Searching for a solar relaxion or scalar particle with XENON1T and LUX

We consider liquid xenon dark matter detectors for searching a light scalar particle produced in the solar core, specifically one that couples to electrons. Through its interaction with the electrons, the scalar particle can be produced in the Sun, mainly through Bremsstrahlung process, and subsequently it is absorbed by liquid xenon atoms, leaving prompt scintillation light and ionization events. Using the latest experimental results of XENON1T and Large Underground Xenon, we place bounds on the coupling between electrons and a light scalar as $g_{\varphi ee}<8\times {10}^{-15}$ from S1-only analysis, and as $g_{\varphi ee}<2\times {10}^{-15}$ from S2-only analysis. These can be interpreted as bounds on the mixing angle with the Higgs boson, $\sin \theta <3\times {10}^{-9}(7\times {10}^{-10})$, for the case of a relaxion that couples to the electrons via this mixing. The bounds are a factor few weaker than the strongest indirect bound inferred from stellar evolution considerations.

Figure 1

Scalar flux from the Sun. The coupling to electrons is chosen to be $g_{\varphi ee}={10}^{-14}$, and the scalar is taken to be massless. The green dashed line is Bremsstrahlung flux from electrons interacting with hydrogen and helium ions, the red dashed line is the flux due to recombination and transitions of bounded electrons in heavier elements, the blue dashed line is the flux from Compton-like scattering, and the black line is the total scalar flux.

Figure 2

The black dots are the data with error bars showing statistical uncertainties ($1\sigma$ Poisson), the blue shaded histogram is the partial background model from XENON1T experiment, and the vertical dashed line shows the S2 threshold [42]. The orange shaded histogram, stacked on the background, is the signal model for a massless scalar, and $g_{\varphi ee}=2\times {10}^{-15}$. There is a peak around $\mathrm{S}2=300–400\mathrm{PE}$, which corresponds to the resonant peak of the scalar spectrum at $\omega \sim 0.2\mathrm{keV}$.

Figure 3

Constraints on the coupling $g_{\varphi ee}$. The result of this work includes the green thick solid line (LUX), the blue thick solid line (XENON1T), the orange thick dashed line (projected sensitivity of XENONnT experiment with $20\mathrm{ton}\times \mathrm{yr}$ exposure), and the black thick solid line (XENON1T S2-only analysis, capturing the resonant production). The thin red, purple, and gray lines represent stellar cooling bounds from red giants, horizontal branch stars with and without in-medium mixing effect [32], respectively.