows for a region in which $g_{a\gamma }$ is relatively large while $g_{ae}$ can be small, reducing the tension with the astrophysical data. Also included are the constraints (95% C.L.) from astrophysical observables including WDLF [68], the tip of RGB [69] and the $R$ parameter (with two models) [64], as well as the constraints from the global fit of the solar data [70], LUX [73], and PandaX [74], with arrows denoting excluded regions. We do not show constraints from CAST [75] and magnetic WDs [71] by assuming the axion mass $\gtrsim 1\mathrm{eV}$. The shaded green region contains $1\sigma$ to $4\sigma$ contours favored by the anomalous stellar cooling [25, 72].