First Search for the Absorption of Fermionic Dark Matter with the PandaX-4T Experiment

Compared with the signature of dark matter elastic scattering off nuclei, the absorption of fermionic dark matter by nuclei opens up a new searching channel for light dark matter with a characteristic monoenergetic signal. In this Letter, we explore the 95.0-day data from the PandaX-4T commissioning run and report the first dedicated searching results of the fermionic dark matter absorption signal through a neutral current process. No significant signal was found, and the lowest limit on the dark matter-nucleon interaction cross section is set to be $1.5\times {10}^{-50}{\mathrm{cm}}^2$ for a fermionic dark matter mass of $40\mathrm{MeV}/c^2$ with 90% confidence level.

Figure 1

Expected fermionic absorption differential rate as a function of energy for xenon target, with a DM mass $m_{\chi }=40\mathrm{MeV}/c^2$ and absorption cross section ${\sigma }_{\chi -\mathrm{N}}^{\mathrm{NC}}=1.0\times {10}^{-49}{\mathrm{cm}}^2$. Various colors represent contributions from different isotopes of xenon. The reconstructed energy is shown in cyan, with the Lindhard factor [27] applied to recover the full energy.

Figure 2

Comparison between the ${}^{83\mathrm{m}}\mathrm{Kr}$ internal conversion electron energy spectrum and the simulation result.

Figure 3

Comparison between the simulated distributions and ${}^{241}\mathrm{Am}\text{−}\mathrm{Be}+D\text{−}D$ calibration data, using $1.5–2.5{\mathrm{keV}}_{\text{ee}}$ ($8.2–13.0{\mathrm{keV}}_{\text{nr}}$) and $15.5–16.5{\mathrm{keV}}_{\text{ee}}$ ($68.2–72.1{\mathrm{keV}}_{\text{nr}}$) energy windows for illustration. Black dot: normalized $S1$ or $S{2}_{\mathrm{b}}$ distribution of ${}^{241}\mathrm{Am}\text{−}\mathrm{Be}$ and $D\text{−}D$ calibration data. Red line: simulated distributions.

Figure 4

The $D\text{−}D$ reconstructed energy spectra from calibration and simulation, with a Gaussian fit performing on the right half part of the back-scatter peak (dashed curves).

Figure 5

Distribution of the selected events in data ROI. The red and cyan solid curves illustrate the 68% and 95% contours of the fermionic DM absorption signal distribution with $m_{\chi }=100\mathrm{MeV}/c^2$. The red dashed line represents the fitted NR median from the end-of-run ${}^{241}\mathrm{Am}\text{−}\mathrm{Be}$ and $D\text{−}D$ calibrations.

Figure 6

The 90% CL upper limits vs $m_{\chi }$ for the absorption cross sections of fermionic DM from the PandaX-4T commissioning data. The green band represents $\pm 1\sigma$ sensitivity band. The gray shaded area describes the constraints from collider experiment for $Z^{\prime }$ [20, 42]. The figure below displays limits and sensitivity bands vs $m_{\chi }$ for the U(1)’ gauge coupling from the PandaX-4T commissioning data. The level of fine-tuning needed in the UV complete model to avoid rapid $\chi \to \nu \nu \nu$ decay is denoted with dashed gray contours labeled ${\mathrm{FT}}_{\nu }$.