Search for New Physics in Electronic Recoil Data from XENONnT

We report on a blinded analysis of low-energy electronic recoil data from the first science run of the XENONnT dark matter experiment. Novel subsystems and the increased 5.9 ton liquid xenon target reduced the background in the (1, 30) keV search region to $(15.8\pm 1.3)\text{events}/(\mathrm{ton}\times \text{year}\times \mathrm{keV})$, the lowest ever achieved in a dark matter detector and $\sim 5$ times lower than in XENON1T. With an exposure of 1.16 ton-years, we observe no excess above background and set stringent new limits on solar axions, an enhanced neutrino magnetic moment, and bosonic dark matter.

Figure 1

Efficiencies in reconstructed energy and the solar axion signal in true (red solid) and reconstructed energy (red dashed). The purple curve represents the detection efficiency dominated by the threefold S1 coincidence requirement. The black curve is the total efficiency, which is a combination of the detection and event-selection efficiencies. The discontinuity at 10 keV is caused by the still-blinded WIMP search region. The bands indicate the $1\sigma$ uncertainty. The black dashed line shows the 1 keV energy threshold of this search. The red solid and dashed lines represent the solar axion signal model before and after accounting for energy smearing and efficiency loss.

Figure 2

Calibration data and models at low energy. Both ${}^{220}\mathrm{Rn}$ and ${}^{37}\mathrm{Ar}$ data are fit using unbinned maximum likelihoods. The ${}^{220}\mathrm{Rn}$ data fit is performed in the energy interval (1, 140) keV, with the low-energy region showing the efficiency near the energy threshold. The ${}^{37}\mathrm{Ar}$ data validate the energy reconstruction and skew-Gaussian smearing model.

Figure 3

Science data (black dots) in the cS1-cS2 space, overlaid on ${}^{220}\mathrm{Rn}$ data (2D histogram). The WIMP search region (orange) is still blinded and not used in this search. Regions (gray shaded) far away from the ER band are excluded to avoid anomalous backgrounds. Iso-energy lines are represented by the gray dashed lines.

Figure 4

Fit to SR0 data using the background model $B_0$. The fit result of $B_0$ is the red line. The subdominant AC background is not shown.

Figure 5

Data and best-fit $B_0$ model below 30 keV. No significant excess above the background was found. The bump at $\sim 10\mathrm{keV}$ is from the LL shell of ${}^{124}\mathrm{Xe}$ $2\nu \mathrm{ECEC}$ [44], while the discontinuity at 10 keV is caused by the blinded WIMP search region; see Figs. 1 and 3. A finer binning than in Fig. 4 is used to show the event rate change near the threshold.

Figure 6

90% C.L. upper limit on different new physics models. Constraints on the axion-electron $g_{\mathrm{ae}}$ and axion-photon $g_{\mathrm{a}\gamma }$ couplings from a search for solar axions are shown in (a). Constraints on solar neutrinos with an enhanced magnetic moment (b), ALP DM (c), and dark photon DM (d) are shown together with the $1\sigma$ (green) and $2\sigma$ (yellow) sensitivity bands estimated with the background-only fit. Constraints between $(39,44)\mathrm{keV}/{\mathrm{c}}^2$ are excluded in (c) and (d) due to the unconstrained ${}^{83\mathrm{m}}\mathrm{Kr}$ background. Selected limits from other experiments [47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61] and astrophysical observations [62, 63, 64, 65, 66] are also shown.